Route to Safety: Navigating Router Pitfalls

IoT 4周前 admin
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Introduction 介绍

Wi-Fi routers have always been an attractive target for attackers. When taken over, an attacker may gain access to a victim’s internal network or sensitive data. Additionally, there has been an ongoing trend of attackers continually incorporating new router exploits into their arsenal for use in botnets, such as the Mirai Botnet.
Wi-Fi 路由器一直是攻击者的诱人目标。接管后,攻击者可能会访问受害者的内部网络或敏感数据。此外,攻击者不断将新的路由器漏洞整合到他们的武器库中,以用于僵尸网络,例如 Mirai 僵尸网络。

Consumer grade devices are especially attractive to attackers, due to many security flaws in them. Devices with lower security often contain multiple bugs that attackers can exploit easily, rendering them vulnerable targets. On the other hand, there are more secure devices that offer valuable insights and lessons to learn from.
消费级设备对攻击者特别有吸引力,因为它们存在许多安全漏洞。安全性较低的设备通常包含多个漏洞,攻击者可以轻松利用这些漏洞,使其成为易受攻击的目标。另一方面,有更安全的设备可以提供有价值的见解和经验教训。

This article gives a technical overview of vulnerabilities in routers for the awareness of security teams and developers, and provides suggestions in ways to avoid making mistakes that could result in such vulnerabilities. We will also look at past vulnerabilities affecting devices of various vendors to learn from their mistakes. Although the following content focuses on routers, the lessons learnt can be applied to other network devices as well.
本文对路由器中的漏洞进行了技术概述,以提高安全团队和开发人员的意识,并就如何避免犯可能导致此类漏洞的错误提供建议。我们还将研究过去影响不同供应商设备的漏洞,以从他们的错误中吸取教训。虽然以下内容侧重于路由器,但所吸取的经验教训也可以应用于其他网络设备。

Disclaimer: This article does not cover all bug classes.
免责声明:本文并未涵盖所有错误类别。

Attack Surface 攻击面

A router’s attack surface may be larger than one might expect. This is because there are various services running on it. Every service that receives requests from an external host, either on the local-area network (LAN) or wide-area network (WAN) interface, presents an attack surface as malformed requests may execute a vulnerable code path in the service. Below, we briefly explore the common services on routers that process external requests.
路由器的攻击面可能比预期的要大。这是因为上面运行着各种服务。在局域网 (LAN) 或广域网 (WAN) 接口上接收来自外部主机的请求的每个服务都会呈现一个攻击面,因为格式错误的请求可能会在服务中执行易受攻击的代码路径。下面,我们将简要介绍处理外部请求的路由器上的常见服务。

Admin Panel 管理面板

The admin panel of a network device hosts a large variety of configurations that could be changed by the device owner/administrator. Every input field is an attack surface as they are processed by the web service running on the device. For example, there may be an input field for blocking traffic to/from a certain IP address. The web service may handle this request in a way that is vulnerable to command injection. In short, the admin panel presents a huge attack surface to an attacker.
网络设备的管理面板托管了设备所有者/管理员可以更改的各种配置。每个输入字段都是一个攻击面,因为它们由设备上运行的 Web 服务处理。例如,可能有一个输入字段用于阻止传入/传出特定 IP 地址的流量。Web 服务可能以易受命令注入影响的方式处理此请求。简而言之,管理面板向攻击者提供了巨大的攻击面。

One may argue that this is not a very concerning attack surface, because an attacker would need to authenticate into the admin panel first in order to access this attack surface. This is true, and therefore many CVEs start with the term “authenticated”, e.g. “authenticated command injection”, which states that authentication is needed to exploit the vulnerability. However, an attacker may find an authentication bypass or there may be some endpoints on the admin panel that do not verify if the user is authenticated. An authentication bypass can be chained with an “authenticated vulnerability”; vulnerabilities on endpoints that do not require authentication are categorized with the term “unauthenticated”, e.g. “unauthenticated buffer overflow”.
有人可能会争辩说,这不是一个非常令人担忧的攻击面,因为攻击者需要先在管理面板中进行身份验证才能访问此攻击面。这是事实,因此许多 CVE 都以术语“authenticated”开头,例如“authenticated command injection”,它指出需要身份验证才能利用该漏洞。但是,攻击者可能会发现身份验证绕过,或者管理面板上的某些端点可能无法验证用户是否已通过身份验证。身份验证绕过可以与“经过身份验证的漏洞”链接;端点上不需要身份验证的漏洞被归类为术语“未经身份验证”,例如“未经身份验证的缓冲区溢出”。

Other Services 其他服务

Besides the admin panel, a router usually also runs other services that process requests for various protocols such as FTP, Telnet, Dynamic Host Configuration Protocol (DHCP) or Universal Plug and Play (UPnP). These services also present an attack surface on the router.
除了管理面板之外,路由器通常还运行其他服务,这些服务处理各种协议的请求,例如 FTP、Telnet、动态主机配置协议 (DHCP) 或通用即插即用 (UPnP)。这些服务还会在路由器上呈现攻击面。

Some services such as DHCP or UPnP do not require authentication. Furthermore, on some devices, some services are accessible from the WAN interface, which means that a remote attacker that is not on the local network can also access these services and exploit any vulnerability on them. For services that are so accessible, it is especially important to ensure that they are secure.
某些服务(如 DHCP 或 UPnP)不需要身份验证。此外,在某些设备上,某些服务可从 WAN 接口访问,这意味着不在本地网络上的远程攻击者也可以访问这些服务并利用它们上的任何漏洞。对于如此可访问的服务,确保它们是安全的尤为重要。

Poor Configurations 配置不良

First, we discuss some configuration mistakes present on some routers. The ones that we will discuss all follow the theme of access, namely
首先,我们讨论一些路由器上存在的一些配置错误。我们将要讨论的那些都遵循访问的主题,即

  • Access via hardcoded credentials
    通过硬编码凭据访问
  • Access to services from a remote network
    从远程网络访问服务
  • Access to root privileges by a running service or normal user
    正在运行的服务或普通用户对 root 权限的访问

Hardcoded Credentials 硬编码凭据

The firmware of a device contains an archive of programs and configuration files that are used by the device for its operations. The same firmware is distributed to and installed on all devices of the same model. Hence, if the credentials for any service (e.g. FTP or Telnet) running on the device is hardcoded in the firmware, the same credentials will be used by every device.
设备的固件包含设备用于其操作的程序和配置文件的存档。相同的固件将分发到同一型号的所有设备上并安装在同一设备上。因此,如果设备上运行的任何服务(例如FTP或Telnet)的凭据在固件中硬编码,则每个设备都将使用相同的凭据。

Impact 冲击

An attacker who has access to the firmware, which can usually be downloaded from the vendor’s website, can extract the files and inspect them to obtain the credentials. If such services are exposed to the Internet, anyone from anywhere in the world may be able to gain a shell on the device (Telnet), access sensitive data (FTP), or manipulate the device’s settings (httpd), depending on which service is vulnerable.
有权访问固件(通常可从供应商网站下载)的攻击者可以提取文件并检查它们以获取凭据。如果此类服务暴露在 Internet 上,则来自世界任何地方的任何人都可以在设备上获取外壳 (Telnet)、访问敏感数据 (FTP) 或操纵设备的设置 (httpd),具体取决于哪个服务易受攻击。

In a less dangerous situation, such vulnerable services may not be exposed to the Internet, but just the LAN instead. It is less devious, but anyone in the same network as the device will be able to access these exposed services. In the case of a router, anyone who is connected to its Wi-Fi network can abuse the hardcoded credentials on the exposed services. This is arguably fine for a router in a home network, since everyone that knows the Wi-Fi password is a family member or trusted friend. However, this is precarious for routers in public spaces like cafes or restaurants .
在不太危险的情况下,此类易受攻击的服务可能不会暴露在互联网上,而只是暴露在局域网中。它不那么狡猾,但与设备位于同一网络中的任何人都可以访问这些公开的服务。对于路由器,连接到其 Wi-Fi 网络的任何人都可以滥用公开服务上的硬编码凭据。对于家庭网络中的路由器来说,这可以说是可以的,因为每个知道Wi-Fi密码的人都是家庭成员或值得信赖的朋友。但是,这对于咖啡馆或餐馆等公共场所的路由器来说是不稳定的。

Examples 例子

Below, we share some examples of hardcoded credentials on routers that were reported by others in the past.
下面,我们将分享一些过去其他人报告的路由器硬编码凭据示例。

CVE-2022-46637 reports that the ProLink PLDT home fiber PRS1841 U V2 router contains hardcoded Telnet and FTP credentials. Furthermore, these services are exposed to the Internet, and vulnerable devices could be found through Shodan.
CVE-2022-46637 报告称,ProLink PLDT 家用光纤PRS1841 U V2 路由器包含硬编码的 Telnet 和 FTP 凭据。此外,这些服务暴露在互联网上,并且可以通过Shodan找到易受攻击的设备。

CVE-2020-29322 details that the telnet service of D-Link router DIR-880L with version 1.07 is initiated with hardcoded username and password. From the advisory, it is not conclusive whether the service is always initiated on startup, and whether the service is exposed to the WAN/Internet. However, if both were to be true, this would serve as an easy backdoor for any attacker to gain a shell on the device.
CVE-2020-29322 详细说明了 D-Link 路由器 DIR-880L 的 1.07 版的 telnet 服务是使用硬编码的用户名和密码启动的。从公告来看,该服务是否总是在启动时启动,以及该服务是否向 WAN/Internet 公开尚无定论。但是,如果两者都是真的,这将成为任何攻击者在设备上获得外壳的简单后门。

CVE-2019-14919 is about the Billion Smart Energy Router (SG600R2. Firmw v3.02.rc6) containing a hardcoded password in the login management binary used by Telnet. It is not mentioned in the writeup whether the service is exposed to WAN or LAN only. Besides this, unrelated to hardcoded credentials, a web shell that runs with root privileges is also found in the admin panel.
CVE-2019-14919 与 Billion Smart Energy Router (SG600R2 有关。Firmw v3.02.rc6) 在 Telnet 使用的登录管理二进制文件中包含硬编码密码。文章中没有提到服务是仅向 WAN 公开还是仅向 LAN 公开。除此之外,与硬编码凭据无关,还可以在管理面板中找到以 root 权限运行的 Web shell。

Suggestions 建议

We recommend that all services that require authentication such as FTP or Telnet be disabled before an administrator sets a strong password for them through the admin panel.
我们建议在管理员通过管理面板为其设置强密码之前,禁用所有需要身份验证的服务(如 FTP 或 Telnet)。

As for the admin panel password, common practice is to randomly generate a password for every device in the factory, and have the password attached as a sticker to the bottom of the device or as a note in the packaging. Upon first login, the administrator would be required to change the password before the admin panel could be used.
至于管理面板密码,通常的做法是为出厂时的每台设备随机生成一个密码,并将密码作为贴纸贴在设备底部或作为包装上的注释。首次登录时,管理员需要更改密码才能使用管理面板。

Services Exposed to the Internet
暴露在互联网上的服务

As mentioned above, attackers can use hardcoded credentials to access services that are exposed to the Internet instead of just being accessible from the LAN. However, if there are no hardcoded credentials, is it then acceptable for services such as the admin panel or FTP server to be exposed to the Internet?
如上所述,攻击者可以使用硬编码凭据来访问向 Internet 公开的服务,而不仅仅是从 LAN 访问的服务。但是,如果没有硬编码的凭据,那么是否可以接受管理面板或 FTP 服务器等服务向 Internet 公开?

There are many services that may run on a router, such as FTP, SSH, UPnP, admin panel, VPN. Although the credentials may not be hardcoded, there could be vulnerabilities in these services that may not require authentication and may lead to RCE on the device. As such, it is still safer to not expose all services to the Internet unless necessary.
有许多服务可以在路由器上运行,例如 FTP、SSH、UPnP、管理面板、VPN。尽管凭据可能未硬编码,但这些服务中可能存在漏洞,这些漏洞可能不需要身份验证,并可能导致设备上的 RCE。因此,除非必要,否则不要将所有服务暴露在互联网上仍然更安全。

Suggestions 建议

It is desirable that services like FTP, Telnet, or SSH not be turned on by default, but only upon the device administrator’s request through the admin panel. If an administrator would like to enable a service, he should be required to specify whether the services are to be exposed to the Internet, or just to the LAN. The device should not be exposing these services to the Internet without the administrator’s knowledge or consent. It is even more desirable for the administrator to be fully aware of such risks and willing to bear them before exposing these services to the Internet.
默认情况下,最好不要打开 FTP、Telnet 或 SSH 等服务,而只能根据设备管理员通过管理面板的请求打开。如果管理员想要启用某项服务,则应要求他指定是将服务公开给 Internet 还是仅公开给 LAN。设备不应在管理员不知情或未同意的情况下将这些服务暴露在 Internet 上。更希望管理员在将这些服务暴露在互联网上之前充分意识到这些风险并愿意承担这些风险。

Services Running as root
服务运行方式 root

On a PC, it is common to separate normal users from superusers (root/administrator). However, this is not the case for many consumer routers. Many of them only have the root user, and every service runs as root. On such devices, the FTP/Telnet/SSH/admin panel web services are running as root. So, when an attacker obtains RCE on a service, he also has a shell with root privileges. There is no privilege separation to prevent the whole device from being compromised when a service is exploited.
在 PC 上,通常将普通用户与超级用户(root/管理员)分开。但是,对于许多消费类路由器来说,情况并非如此。其中许多只有 root 用户,并且每个服务都以 root .在此类设备上,FTP/Telnet/SSH/管理面板 Web 服务以 root .因此,当攻击者在服务上获得 RCE 时,他也拥有具有 root 权限的 shell。没有权限分离来防止整个设备在被利用时受到损害。

We list some examples below:
我们在下面列出一些例子:

The writeups above show that a root shell could be obtained without a privilege escalation exploit, which means that there is no privilege separation.
上面的文章表明,可以在没有权限升级漏洞的情况下获得 root shell,这意味着没有权限分离。

Suggestions 建议

Every process/service should be running as a different user, applying the principle of least privileges. For example, the httpd service runs as the web user; the upnpd service runs as the upnp user; the ftp service runs as the ftp user, and so on. Under this configuration, when a service is exploited, the attacker cannot compromise other services or the whole device without a privilege escalation exploit.
每个进程/服务都应以不同的用户身份运行,并应用最小权限原则。例如, httpd 服务以 web 用户身份运行; upnpd 服务以 upnp 用户身份运行; ftp 服务以 ftp 用户身份运行,依此类推。在此配置下,当某项服务被利用时,攻击者无法在不利用权限提升的情况下破坏其他服务或整个设备。

Password-less sudo?
无 sudo 密码?

On some of the routers that we have inspected, they do not run their admin panel web service as root but a normal user. This is good. However, to perform some system-level operations, they want to make use of shell commands which require root privileges. For example, the iptables command.
在我们检查过的一些路由器上,它们不会以普通用户的身份 root 运行其管理面板 Web 服务。这很好。但是,要执行某些系统级操作,他们希望使用需要 root 权限的 shell 命令。例如, iptables 命令。

Bad Example 坏例子

Consider the scenario where the admin panel supports blocking traffic to and from a certain IP address using iptables (requires superuser privileges). To achieve this, developers may be tempted to introduce a SUID binary that works like sudo but does not need a password. In other words, a SUID binary that takes a command string as argument and runs that command string as a shell command with root privileges. A simple example of this is shown below (let’s refer to it as custom_sudo in the following examples):
考虑以下情况:管理面板支持使用 iptables (需要超级用户权限)阻止进出特定 IP 地址的流量。为了实现这一点,开发人员可能会想引入一个 SUID 二进制文件,它的工作方式类似于 sudo 密码,但不需要密码。换言之,一个 SUID 二进制文件,它采用命令字符串作为参数,并将该命令字符串作为具有 root 权限的 shell 命令运行。下面显示了一个简单的示例(让我们在 custom_sudo 以下示例中引用它):

int main(int argc, char** argv)
{
  setuid(geteuid());
  system(argv[1]);
  return 0;
}

Then, the developers may use custom_sudo to run an iptables command as follows.
然后,开发人员可以使用 custom_sudo 以下命令运行命令 iptables 。

snprintf(cmd, 255, "iptables -A INPUT -s '%s' -j DROP", ip_addr_to_block);
execve("/usr/sbin/custom_sudo", { "/usr/sbin/custom_sudo", cmd, 0 }, 0);

Obviously, having such a program defeats the purpose of having different users, when all users can just use this program to run any commands as root, violating the principle of least privileges.
显然,拥有这样的程序违背了拥有不同用户的目的,因为所有用户都可以使用这个程序来运行任何命令,违反了 root 最小权限原则。

Suggestions 建议

It is not recommended that operations requiring root privileges be executed by means of providing a whole command string to such a SUID program as sudo or custom_sudo. Instead, we suggest having a SUID program that only takes in the necessary values as arguments and then use them in carrying out the desired operations.
不建议通过向 SUID 程序提供整个命令字符串(如 sudo 或 custom_sudo )来执行需要 root 特权的操作。相反,我们建议使用一个 SUID 程序,该程序仅将必要的值作为参数,然后使用它们来执行所需的操作。

For the example above on blocking an IP address, we suggest having a SUID program called block_ip_addr that just takes in the IP address as an argument, then performs the iptables operation with the given IP address string internally. For example:
对于上面关于阻止 IP 地址的示例,我们建议调用 block_ip_addr 一个 SUID 程序,该程序仅将 IP 地址作为参数,然后在内部使用给定的 IP 地址字符串执行 iptables 操作。例如:

execve("/usr/sbin/block_ip_addr", { "/usr/sbin/block_ip_addr", ip_addr_to_block, 0 }, 0);

Then, the implementation of block_ip_addr can be as follows:
然后,实现 block_ip_addr 可以如下:

char* ip_addr_to_block = argv[1];
execve("/bin/iptables", { "/bin/iptables", "-A", "INPUT", "-s", ip_addr_to_block, "-j", "DROP", 0 }, 0);

If this is done this for all commands that require root privileges, the custom_sudo program is no longer necessary and can be removed entirely from the system.
如果对所有需要 root 权限的命令都执行此操作,则不再需要该 custom_sudo 程序,并且可以从系统中完全删除。

However, if a developer strongly insists on using such a custom_sudo program, please at least verify that the program that will be executed is among a list of allowed programs. For example:
但是,如果开发人员强烈坚持使用此类 custom_sudo 程序,请至少验证将要执行的程序是否在允许的程序列表中。例如:

if (strncmp(argv[1], "/bin/iptables ", strlen("/bin/iptables "))) {
  // if strncmp returns a non-zero value, the command string does not start with the expected `iptables `
  // abort
  ...
}

Note that in the check above, /bin/iptables ends with a space character. This is to make sure that the command string indeed is running the /bin/iptables program, and not something like iptablesfake. Also, use the absolute path (e.g. /bin/iptables) and not just the program name (e.g. iptables), to prevent the invocation of the wrong program, as the program path is determined according to the PATH environment variable. For example /home/user/iptables created by an attacker may be executed instead of /bin/iptables if the PATH variable is configured as so.
请注意,在上面的检查中, /bin/iptables 以空格字符结尾。这是为了确保命令字符串确实正在运行程序 /bin/iptables ,而不是类似 iptablesfake .此外,使用绝对路径(例如 /bin/iptables )而不仅仅是程序名称(例如 iptables ),以防止调用错误的程序,因为程序路径是根据 PATH 环境变量确定的。例如 /home/user/iptables , /bin/iptables 如果 PATH 变量配置为这样,则攻击者创建的变量可能会被执行。

Also, make sure that there are no command injection vulnerabilities that could be exploited to bypass such checks. This could be done by running the desired command with execve instead of system.
此外,请确保没有可利用的命令注入漏洞来绕过此类检查。这可以通过运行 execve 所需的命令而不是 system 来完成。

In short, when considering the implementation of operations that require superuser privileges, the principle of least privileges should be followed to ensure that the chosen implementation does not provide a user with more privileges than necessary.
简而言之,在考虑实现需要超级用户权限的操作时,应遵循最小权限原则,以确保所选实现不会为用户提供不必要的权限。

Summary 总结

All of the misconfigurations above have straightforward solutions. However, the implementation of these solutions incur extra development and testing time. For the consumers, it is desirable that all router vendors consider these improvements as “must have” and not just “good to have”.
上面的所有错误配置都有简单的解决方案。但是,这些解决方案的实施会产生额外的开发和测试时间。对于消费者来说,希望所有路由器供应商都认为这些改进是“必须拥有的”,而不仅仅是“好的”。

Vulnerability Classes 漏洞类别

In this section, we will discuss the following vulnerabilities affecting services running on routers.
在本节中,我们将讨论以下影响路由器上运行服务的漏洞。

  • Authentication Bypass 身份验证绕过
  • Command Injection 命令注入
  • Buffer Overflow 缓冲区溢出
  • Format String Bug 格式字符串错误

Command injection and buffer overflow are the two main vulnerability classes present in routers that lead to RCE. Format string vulnerabilities may also result in RCE, but are very rarely seen in the recent years.
命令注入和缓冲区溢出是路由器中存在的导致 RCE 的两个主要漏洞类别。格式字符串漏洞也可能导致 RCE,但近年来很少见。

Authentication Bypass 身份验证绕过

The attack surface of a service is greatly reduced if the service requires authentication. Typically, an unauthenticated client would only be able to send requests related to authentication, or for querying some information about the service or the device. Therefore, an authentication bypass vulnerability is valuable to attackers as it opens up the whole remaining attack surface.
如果服务需要身份验证,则服务的攻击面将大大减少。通常,未经身份验证的客户端只能发送与身份验证相关的请求,或者用于查询有关服务或设备的某些信息。因此,身份验证绕过漏洞对攻击者很有价值,因为它会打开整个剩余的攻击面。

Besides that, even without RCE, a non-administrator could still perform an authentication bypass to disclose and control sensitive settings on the admin panel. An authentication bypass on other services that require authentication such as FTP, SSH or Telnet could also lead to shell access or disclosure of sensitive information. Hence, authentication bypass should not be taken lightly.
除此之外,即使没有 RCE,非管理员仍然可以执行身份验证绕过,以披露和控制管理面板上的敏感设置。对其他需要身份验证的服务(如 FTP、SSH 或 Telnet)的身份验证绕过也可能导致 shell 访问或敏感信息泄露。因此,不应掉以轻心。

Examples 例子

In the following sub-sections, we share examples of authentication bypass bugs on routers that were reported in the past.
在以下小节中,我们将分享过去报告的路由器上的身份验证绕过错误的示例。

CVE-2021-32030: Mistake in authentication logic
CVE-2021-32030:身份验证逻辑错误CVE-2021-32030:身份验证逻辑错误

CVE-2021-32030 reports an authentication bypass on the ASUS GT-AC2900 router. In short, the vulnerability arises due to a mistake in the authentication flow as simplified below:
CVE-2021-32030 报告了 ASUS GT-AC2900 路由器上的身份验证绕过。简而言之,该漏洞是由于身份验证流程中的错误而产生的,简化如下:

  1. A header field asus_token should be supplied by the client for authenticating into the admin panel.
  2. asus_token is compared with the ifttt_token value retrieved from *nvram.
  3. If nvram does not contain ifttt_token, it returns an empty string, i.e. a null-byte.
  4. If asus_token is a null byte, the comparison succeeds and the client is authenticated successfully.
    如果 asus_token 为 null 字节,则比较成功,客户端已成功通过身份验证。

*nvram stands for non-volatile RAM. It is used by many routers to store configuration values that should persist over device reboot. Hence the term non-volatile, as the contents persist, unlike for normal RAM in which all its contents will be cleared when the device is turned off.
nvram 代表非易失性 RAM。许多路由器使用它来存储应在设备重新启动后保留的配置值。因此,术语非易失性,因为内容持续存在,这与普通RAM不同,在普通RAM中,当设备关闭时,其所有内容都将被清除。

Suggestions 建议

In this case, the vulnerability is caused by programmer error, in which an unexpected edge case input breaks the authentication logic. The relevant function should first ensure that ifttt_token is not an empty string, before comparing it with the client-supplied asus-token. To be extra careful, the developers may also add an additional check to ensure that asus-token is not an empty string, in case it may be compared with another token that is also retrieved from nvram in the future.
在这种情况下,该漏洞是由程序员错误引起的,其中意外的边缘情况输入破坏了身份验证逻辑。相关函数应首先确保它不是 ifttt_token 空字符串,然后再将其与客户端提供的 asus-token .为了格外小心,开发人员还可以添加一个额外的检查,以确保它不是 asus-token 空字符串,以防将来可能将其与另一个从nvram中检索到的令牌进行比较。

CVE-2020-8864: Mistake in authentication logic
CVE-2020-8864:身份验证逻辑错误

CVE-2020-8864 reports an authentication bypass on the D-Link DIR-882, DIR-878 and DIR-867 routers. The exploit is the same as the one above in the ASUS router, although the implementation of authentication logic is different, as simplified below:
CVE-2020-8864 报告了 D-Link DIR-882、DIR-878 和 DIR-867 路由器上的身份验证绕过。该漏洞与华硕路由器中的上述漏洞相同,但身份验证逻辑的实现不同,简化如下:

  1. LoginPassword is provided by the client for authentication.
    LoginPassword 由客户端提供用于身份验证。
  2. strncmp is used to compare the client-supplied password with the correct password, as shown below:
    strncmp 用于将客户端提供的密码与正确的密码进行比较,如下所示:

    strncmp(db_password, attacker_provided_password, strlen(attacker_provided_password));
    
  3. If an attacker submits a login request with an empty password, strncmp will have 0 as its 3rd argument (length), and that returns 0, meaning the two strings compared are equal, which is correct because the first 0 characters of both strings are the same. As a result, authentication is successful.
    如果攻击者使用空密码提交登录请求, strncmp 则其第三个参数(长度)将为 0,并且返回 0,这意味着比较的两个字符串相等,这是正确的,因为两个字符串的前 0 个字符相同。因此,身份验证成功。
Suggestions 建议

Again, the vulnerability is caused by programmer error. The fix here is by passing strlen(db_password) as the 3rd argument, instead of strlen(attacker_provided_password).
同样,该漏洞是由程序员错误引起的。这里的解决方法是作为第 3 个参数传递 strlen(db_password) ,而不是 strlen(attacker_provided_password) .

CVE-2020-8863: Expected password value is controlled by attacker
CVE-2020-8863:攻击者控制预期密码值CVE-2020-8863: Expected password value is control by attacker

CVE-2020-8863 reports another authentication bypass on the D-Link DIR-882, DIR-878 and DIR-867 routers.
CVE-2020-8863 报告了 D-Link DIR-882、DIR-878 和 DIR-867 路由器上的另一个身份验证绕过。

The subsection above on CVE-2020-8864 was simplified by omitting some details about the authentication flow. Here, we describe it in detail so that we can accurately describe this authentication bypass later.
上面关于 CVE-2020-8864 的小节通过省略有关身份验证流程的一些详细信息进行了简化。在这里,我们详细描述它,以便我们以后可以准确地描述这种身份验证绕过。

These routers use the Home Network Administration Protocol (HNAP), a SOAP-based protocol for the requests on the admin panel from the client to the web server. The authentication process is as follows:
这些路由器使用家庭网络管理协议 (HNAP),这是一种基于 SOAP 的协议,用于管理面板上从客户端到 Web 服务器的请求。认证过程如下:

  1. The client sends a request message and obtains an authentication challenge from the server.
  2. The server responds to the request with the values: Challenge and PublicKey.
  3. The client should combine the PublicKey with the password to create the PrivateKey. Then, use the PrivateKey and Challenge to generate a challenge response that is to be submitted as LoginPassword to the server.
  4. The server will perform the same computations, and if the LoginPassword matches, it means that the client knows the correct password, and authentication succeeds.

The description above is taken from this writeup on the ZDI blog. Check it out for more details and code examples.
以上描述摘自ZDI博客上的这篇文章。查看更多详细信息和代码示例。

In the web server binary, it is discovered that there is a code path that checks a PrivateLogin field in the login request. It is as follows:
在 Web 服务器二进制文件中,发现存在一个代码路径,用于检查登录请求中的 PrivateLogin 字段。具体如下:

//  If PrivateLogin != NULL && PrivateLogin  == "Username"  Then Password = Username
    if ((PrivateLogin == (char *)0x0) || (iVar1 = strncmp(PrivateLogin,"Username",8), iVar1 != 0)) {
      GetPassword(Password,0x40);  // [1]
    }   
    else {
      strncpy(Password,Username,0x40);  // [2]
    }   
    GenPrivateKey(Challenge,Password,Publickey,PrivateKey,0x80);

If the submitted PrivateLogin field contains “Username”, then the submitted Username value is used as the password ([2]) for generating the expected LoginPassword value (challenge response), instead of using the user’s actual password by calling GetPassword ([1]). To put it in simple terms, the client can control the password that is used to generate the expected challenge response, therefore bypassing authentication.
如果提交 PrivateLogin 的字段包含“用户名”,则提交 Username 的值将用作生成预期 LoginPassword 值(质询响应)的密码 ([2]),而不是通过调用 GetPassword ([1]) 使用用户的实际密码。简单来说,客户端可以控制用于生成预期质询响应的密码,从而绕过身份验证。

It is unclear what the purpose of the PrivateLogin field is. As HNAP is an obsolete proprietary protocol with no documentation online, it is hard for us to determine the original purpose of this field.
目前尚不清楚该领域 PrivateLogin 的目的是什么。由于HNAP是一个过时的专有协议,没有在线文档,我们很难确定该领域的最初目的。

As a takeaway, ensure that when implementing an authentication protocol, secrets such as passwords should not be mixed with values submitted by the client.
总而言之,请确保在实现身份验证协议时,密码等机密不应与客户端提交的值混合。

Summary 总结

Authentication bypass vulnerabilities on the admin panel arise from programmer mistakes, as they fail to consider edge case inputs such as empty passwords that may break the authentication logic. Besides that, there may also be flawed implementations of a protocol. Special attention should be given to reviewing the implementation of an authentication routine to catch unintended bypasses due to such mistakes.
管理面板上的身份验证绕过漏洞是由程序员的错误引起的,因为它们没有考虑可能破坏身份验证逻辑的边缘情况输入,例如空密码。除此之外,协议的实现也可能存在缺陷。应特别注意审查身份验证例程的实现,以捕获由于此类错误而导致的意外绕过。

Command Injection 命令注入

Command injection is a commonly seen vulnerability in routers or other network and IoT devices. In this section, we discuss the vulnerable code pattern, reason behind the ubiquity of such vulnerability, guidelines to prevent them, and some examples of them in various routers in the past.
命令注入是路由器或其他网络和 IoT 设备中常见的漏洞。在本节中,我们将讨论易受攻击的代码模式、此类漏洞无处不在的原因、防止它们的指南,以及过去在各种路由器中的一些示例。

Root Cause 根源

Command injection is possible because a command string that contains unsanitized user input is executed as a shell command, by means of system or popen in C, os.system in Python, os.execute or io.popen in Lua. For example,
命令注入是可能的,因为包含未经审查的用户输入的命令字符串是作为 shell 命令执行的,通过 C system 或 popen 在 C 中、 os.system Python os.execute 或 io.popen Lua 中执行。例如

sprintf(cmd, "ping %s", ip_addr);
system(cmd);
os.system(f"ping {ip_addr}")

In the examples above, an input IP address such as 127.0.0.1; reboot will result in a command string of ping 127.0.0.1; reboot, which escapes the intended ping command and executes arbitrary commands such as reboot.

Rationale for using shell commands

Running shell commands to perform various system-level operations is definitely not the norm in software development. For performance and compatibility reasons, in software running on personal computers, it is very rare to see system-level operations such as filesystem or network operations being carried out through running shell commands. However, this is almost ubiquitous in the world of embedded devices such as routers.

The reasons behind this phenomenon are somewhat acceptable. In routers, performance is not a concern because said operations are not very frequently performed. Compatibility is also not affected because programs only run on the vendor’s own devices. Without such factors in mind, it is tempting to find the simplest way to implement a required feature, and such simplest way may be flawed in security.

For example, look at the following function from [NETGEAR’s pufwUpgrade binary](https://research.nccgroup.com/2024/02/09/puckungfu-2-another-netgear-wan-command-injection/#:~:text=int-,saveCfuLastFwpath,-(char%20*).

int saveCfuLastFwpath(char *fwPath)
{
    char command [1024];
    memset(command, 0, 0x400);
    snprintf(command, 0x400, "rm %s", "/data/cfu_last_fwpath");
    system(command);
    // Command injection vulnerability
    snprintf(command, 0x400, "echo \"%s\" > %s", fwPath, "/data/cfu_last_fwpath");
    DBG_PRINT(DAT_0001620f, command);
    system(command);
    return 0;
}

There are proper C library functions for deleting a file (unlink), as well as for writing to a file (fopen and fwrite). But the developers instead chose to create shell command strings starting with rm and echo and run them with system. Admittedly, shell commands are easier to remember and use without needing to refer back to the C documentation.
有适当的 C 库函数用于删除文件 ( unlink fopen ) 以及写入文件 ( 和 fwrite )。但开发人员选择创建以 rm 和 echo 开头的 shell 命令字符串,并使用 system .诚然,shell 命令更易于记忆和使用,而无需参考 C 文档。

Another such example is when a router firmware developer inserts a user-entered password into a command string to calculate the password hash using md5sum. It takes more time and effort to write C code that achieves the same goal.
另一个这样的例子是,当路由器固件开发人员将用户输入的密码插入到命令字符串中时,请使用 md5sum 来计算密码哈希。编写实现相同目标的 C 代码需要更多的时间和精力。

To compensate for such reduced effort, at least some device vendors do sanitize their inputs before inserting them into a command string and calling system. This is good. However, it just takes a small mistake, such as forgetting to sanitize an input field, to introduce a command injection vulnerability allowing RCE on the device. A vulnerability may also be introduced due to certain manipulations performed on the command string, or some unexpected reasons due to the way the command is written. For example, CVE-2024-1837 for which we will publish the advisory soon. Found by me 🙂
为了弥补这种减少的工作量,至少一些设备供应商在将输入插入命令字符串并调用 system .这很好。但是,只需要一个小错误,例如忘记清理输入字段,就会引入允许在设备上进行 RCE 的命令注入漏洞。由于对命令字符串执行了某些操作,或者由于命令的编写方式而导致了一些意外原因,也可能引入漏洞。例如,CVE-2024-1837,我们将很快发布公告。由我发现:)

From my limited exposure, I have noticed that the more expensive Cisco and ASUS routers do not take shortcuts by performing OS-level operations through shell commands, but instead they properly implement them with the corresponding API functions. If they were to execute external programs with user inputs, they only use safe functions such as execve that are not susceptible to command injection. With such efforts, they eradicate even the smallest possibility of command injection on the admin panel.
从我有限的曝光中,我注意到更昂贵的思科和华硕路由器不会通过shell命令执行操作系统级操作来走捷径,而是通过相应的API功能正确地实现它们。如果它们要使用用户输入执行外部程序,则它们只使用安全功能,例如 execve 不易受命令注入影响的功能。通过这样的努力,他们甚至消除了管理面板上命令注入的最小可能性。

In the next section, we share some suggestions to prevent command injection in the event where external programs need to be executed.
在下一节中,我们将分享一些建议,以防止在需要执行外部程序的情况下进行命令注入。

Prevention 预防

In this subsection, we share secure code design guidelines to prevent command injection. First of all, as mentioned repeatedly in the subsections above, not all operations need to be performed through shell commands, so please avoid doing so unless necessary.
在本小节中,我们将分享安全代码设计指南,以防止命令注入。首先,正如上面小节中反复提到的,并非所有操作都需要通过 shell 命令执行,因此除非必要,否则请避免这样做。

Avoid system commands 避免 system 命令

The decision to run external programs using a shell command is the cause for potential command injection bugs. On top of just recommending developers to not write such code, security teams could help them avoid the usage of functions that run shell commands by raising warnings where such functions are called.
使用 shell 命令运行外部程序的决定是导致潜在命令注入错误的原因。除了建议开发人员不要编写此类代码之外,安全团队还可以通过在调用此类函数时发出警告来帮助他们避免使用运行 shell 命令的函数。

We list below such functions that should be avoided from the codebase. Note that the list is not exhaustive. Security teams should check if there are other such functions supported by the standard library or any third party libraries imported by the codebase.

  • C: systempopen
  • Python: os.systemsubprocess.Popen/subprocess.call/subprocess.run with shell=True argument

Do not worry about whether such a rule may break compatibility, because it won’t. We provide the safe alternative below, which can do the same things that a shell command can do.

Run executable with argument list

There is a safe way to run external scripts or binaries, by specifying a program and providing an argument list, by using execve in C or subprocess.Popen/subprocess.run/subprocess.call (without the shell=True argument) in Python. For example,

execve("/bin/ping", { "/bin/ping", ip_addr, 0 }, 0);
subprocess.Popen(["/bin/ping", ip_addr])

This eliminates the possibility of command injection on the command directly. However, beware that this does not make any guarantees about whether there will be command injection in the target executable, for example /bin/ping as in the example above.
这消除了直接在命令上注入命令的可能性。但是,请注意,这并不能保证目标可执行文件中是否会有命令注入,例如 /bin/ping ,如上例所示。

Note that in C, execve replaces the current running process with the target executable. That is, if execve is called with /bin/ping by the admin panel server, the whole service will be gone, replaced by ping. This is certainly not the intended behaviour. Remember to fork the process before calling execve.
请注意,在 C 语言中, execve 将当前正在运行的进程替换为目标可执行文件。也就是说,如果 execve /bin/ping 被管理面板服务器调用,则整个服务将消失,取而代之的是 ping 。这当然不是预期的行为。在调用 execve 之前, fork 请记住该过程。

However, watch out for code as in the example below. It defeats the purpose since it runs a shell command again.
但是,请注意以下示例中的代码。它违背了目的,因为它再次运行 shell 命令。

sprintf(cmd, "ping %s", ip_addr);
execve("/bin/sh", { "/bin/sh", "-c", cmd, 0 }, 0);
Custom execve 习惯 execve

In languages such as Lua, there may not be a library function such as execve for running a specific program with an argument list. In such unfortunate scenario, there is no choice but to use the system or popen equivalent that is available in this language. In Lua, that is os.execute.
在 Lua 等语言中,可能没有库函数,例如 execve 用于运行带有参数列表的特定程序。在这种不幸的情况下,别无选择,只能使用这种语言中可用的 system or popen 等效项。在 Lua 中,即 os.execute .

To protect the developers from crafting command strings prone to command injection, the development team may create a function similar to execve that takes in an executable path and argument list, then crafts the command string with these values, and passes it to system for execution. The executable path can be concatenated together with all the arguments, but there are two important things to take note:
为了防止开发人员制作容易进行命令注入的命令字符串,开发团队可以创建一个类似于该函数 execve 的函数,该函数接受可执行路径和参数列表,然后使用这些值制作命令字符串,并将其传递给 system 执行。可执行路径可以与所有参数连接在一起,但有两件重要的事情需要注意:

Wrap every argument with single quotes. This is to prevent command substitution, because in a shell command, contents within single quotes will be passed verbatim as an argument. With single quotes, any sequence of characters in the form $(...) or `...` will not be evaluated. Do not wrap the arguments in double quotes. Command substitution will still apply for contents within double quotes.
用单引号括住每个参数。这是为了防止命令替换,因为在 shell 命令中,单引号内的内容将作为参数逐字传递。使用单引号时,将不计算形式 $(...) or `...` 中的任何字符序列。不要用双引号括括参数。命令替换仍将适用于双引号内的内容。

Escape the single quotes in every argument. If an argument contains single quotes, it will close the opening single quote before it, and any command substitution payload after it will be evaluated. Make sure all single quotes in every argument are escaped by prepending them with a backslash character, so that they do not close the opening single quote before the argument.
对每个参数中的单引号进行转义。如果参数包含单引号,它将关闭它前面的左单引号,并且将计算它之后的任何命令替换有效负载。确保每个参数中的所有单引号都通过在它们前面加上反斜杠字符进行转义,以便它们不会在参数之前关闭开始的单引号。

The example below demonstrates how the operations above could be implemented in Lua.
下面的示例演示了如何在 Lua 中实现上述操作。

function custom_execute(executable_path, args)
    -- Escape single quotes within arguments
    local escape_single_quotes = function(str)
        return string.gsub(str, "'", "\\'")
    end

    -- Quote and escape each argument
    local quoted_args = {}
    for _, arg in ipairs(args) do
        table.insert(quoted_args, "'" .. escape_single_quotes(tostring(arg)) .. "'")
    end

    -- Concatenate executable path and quoted arguments
    local command = executable_path .. " " .. table.concat(quoted_args, " ")

    -- Execute the command using os.execute
    os.execute(command)
end

-- Example usage
local echo_path = "/bin/echo"
local args = {"hello", "world", "hey"}
custom_execute(echo_path, args)

The custom_execute function above removes the possibility of command injection, regardless of any user input that may be part of the argument list. Such a function gives developers a peace of mind when it is necessary to execute external programs, and removes the burden from them to consider any sanitization that is required for the user input.

Avoid eval in shell scripts
避免 eval 在 shell 脚本中

The suggestions above are applicable to services that receive input from a client request and use this input value in the execution of another program on the system. The protections above ensure that handlers for client requests are safe from command injection. However, they do not make any guarantees about the safety of the external program that is executed.
上述建议适用于从客户端请求接收输入并在系统上执行另一个程序时使用此输入值的服务。上述保护措施可确保客户端请求的处理程序不会受到命令注入的影响。但是,他们不对执行的外部程序的安全性做出任何保证。

Consider the following example where /usr/sbin/custom_script is a shell script that is given a user input value as an argument. There is no command injection in executing the script. However, there could be command injection within the script that is being executed.
请考虑以下示例,其中 /usr/sbin/custom_script 是一个 shell 脚本,它被赋予一个用户输入值作为参数。执行脚本时没有命令注入。但是,正在执行的脚本中可能存在命令注入。

execve("/usr/sbin/custom_script", { "/usr/sbin/custom_script", user_input, 0 }, 0);

Consider the following shell script that inserts an argument ($1) into a command string and executes it in various ways.
请考虑以下 shell 脚本,该脚本将参数 ( $1 ) 插入命令字符串并以各种方式执行它。

  1. Using eval. 使用 eval .
  2. Using $(...) (command substitution).
    使用 $(...) (命令替换)。
  3. Using `...` (command substitution).
    使用 `...` (命令替换)。
#!/bin/sh
cmd="echo $1"

files=`eval "$cmd"`
echo $files

files=$($cmd)
echo $files

files=`$cmd`
echo $files

The output of the script above when executed with aaa;whoami as an argument is as follows.
当作为 aaa;whoami 参数执行时,上述脚本的输出如下所示。

$ ./script 'aaa;whoami'
aaa user        // command injection occured
aaa;whoami
aaa;whoami

Notice that when command substitution (2nd and 3rd example) is performed, there is no command injection. This is because the argument $1 is passed as an argument to the echo program as written in cmd. This means that the whole argument string containing aaa;whoami is passed to echo as an individual argument.
请注意,执行命令替换(第 2 个和第 3 个示例)时,没有命令注入。这是因为该参数 $1 是作为参数传递给程序的 echo ,如 中 cmd 编写的那样。这意味着包含 aaa;whoami 的整个参数字符串将作为单个参数传递给 echo 。

On the other hand, in the case of eval$1 is interpolated, that is, expanded as a string and inserted into the command string, resulting in echo aaa;whoami being the command that is executed. Command injection is present in this case, as seen in the output attached above.
另一方面,在 eval 的情况下, $1 是插值的,即作为字符串展开并插入到命令字符串中,从而 echo aaa;whoami 成为执行的命令。在这种情况下存在命令注入,如上面所附的输出所示。

Therefore, avoid the usage of eval in shell scripts, to prevent any potential command injection vulnerabilities due to mishandling of arguments which may come from user input.
因此,请避免使用 eval in shell 脚本,以防止由于错误处理可能来自用户输入的参数而导致任何潜在的命令注入漏洞。

Actionable Steps

To summarize the suggestions above, we recommend development and security teams to impose the following rules on their codebase:

  1. Avoid dangerous functions that execute a command string directly, e.g. systempopeneval. Only allow the execution of an external program by passing an argument list.
  2. A thorough review should be conducted to decide whether a function is safe or necessary.
  3. If an unsafe function is necessary (such as os.execute in Lua), use custom wrapper functions that call the function in a safe manner. For example, the custom_execute wrapper for os.execute in Lua shown above.
    如果需要不安全的函数(例如 os.execute 在 Lua 中),请使用以安全方式调用该函数的自定义包装函数。例如,如上所示的 custom_execute Lua os.execute 中的包装器。

Examples 例子

In the following sub-sections, we show examples of command injection in various routers, in implementations using different programming languages, in various services, to show that this vulnerability can manifest itself under different contexts.
在以下小节中,我们展示了各种路由器、使用不同编程语言的实现、各种服务中的命令注入示例,以表明此漏洞可以在不同的上下文中表现出来。

In 2021, I discovered some command injection vulnerabilities in the DIR-1960/1360/2660/3060 and DIR-X1560 devices. The vulnerabilities were found in code that handles web requests from the admin panel.
2021 年,我在 DIR-1960/1360/2660/3060 和 DIR-X1560 设备中发现了一些命令注入漏洞。这些漏洞是在处理来自管理面板的 Web 请求的代码中发现的。

Some were due to complete lack of sanitization of user input, inserting them in command strings to run programs like sendmailsmbpasswd and iptables. Such code is written as operations related to email or SMB can be rather complicated to implement, and a simpler solution would be to use the relevant programs that are already present on the system. The code for running such commands with user input had insufficient or no sanitization performed on the input values, resulting in command injection attacks being possible through the corresponding web endpoints.
有些是由于完全没有对用户输入进行清理,将它们插入命令字符串中以运行 和 smbpasswd iptables . sendmail 此类代码编写为与电子邮件或 SMB 相关的操作可能相当复杂,更简单的解决方案是使用系统上已经存在的相关程序。使用用户输入运行此类命令的代码未对输入值执行充分或未执行审查,从而导致可能通过相应的 Web 终结点进行命令注入攻击。

Failed validation of IP range string
IP 范围字符串验证失败

There was a rather interesting case of flawed input validation done before inserting a user input string into an iptables command string. The user input value is an IP address range, e.g. 123.123.123.123/24. The handler function did check if the user input follows the a.b.c.d/subnet format, but not correctly.
有一个相当有趣的案例,即在将用户输入字符串插入命令字符串之前,进行了有缺陷的 iptables 输入验证。用户输入值是一个 IP 地址范围,例如 123.123.123.123/24 .处理程序函数确实检查了用户输入是否遵循 a.b.c.d/subnet 格式,但不正确。

  1. It calls the inet_addr C function to verify that the front part (a.b.c.d) is a valid IP address.
    它调用 inet_addr C 函数来验证前端 ( a.b.c.d ) 是否为有效的 IP 地址。
  2. Then, it calls strtolon the part after the / (subnet) to check if it is a positive number.
    然后,它调用 strtol / ( subnet ) 后面的部分来检查它是否为正数。
  3. On first glance, this is useful because a string that starts with alphabets or symbols will result in 0 being returned.
    乍一看,这很有用,因为以字母或符号开头的字符串将导致返回 0。
  4. However, a string like 16 abc will let strtol return 16 which is considered valid.
    但是,像这样的 16 abc 字符串将返回 strtol 16,这被认为是有效的。

As a result, a user input like 123.123.123.123/16 $(reboot) will successfully perform command injection when it is inserted into an iptables command string.
因此,当用户输入(如 123.123.123.123/16 $(reboot) )插入 iptables 到命令字符串中时,它将成功执行命令注入。

This is an example of failed validation that could be potentially caused by an incomplete understanding of how library functions such as strtol works.
这是一个验证失败的例子,可能是由于对库功能(如) strtol 的工作原理理解不完全所致。

Zyxel (Python) 合勤 (Python)

Now, let’s look at the Zyxel NAS whose web management interface runs on Python. This is not a router, but serves as a good case study.
现在,让我们看一下 Zyxel NAS,其 Web 管理界面运行在 Python 上。这不是路由器,但可以作为一个很好的案例研究。

CVE-2023-4473 was reported by IBM X-Force regarding an OS command injection in the following form:
IBM X-Force 报告了 CVE-2023-4473,涉及以下形式的操作系统命令注入:

mail_hour = pyconf.get_conf_value(MAINTENANCE_LOG_MAIL, 'hour')
mail_minute = pyconf.get_conf_value(MAINTENANCE_LOG_MAIL, 'miniute')
cmd = '/usr/sbin/zylog_config mail 1 schedule daily hour %s minute %s' % (mail_hour, mail_minute)
os.system(cmd)

The values mail_hour and mail_minute are obtained from user input as provided in the POST request below.

curl -s -X POST \
  –data-binary 'schedulePeriod=daily&scheduleHour=0&scheduleMinute=0%60cmd60' \
  'http://10.20.17.122/cmd,/ck6fup6/zylog_main/configure_mail_syslog'
Fix

os.system should never ever be used with a string that contains user input. As suggested in the earlier section, use subprocess.Popen instead, by providing the executable path and its arguments as an argument list. Doing so removes the possibility of command injection because the command string is no longer executed as a shell command. For example:
os.system 不应与包含用户输入的字符串一起使用。如上一节所述,改用 subprocess.Popen 可执行文件路径及其参数作为参数列表。这样做可以消除命令注入的可能性,因为命令字符串不再作为 shell 命令执行。例如:

cmd = '/usr/sbin/zylog_config mail 1 schedule daily hour %s minute %s' % (mail_hour, mail_minute)
subprocess.Popen(cmd.split(" "))

TP-Link’s routers use a fork of OpenWrt’s LuCI, a configuration interface based on Lua, as the backend for their admin panel.
TP-Link 的路由器使用 OpenWrt 的 LuCI(一种基于 Lua 的配置界面)的分支作为其管理面板的后端。

Their Lua source code is compiled to bytecode and stored on the router, and the code is invoked according to the requests made by the client. Any security researcher who is interested in analyzing the admin panel backend code would need to decompile the Lua bytecode. Decompiling Lua bytecode is not as difficult a task as decompiling binaries compiled from C, as bytecode-based languages such Lua, Python, or Java contain more information that make them easier to decompile. There is an open source Lua decompiler luadec.
他们的 Lua 源代码被编译成字节码并存储在路由器上,代码根据客户端发出的请求被调用。任何对分析管理面板后端代码感兴趣的安全研究人员都需要反编译 Lua 字节码。反编译 Lua 字节码并不像反编译从 C 编译的二进制文件那样困难,因为基于字节码的语言(如 Lua、Python 或 Java)包含更多信息,使它们更容易反编译。有一个开源的 Lua 反编译器 luadec。

However, as mentioned, TP-Link uses a fork of LuCI, and they made some changes to it, including the underlying Lua compiler. According to this article Unscrambling Lua, TP-Link changed the bytecode opcodes emitted by its Lua compiler so that luadec would not work properly. One would have to reverse engineer the changes made, and apply the same changes to luadec, to have a working decompiler for TP-Link’s LuCI’s bytecode. There is another open source project luadec-tplink by superkhung which is not perfect, but works somewhat okay for reverse engineering TP-Link’s request handlers stored in bytecode form.
然而,如前所述,TP-Link 使用了 LuCI 的一个分支,他们对它进行了一些更改,包括底层的 Lua 编译器。根据这篇文章 Unscramblebling Lua,TP-Link 更改了其 Lua 编译器发出的字节码操作码,使 luadec 无法正常工作。必须对所做的更改进行逆向工程,并将相同的更改应用于 luadec,以便为 TP-Link 的 LuCI 字节码提供有效的反编译器。superkhung 还有另一个开源项目 luadec-tplink,它并不完美,但对于以字节码形式存储的 TP-Link 请求处理程序进行逆向工程来说,它确实可以。

ZDI-23-451/CVE-2023-1389 reports an unauthenticated command injection that could be exploited by attackers on the LAN interface to gain RCE. The vulnerable endpoint is responsible for setting the admin panel’s locale, in particular through the country parameter.
ZDI-23-451/CVE-2023-1389 报告了未经身份验证的命令注入,攻击者可以在 LAN 接口上利用该命令注入来获取 RCE。易受攻击的端点负责设置管理面板的区域设置,特别是通过 country 参数。

POST /cgi-bin/luci/;stok=/locale?form=country HTTP/1.1
Host: <target router>
Content-Type: application/x-www-form-urlencoded

operation=write&country=$(id>/tmp/out)

An RCE can be gained from a request to change the country setting without requiring authentication.
RCE 可以从更改国家/地区设置的请求中获得,而无需身份验证。

The situation is made worse by ZDI-23-452/CVE-2023-27359 which is a race condition vulnerability in the firewall service hotplugd, that allows an attacker to access the vulnerable endpoint above through the WAN.
ZDI-23-452/CVE-2023-27359 使情况变得更糟,这是防火墙服务 hotplugd 中的竞争条件漏洞,允许攻击者通过 WAN 访问上述易受攻击的端点。

Suggestions 建议

Although we do not have the code, it is very likely that the country parameter was inserted into a command string, then passed to either os.execute or io.popen, resulting in a command injection vulnerability. As recommended in the earlier section, it is best to create a wrapper for os.execute that wraps all arguments with single quotes and escapes all single quotes within them.
虽然我们没有代码,但很可能该 country 参数入到命令字符串中,然后传递给 or os.execute io.popen ,从而导致命令注入漏洞。如上一节所述,最好创建一个包装器 os.execute ,该包装器使用单引号包装所有参数,并转义其中的所有单引号。

On this vulnerable router, LuCI may be running as root (according to the linked Tenable advisory), allowing any injected commands to be executed as root. As described in Services Running as root above, it is a poor practice to run any service with root privileges, as that would mean a compromise of the whole device once that service is exploited.

Remarks

TP-Link may obfuscate their Lua bytecode to prevent others from reverse engineering their code. This adds extra work for not only threat actors, but also security researchers in detecting vulnerabilities in their devices. It may form an illusion that these devices are secure when in reality there were just very few people who spent time inspecting these devices.
TP-Link 可能会混淆他们的 Lua 字节码,以防止其他人对他们的代码进行逆向工程。这不仅为威胁参与者增加了额外的工作,也为安全研究人员在检测其设备中的漏洞方面增加了额外的工作。这可能会形成一种错觉,即这些设备是安全的,而实际上很少有人花时间检查这些设备。

DHCP server (C) DHCP 服务器 (C)

Command injection is not limited to just the admin panel. Earlier, our team discovered a command injection vulnerability in the DHCP server of the NETGEAR RAX30 as shared in this writeup.
命令注入不仅限于管理面板。早些时候,我们的团队在 NETGEAR RAX30 的 DHCP 服务器中发现了一个命令注入漏洞,如本文所述。

The vulnerable code is as follows:
易受攻击的代码如下:

int __fastcall send_lease_info(int a1, dhcpOfferedAddr *lease) 
{
// truncated...
  if ( !a1 )
  {
// truncated ...
    if ( body.hostName[0] )
    {
      strncpy((char *)HostName, body.hostName, 0x40u); // [1]
      snprintf((char *)v11, 0x102u, "%s", body.vendorid);
    }
    else
    {
      strncpy((char *)v10, "unknown", 0x40u);
      strncpy((char *)v11, "dhcpVendorid", 0x102u);
    }
    sprintf(
      command,
      "pudil -a %s %s %s %s \"%s\"",
      body.macAddr,
      body.ipAddr,
      (const char *)HostName,  // [2]
      body.option55,
      (const char *)v11);
    system(command);   // [3]
  }
//...
}

At [1], the hostName parameter of the DHCP request body is copied into HostName, then inserted into command at [2], and executed by system at [3]. There is no sanitization done on this user input.
在 [1] ,DHCP 请求体的 hostName 参数被复制到 HostName ,然后插入到 command at [2] 中,并由 system at [3] 执行。未对此用户输入进行清理。

Patch 补丁

The vulnerability was fixed by calling execve instead of system to execute the command.
该漏洞已通过调用 execve 而不是 system 执行命令来修复。

Buffer Overflow 缓冲区溢出

Buffer overflow is a common issue for programs written in the C programming language, and most services in routers are written in C. Buffer overflow vulnerabilities could be exploited by an attacker to gain RCE on the underlying device.
缓冲区溢出是用 C 编程语言编写的程序的常见问题,路由器中的大多数服务都是用 C 语言编写的。

Over the years, buffer overflow has gotten increasingly difficult to exploit due to mitigations such as ASLR, PIE, RELRO, stack canary and NX. ASLR is enforced by the OS, while PIE, RELRO, stack canary and NX are protections added by the compiler by default. However, in the recent years, many routers are still observed to lack such mitigations. This could be due to the vendors’ still using very old versions of GCC (or other compilers) in their deployment process, which could be missing the ability to add said mitigations to the resulting binary.
多年来,由于 ASLR、PIE、RELRO、堆栈金丝雀和 NX 等缓解措施,缓冲区溢出变得越来越难以利用。ASLR 由操作系统强制执行,而 PIE、RELRO、堆栈金丝雀和 NX 是编译器默认添加的保护。然而,近年来,仍然观察到许多路由器缺乏这种缓解措施。这可能是由于供应商在部署过程中仍在使用非常旧版本的 GCC(或其他编译器),这可能缺少将上述缓解措施添加到生成的二进制文件的能力。

With the mitigations listed above, attempts to exploit a buffer overflow vulnerability could be prevented most of the time, unless the vulnerability satisfies conditions that are favorable for exploitation. However, in successfully preventing the exploitation attempt, the mitigations will halt the running service because its internal state has been corrupted. This results in a DoS which is also not desirable. Hence, knowing that the mitigations do not guarantee full protection against all exploitation attempts, it is still most preferable that buffer overflow vulnerabilities are avoided through secure coding practices and design, which we will discuss in detail in this section.
通过上面列出的缓解措施,大多数情况下可以防止利用缓冲区溢出漏洞的尝试,除非该漏洞满足有利于利用的条件。但是,在成功阻止攻击尝试时,缓解措施将停止正在运行的服务,因为其内部状态已损坏。这会导致 DoS,这也是不可取的。因此,知道缓解措施并不能保证完全防止所有漏洞利用尝试,因此最好还是通过安全编码实践和设计来避免缓冲区溢出漏洞,我们将在本节中详细讨论。

Root Cause 根源

The root cause of a buffer overflow bug is straightforward. A buffer is allocated a certain size, but data longer than that size is written into the buffer. As a result, other values in adjacent memory are corrupted with user-controlled values.
缓冲区溢出 bug 的根本原因很简单。为缓冲区分配了特定大小,但超过该大小的数据将写入缓冲区。因此,相邻内存中的其他值会被用户控制的值损坏。

In routers, this mistake is commonly observed in the usage of functions that copy memory contents. We list examples in the code snippet below. For the examples below, suppose that websGet is a function that takes in a key and returns the corresponding value in the query string of an incoming web request of the router admin panel.
在路由器中,在使用复制内存内容的函数时通常会观察到此错误。我们在下面的代码片段中列出了示例。对于下面的示例,假设这是一个 websGet 函数,它接受一个键,并在路由器管理面板的传入 Web 请求的查询字符串中返回相应的值。

char* contents = websGet("contents");
int size = websGet("size");

char buffer[128];

strcpy(buffer, contents);
strncpy(buffer, contents, size);

sprintf(buffer, "%s", contents);
snprintf(buffer, "%s", contents);

buffer[0] = 0;
strcat(buffer, contents);
strncat(buffer, content, size);

memcpy(buffer, contents, size);

In the code snippet above, strcpysprintf and strcat copy the whole user-supplied contents string into buffer without restricting the length. If the length of contents exceeds the size allocated for buffer (i.e. 128), buffer overflow occurs.
在上面的代码片段中, strcpy sprintf 将用户提供 contents 的整个字符串 strcat 复制到 , buffer 而不限制长度。如果 的 contents 长度超过分配的大小 buffer (即 128),则会发生缓冲区溢出。

The example above also assumes a scenario in which the user also specifies the size of contents to copy through strncpysnprintfstrncat and memcpy. Similarly, if size exceeds 128, buffer overflow occurs.
上面的示例还假设了这样一种情况:用户还指定要通过 strncpy 、 snprintf strncat 和 memcpy 复制 size 的 。 contents 同样,如果 size 超过 128,则会发生缓冲区溢出。

Aside from functions that perform copying, code for manually copying memory contents can also be vulnerable to buffer overflow, as shown in the example below, in which the size field is user-supplied and could be greater than the size allocated for buffer.
除了执行复制的函数之外,用于手动复制内存内容的代码也容易受到缓冲区溢出的影响,如以下示例所示,其中字段 size 是用户提供的,并且可能大于分配给 buffer 的大小。

for (int i = 0; i < size; ++i) buffer[i] = contents[i];

All of the vulnerable code examples above share a common theme. They do not restrict the size of the contents being copied/written. Implicitly, they had allowed the user (attacker) to choose the size. To prevent buffer overflow, do not use functions that do not restrict the length, e.g. strcpysprintf and strcat. Instead, use strncpysnprintfstrncat or memcpy, and make sure that the length argument is not a user-controlled value.
上面所有易受攻击的代码示例都有一个共同的主题。它们不限制要复制/写入的内容的大小。隐含地,他们允许用户(攻击者)选择大小。为防止缓冲区溢出,请勿使用不限制长度的函数,例如 strcpy 和 sprintf strcat 。请改用 strncpy 、 snprintf 或 strncat memcpy ,并确保 length 参数不是用户控制的值。

The examples above are contrived, as they are just intended for demonstrating the insecure code patterns. In a more complex codebase, a user-submitted value may be stored and retrieved and manipulated repeatedly, at various locations in the code, before finally being used in the copying operation. Under such situations, it can be difficult to ensure that all such memory-writing operations are immune to buffer overflow. We provide suggestions below in the form of secure design and practices to systematically protect your code against buffer overflow bugs.
上面的示例是人为的,因为它们只是为了演示不安全的代码模式。在更复杂的代码库中,用户提交的值可以在代码的不同位置重复存储、检索和操作,然后最终用于复制操作。在这种情况下,可能很难确保所有此类内存写入操作都不受缓冲区溢出的影响。我们在下面以安全设计和实践的形式提供建议,以系统地保护您的代码免受缓冲区溢出错误的影响。

Prevention 预防

Buffer overflow bugs can be considered as caused by human mistakes. The complexity of a codebase increases the likelihood of the occurrence of such mistakes. Through careful design and restrictions imposed on the codebase, we can do our best to protect developers against unintentionally introducing buffer overflow bugs into the program.
缓冲区溢出错误可以被认为是由人为错误引起的。代码库的复杂性增加了发生此类错误的可能性。通过仔细的设计和对代码库的限制,我们可以尽最大努力保护开发人员,防止无意中将缓冲区溢出错误引入程序。

Use bounded functions for copying
使用有界函数进行复制

The usage of memory-copying functions without a length limit such as strcpystrcat and sprintf should not be allowed in the codebase. Instead, use the bounded alternatives such as strncpystrncatsnprintf or memcpy. There is no imaginable scenario where the unbounded functions (e.g. strcpy) will be more useful than their bounded alternatives (e.g. strncpy).
在代码库中不应允许使用没有长度限制的内存复制函数,例如 strcpy 。 strcat sprintf 请改用有界替代项,例如 strncpy 、 strncat 或 snprintf memcpy 。没有可以想象的场景,其中无界函数(例如 strcpy )将比它们的有界替代函数(例如)更有用 strncpy 。

Be mindful that when using functions like strncpy or memcpy, do not use strlen to determine the length to copy, as listed in the example below, as this is no different from just calling strcpy. The length argument should be independent of the user input, but based on the allocated size of the destination buffer instead.
请注意,在使用 或 memcpy 等 strncpy 函数时,请勿用于 strlen 确定要复制的长度,如以下示例所示,因为这与仅调用 strcpy 没有什么不同。length 参数应独立于用户输入,但应基于目标缓冲区的分配大小。

char buffer[128];
// bad
strncpy(buffer, contents, strlen(contents));
// good
strncpy(buffer, contents, 128);
strncpy(buffer, contents, sizeof(buffer));
Pass buffer size as function argument
将缓冲区大小作为函数参数传递

Even when developers put in conscious effort to ensure that memory is only copied into within a buffer’s bounds, there is another challenge: it is difficult to know what exactly is the size allocated for a buffer. The following example illustrates this problem.
即使开发人员有意识地努力确保内存只在缓冲区的边界内复制,也存在另一个挑战:很难知道为缓冲区分配的大小到底是多少。下面的示例说明了此问题。

void get_name(char* buf)
{
  char* name = websGet("name");
  memcpy(buf, name, ???);
}

void f2(char* buf) { get_name(buf); }
void f3(char* buf) { f2(buf); }

void store_input() {
  char* buf = (char*) malloc(64);
  f3(buf);
}

In the example above, store_input allocates 64 bytes for buf. Then, buf is passed through f3 then f2 to get_name which copies a user-provided string into it. In get_name, it is no longer obvious what was the size allocated for buf. The developer would need to find references to get_name, see that it is called by f2, then again find references to f2, see that it is called by f3, then finally get to store_input and learn that the allocated size is 64. If there are way more functions calling get_namef2 or f3, this would be madness. The developer would have to make sure that the length specified in get_name is safe for all its callers.
在上面的示例中, store_input 为 buf 分配了 64 个字节。然后, buf 通过 f3 将 f2 用户提供的字符串复制到 get_name 其中。在 get_name 中,分配的大小不再明显 buf 。开发人员需要找到对 get_name 的引用,看到它被调用, f2 然后再次找到对 f2 的引用,看到它被调用, f3 最后到达 store_input 并了解分配的大小是 64。如果有更多的函数调用 get_name , f2 或者 f3 ,这将是疯狂的。开发人员必须确保 中 get_name 指定的长度对其所有调用方都是安全的。

Furthermore, any changes made to the allocation size in store_input also has to be made to get_name. If the developer modifying store_input was not aware of get_name, he will miss this out, resulting in get_name calling memcpy with the wrong length. In general, it is bad practice to have values serving the same purpose hardcoded in different locations.
此外,对 中的分配大小所做的任何更改 store_input 也必须对 get_name 进行。如果修改的开发人员 store_input 不知道 get_name ,他将错过这一点,从而导致 get_name 调用 memcpy 的长度错误。通常,在不同位置硬编码用于相同目的的值是一种不好的做法。

One may consider having a global size constant SIZE that is used by the allocation in store_input and memcpy in get_name. This works well, if get_name is ever only given a buffer that is allocated SIZE bytes. This may be the case in the short term. However, could this still hold 2 years later if most of the development team has changed? For example, a new developer may decide to call get_name with a buffer of a smaller size, without being aware of the memcpy length.
可以考虑有一个全局大小常量,该常 SIZE 量由 in store_input 和 memcpy in get_name 中的分配使用。如果 get_name 只给分配了 SIZE 字节的缓冲区,这很有效。短期内可能会出现这种情况。但是,如果大多数开发团队都发生了变化,这种情况在 2 年后还能成立吗?例如,新开发人员可能决定使用较小大小的缓冲区进行调用 get_name ,而不知道 memcpy 长度。

We suggest designing a codebase that is resilient to the changes above, that is, by passing the destination buffer’s allocated size as a function argument. The following code snippet shows how this can be applied on the example above.
我们建议设计一个能够灵活应对上述更改的代码库,即通过将目标缓冲区的分配大小作为函数参数传递。以下代码片段演示如何在上面的示例中应用此功能。

void get_name(char* buf, size_t size)
{
  char* name = websGet("name");
  memcpy(buf, name, size);
}

void f2(char* buf, size_t size) { get_name(buf, size); }
void f3(char* buf, size_t size) { f2(buf, size); }

void store_input() {
  size_t size = 64;
  char* buf = (char*) malloc(size);
  f3(buf, size);
}

In doing so, there is no room for any uncertainties in the size when memcpy is called by get_name. It is guaranteed to get_name that the size given to memcpy must be the allocated size for the buffer. When a developer works on the code in get_name, he does not need to check all of its callers to ensure that the size is suitable. Similarly, a developer working on store_input also does not need to worry whether f2f3 or get_name will write out of bounds, assuming that the developers of those functions have used the provided size argument correctly.
这样一来,当被 get_name 调用时 memcpy ,大小没有任何不确定性的余地。可以保证 get_name ,给定的大小 memcpy 必须是为缓冲区分配的大小。当开发人员处理 中的 get_name 代码时,他不需要检查它的所有调用者来确保大小合适。同样,开发人员 store_input 也不需要担心 f2 , f3 或者 get_name 会写出越界,假设这些函数的开发人员已经正确使用了提供的 size 参数。

Also, note that store_input passes the same size variable to malloc and f3, instead of hardcoding the value 64 in both function calls. This ensures that when the allocation size is changed, the change will immediately apply to both malloc and f3. Such practice removes the possibility of mistakes.
另请注意,将相同的 size 变量 store_input 传递给 malloc 和 f3 ,而不是在两个函数调用中对值 64 进行硬编码。这可确保在更改分配大小时,更改将立即应用于 malloc 和 f3 。这种做法消除了出错的可能性。

For code reviewers, potentials bugs are also easier to detect. In the original example, the reviewer would have to follow the flow from store_input to get_name to ensure that the size given to memcpy is within bounds. In a big codebase, there may be tens or hundreds of such flows to review whenever the implementation of a function such as get_name changes. 

In this improved implementation, the reviewer just needs to ensure that functions like get_name correctly uses the size argument that is given to it, and ensure that functions like store_input provide the correct size. 

Caveat: strncpy 警告: strncpy

There are caveats for using strncpy and strncat. We will discuss strncpy first. 

Note that for strncpy, if the requested length to copy is smaller than the length of the source string, the copied string will not be null-terminated. Consider the following example. 

char* hello = "HELLO WORLD";
char dest[10];
memset(dest, '\xAA', 10);
strncpy(dest, hello, 5);

// to print contents of `dest` in hex
for (int i = 0; i < 10; ++i) printf("%hhx ", dest[i]);
printf("\n");

The result of running the code above is as follows. 

48 45 4c 4c 4f aa aa aa aa aa

As strncpy was given 5 as the length to copy, it correctly copies 5 characters, and does nothing more than that, such as adding a null byte to terminate the destination buffer. The string in dest is therefore not null-terminated as shown in the program output above. 

The consequences of this may not be directly observed. By itself, there is no memory corruption, because nothing is read or written out of bounds. However, if a future operation that uses dest assumes that it is null-terminated, but in reality it may not be so, unexpected behaviour may occur. Referring to the example above, if strlen was applied on dest, it does not return 5 because there is no null-byte at the 6th position (i.e. right after the 5th position), even though it is expected to return 5. This discrepancy may affect subsequent operations in unpredicted ways. 

Another example would be a scenario where the contents in dest were to be copied as part of the service’s response to the client. Similarly, as dest was not null-terminated, the program may copy adjacent memory contents (contents of other variables) or leftover memory contents in the buffer written by previous operations (i.e. \xaa\xaa\xaa in the example above). This constitutes an information leakage bug. Since there is no out-of-bounds memory write, there is no danger of RCE. However, sensitive values could be leaked, for example pointers to bypass ASLR, or secrets like passwords to bypass authentication. 

In a more severe scenario, a buffer overflow may be possible too, although unlikely if proper precautions were already enforced to ensure that memory-copying operations do not rely on the position or presence of the null byte, as per the advice given above. 

Custom strncpy 习惯 strncpy

With this knowledge about strncpy and the implications, the developer should remember to insert a null byte to terminate the copied string, while also making sure not to write the null byte beyond the buffer’s bounds. For example: 

char* hello = "HELLO WORLD";
char dest[5];

// bad, writing out of bounds
strncpy(dest, hello, 5);
dest[5] = 0;

// good, within bounds
strncpy(dest, hello, 4);
dest[4] = 0;

This does not look good. While the usage of strncpy was meant for safety purposes, it has introduced a new problem for developers to be wary of. Now, developers are required to remember adding a null byte, while also being careful about the position it is added (need to minus one from the buffer size). A lapse in concentration by a developer and a code reviewer will result in an off-by-one write bug, which under the right conditions could be exploitable by an attacker to gain RCE. 

There is a reliable solution to this. The development team could create a custom version of strncpy that best suits their needs. For example, let’s call it my_strncpy. The custom my_strncpy could behave differently from strncpy by ensuring that a null byte is always added at the last position, that is, length minus one. The tradeoff would be that only length minus one characters are copied from the source string, but this is not a security concern. Internal documentation should then be maintained to ensure clarity of my_strncpy’s behaviour. Furthermore, usage of strncpy should be avoided since there would not be any good reason for it to be used anymore.
有一个可靠的解决方案。开发团队可以创建最适合他们需求的 strncpy 自定义版本。例如,我们称之为 my_strncpy .该自定义 my_strncpy 的行为可能与 strncpy 确保始终在最后一个位置添加 null 字节(即长度减去 1)不同。权衡是仅从源字符串复制长度减去一个字符,但这不是安全问题。然后应保留内部文件,以确保 my_strncpy 行为的清晰性。此外,应避免使用 OF strncpy ,因为没有任何充分的理由再使用它。

By doing the above, security is decoupled from the unexpected intricacies of standard library functions, so that developers and code reviewers are protected from making mistakes caused by unintended behaviour.
通过执行上述操作,安全性与标准库函数的意外复杂性分离,从而保护开发人员和代码审查者免于因意外行为而导致的错误。

Caveat: strncat 警告: strncat

The problem caused by strncat is the opposite of strncpy’s. Unlike strncpy which does not add a null byte to the end of the copied string, strncat adds a null byte after the end of the copied string. For example:
引起的 strncat 问题与 strncpy 的相反。与此不同 strncpy ,它不会在复制的字符串末尾添加 null 字节, strncat 而是在复制的字符串末尾添加 null 字节。例如:

char* hello = "HELLO WORLD";
char dest[10];
memset(dest, '\xAA', 10);

dest[0] = 0;
strncat(dest, hello, 5);

// to print contents of `dest` in hex
for (int i = 0; i < 10; ++i) printf("%hhx ", dest[i]);
printf("\n");

Even though in the code above strncat was given 5 as the length to copy, it had also added a null byte at the 6th position, after the end of the copied string.
尽管在上面 strncat 的代码中给出了 5 作为复制的长度,但它也在复制的字符串末尾的第 6 个位置添加了一个空字节。

48 45 4c 4c 4f 0 aa aa aa aa

This is much more dangerous than the case of strncpy, because there is actually an out-of-bounds memory write. Consider the contrived example below:
这比 strncpy 的情况危险得多,因为实际上存在越界内存写入。考虑下面人为的例子:

char* hello = "HELLO WORLD HELLO WORLD";
char dest[12];
int needs_auth = 1;
dest[0] = 0;
strncat(dest, hello, 12);

Here, it depends on how the compiler allocates the variables on the stack. If needs_auth is allocated after dest, then as strncat is given length 12, it would write 12 bytes into dest, and a null byte into needs_auth which is stored after it. As a result, needs_auth would contain the value 0.
在这里,这取决于编译器如何在堆栈上分配变量。如果 needs_auth 在 之后 dest 分配,则 strncat 给定长度为 12,它会将 12 个字节写入 dest ,并在其后存储一个空字节 needs_auth 。因此, needs_auth 将包含值 0。

Such an off-by-one bug to write memory out of bounds may not appear as intimidating as a conventional buffer overflow, but it is still a buffer overflow nonetheless, despite just overflowing by just one null byte. The consequences can be dire if the null byte is written into a variable that is part of critical logic. For example, authentication could be bypassed as seen in the example above; or if values related to bounds checking were overwritten, a buffer overflow may occur.
这种将内存写出界的偏离 1 的错误可能不像传统的缓冲区溢出那样令人生畏,但它仍然是一个缓冲区溢出,尽管只是溢出了一个空字节。如果将 null 字节写入作为关键逻辑一部分的变量中,后果可能会很严重。例如,可以绕过身份验证,如上例所示;或者,如果与边界检查相关的值被覆盖,则可能会发生缓冲区溢出。

Custom strncat 习惯 strncat

Here, we give a similar suggestion as we did for strncpy to protect the developers from making mistakes caused by this tricky behaviour. Again, although it is part of the developers’ responsibility to make sure the code is correct, it is not reliable to expect such intricacies to always be on their mind, as they may be very focused on implementing the feature requirements and have forgotten about the memory side effects of their code. Thus, it is very beneficial to set up a safe development environment that takes this burden off the developers’ shoulders.
在这里,我们给出了一个与我们类似的建议, strncpy 以保护开发人员免于犯这种棘手行为造成的错误。同样,尽管确保代码正确是开发人员的责任之一,但期望这些复杂性始终在他们的脑海中是不可靠的,因为他们可能非常专注于实现功能要求,而忘记了代码的内存副作用。因此,建立一个安全的开发环境来减轻开发人员的负担是非常有益的。

Similar to my_strncpy, the development team could create a custom strncat as well, e.g. my_strncat, that works according to their needs. One possible implementation for my_strncat is to add the null byte at the last position, that is, for example if the length field is 5, the null byte will be added at the 5th position (1-indexed). This means that only length minus one characters are concatenated to the destination string. Such behaviour should then be clearly written in the team’s internal documentation to ensure there is no ambiguity. Furthermore, usage of strncat from the standard library should be avoided since there is no longer a good reason for it to be used.
与 类似, my_strncpy 开发团队也可以创建一个自定义 strncat ,例如 my_strncat ,根据他们的需求工作。一种可能的实现 my_strncat 是在最后一个位置添加 null 字节,也就是说,例如,如果长度字段为 5,则 null 字节将添加到第 5 个位置(1 索引)。这意味着只有长度减去一个字符才会连接到目标字符串。然后,这种行为应该清楚地写在团队的内部文档中,以确保没有歧义。此外,应避免使用标准库中的, strncat 因为不再有充分的理由使用它。

Actionable Steps 可操作的步骤

To summarize the suggestions above, we advise development and security teams to impose the following rules on their codebase:
总结上述建议,我们建议开发和安全团队对其代码库实施以下规则:

  1. Avoid unbounded memory-copying functions (strcpysprintfstrcat) and use bounded functions that do the same.
    避免使用无界内存复制函数 ( strcpy , sprintf , , strcat ),并使用执行相同操作的有界函数。
  2. Create and use your own custom implementation of library functions (such as strncpy or strncat) so that you have full control and understanding of their behaviour, to avoid pitfalls caused by unexpected intricacies of the library functions. Then, avoid using the library functions.
    创建和使用您自己的库函数(如 strncpy 或 strncat )的自定义实现,以便您可以完全控制和理解它们的行为,以避免因库函数的意外复杂性而导致的陷阱。然后,避免使用库函数。
  3. Avoid hardcoding sizes for allocation and memory-copying operations. For a function that takes in a buffer pointer and writes to it, ensure that the buffer’s allocation size is also taken as an argument. This removes uncertainties about the size, for both the caller and the callee.
    避免对分配和内存复制操作的大小进行硬编码。对于接受缓冲区指针并写入该指针的函数,请确保缓冲区的分配大小也用作参数。这消除了呼叫者和被调用方的大小不确定性。

The suggestions above all follow the same principles. We want to change the problem from “are we using this function correctly” to “are we implementing this function correctly”. There will be significantly less room for error, as the former requires reviewing every single function and trying to catch incorrect usages such as the off-by-one examples given; whereas the latter removes the possibility of such pitfalls, and only requires reviewing the custom implementations to ensure that they are implemented correctly.
上述建议都遵循相同的原则。我们想将问题从“我们是否正确使用此函数”更改为“我们是否正确实现此函数”。出错的余地将大大减少,因为前者需要审查每个函数并试图捕获不正确的用法,例如给出的偏离示例;而后者消除了此类陷阱的可能性,只需要检查自定义实现以确保它们正确实现。

In other words, instead of worrying about “what are the possible pitfalls of using this function, is there some specific scenario that I have missed”, we can now call memory-writing functions with a piece of mind and just care about “is this function implemented correctly”.
换句话说,我们不再担心“使用这个函数可能有什么陷阱,我是否错过了一些特定的场景”,我们现在可以放心地调用内存写入函数,只关心“这个函数是否正确实现”。

In routers that are higher on the price range, we have observed a widespread application of the rules above in their codebase. As a result, it was more difficult to find bugs on these devices that are typically considered as low hanging fruits on cheaper routers.
在价格较高的路由器中,我们观察到上述规则在其代码库中的广泛应用。因此,在这些设备上发现错误变得更加困难,这些错误通常被认为是廉价路由器上唾手可得的果实。

Examples 例子

The following are examples where a buffer overflow due to improper (or the lack of) bounds checks was exploitable to gain RCE due to the lack of mitigations such as ASLR, PIE, NX or canary.
以下示例显示,由于缺乏 ASLR、PIE、NX 或 Canary 等缓解措施,由于边界检查不当(或缺少)而导致的缓冲区溢出可利用来获取 RCE。

These show that even in the recent years having modern mitigations, simple bugs are still present in routers and could be easily exploited.
这些表明,即使在最近几年具有现代缓解措施,路由器中仍然存在简单的错误,并且很容易被利用。

Format String Bug 格式字符串错误

A format string vulnerability could be exploited to leak pointers, perform buffer overflow, or write to certain memory locations. It is a very powerful bug. However, security teams need not be too worried about this vulnerability class, as it is one that is very easy to prevent. We explain why this is the case below.
可利用格式字符串漏洞泄漏指针、执行缓冲区溢出或写入特定内存位置。这是一个非常强大的错误。但是,安全团队不必太担心这种漏洞类别,因为它很容易预防。我们在下面解释为什么会这样。

Prevention 预防

Format string bugs are very easy to prevent, as it can only occur when a program passes user input directly into the format string argument of printf or its variant functions. An example of vulnerable code is as follows:
格式字符串错误非常容易防止,因为只有当程序将用户输入直接传递到格式字符串参数 printf 或其变体函数中时,才会发生这种错误。易受攻击的代码示例如下:

printf(username);
fprintf(fp, username);

The correct way to print a string with printf is by using the %s specifier, as shown below:
打印字符串 printf 的正确方法是使用 %s 说明符,如下所示:

printf("%s", username);
fprintf(fp, "%s", username);

This mistake is very easy to catch. Even if a programmer actually missed it, modern C compilers will also raise a warning about it under default settings. By providing the -Werror=format-security flag to gcc, the compilation will fail with these warnings treated as errors.
这个错误很容易被发现。即使程序员真的错过了它,现代 C 编译器也会在默认设置下发出警告。通过向 提供 -Werror=format-security 标志 gcc ,编译将失败,并将这些警告视为错误。

$ cat fs.c
#include <stdio.h>

int main(int argc, char** argv)
{
  printf(argv[1]);
  return 0;
}

$ gcc fs.c
fs.c: In function ‘main’:
fs.c:5:3: warning: format not a string literal and no format arguments [-Wformat-security]
    5 |   printf(argv[1]);
      |   ^~

$ gcc fs.c -Werror=format-security
fs.c: In function ‘main’:
fs.c:5:3: error: format not a string literal and no format arguments [-Werror=format-security]
    5 |   printf(argv[1]);
      |   ^~~~~~
cc1: some warnings being treated as errors

As shown above, this bug is easy to catch and fix. However, some router binaries might have been compiled decades ago, and at that time developers might not have had such awareness of format string vulnerabilities, or the compiler used might not have emitted such warnings. There is a chance that some of these vulnerable code have persisted until today.
如上所示,此错误很容易捕获和修复。但是,某些路由器二进制文件可能是几十年前编译的,当时开发人员可能没有意识到格式字符串漏洞,或者使用的编译器可能没有发出此类警告。其中一些易受攻击的代码有可能一直持续到今天。

Actionable Steps 可操作的步骤

For security teams, we advise you to check your old codebases and ensure that there are no more such bugs. We recommend adding the -Werror=format-security flag to gcc in the deployment process, to catch any such bugs that persisted from the past as well as the ones that may be accidentally introduced in the future.
对于安全团队,我们建议您检查旧的代码库,并确保不再有此类错误。我们建议 gcc 在部署过程中添加该 -Werror=format-security 标志,以捕获过去持续存在的任何此类错误以及将来可能意外引入的错误。

There is a caveat to take careful note of. The -Werror=format-security flag only works on GCC 4.3.x or newer (source). Please make sure that your GCC is of a sufficiently new version to support this flag. At the point of writing this, the latest version of GCC is 13.2. GCC 4.3.6, the latest version under 4.3.x, was released in 2011.
有一个注意事项需要仔细注意。该 -Werror=format-security 标志仅适用于 GCC 4.3.x 或更高版本(源代码)。请确保您的 GCC 版本足够新,可以支持此标志。在撰写本文时,GCC 的最新版本是 13.2。GCC 4.3.6 是 4.3.x 下的最新版本,于 2011 年发布。

Example 

CVE-2018-14713 reports a format string vulnerability on the ASUS RT-AC3200 router, in which the web server might have been written back in 1999 (according to the linked article). The format string vulnerability could be used to leak pointers in memory to bypass ASLR. Then, since there is no stack canary, a buffer overflow could be exploited by using ROP to call system in libc to gain RCE.
CVE-2018-14713 报告了 ASUS RT-AC3200 路由器上的格式字符串漏洞,其中 Web 服务器可能在 1999 年被写入(根据链接的文章)。格式字符串漏洞可用于泄漏内存中的指针以绕过 ASLR。然后,由于没有堆栈金丝雀,因此可以通过使用 ROP 调用 system libc 来获取 RCE 来利用缓冲区溢出。

Besides the admin panel, other services on a router may be vulnerable to a format string vulnerability as well. In 2013, a format string vulnerability was discovered in the Universal Plug and Play (UPnP) service of many routers, described in great detail in the article From Zero to ZeroDay Journey: Router Hacking (WRT54GL Linksys Case). The service does not require any authentication and accepts requests from the WAN interface.
除了管理面板之外,路由器上的其他服务也可能容易受到格式字符串漏洞的影响。2013 年,在许多路由器的通用即插即用 (UPnP) 服务中发现了一个格式字符串漏洞,在文章《从零到零日之旅:路由器黑客攻击(WRT54GL Linksys 案例)中进行了详细描述。该服务不需要任何身份验证,并接受来自 WAN 接口的请求。

The scary part about this vulnerability is that it is found in the Broadcom UPnP stack, which is code that is reused by many other router vendors. At the end of the linked article, there is a long list of routers by different vendors that are affected by this vulnerability.
这个漏洞的可怕之处在于它是在 Broadcom UPnP 堆栈中发现的,该堆栈是许多其他路由器供应商重复使用的代码。在链接文章的末尾,有一长串受此漏洞影响的不同供应商提供的路由器。

Conclusion 结论

In this article, we have discussed the attack surface of a router as well as misconfigurations and vulnerability classes that affect exposed services. We also provided suggestions in the form of best practices and secure code design to prevent attackers from abusing these services. By designing the development environment intentionally through enforcing the usage of only secure functions, the development team can be protected from making mistakes that may have dire consequences.
在本文中,我们讨论了路由器的攻击面以及影响暴露服务的错误配置和漏洞类别。我们还以最佳实践和安全代码设计的形式提供了建议,以防止攻击者滥用这些服务。通过强制使用安全功能来有意识地设计开发环境,可以保护开发团队免于犯可能产生可怕后果的错误。

There are other problems not covered in this article. Firstly, we have observed that for some vendors, many of their devices share the same codebase. However, when a vulnerability is reported for a device and remediated for that device, the fixes are not applied to the other devices which share similar code and have the same vulnerability. This is likely due to the vendors’ lack of knowledge about which devices share the same code, or if they do know, it may be because of the high cost of testing such devices.
本文未涵盖其他问题。首先,我们观察到,对于某些供应商来说,他们的许多设备共享相同的代码库。但是,当报告某个设备的漏洞并针对该设备进行修正时,修补程序不会应用于共享类似代码并具有相同漏洞的其他设备。这可能是由于供应商不了解哪些设备共享相同的代码,或者如果他们知道,可能是因为测试此类设备的成本很高。

When vendors leave vulnerabilities unpatched on similar devices while fixing them on only one device, attackers who notice a security update published for that one device can quickly analyse the patch and write exploits for the other unpatched devices. This leaves many consumers at risk of being compromised.
当供应商在类似设备上留下未修补的漏洞,而只在一台设备上修复漏洞时,发现为该设备发布的安全更新的攻击者可以快速分析补丁并为其他未修补的设备编写漏洞。这使许多消费者面临被损害的风险。

Another problem comes from the usage of open source projects. It is reasonable and beneficial for developers to import open source libraries to save development time. However, these open source projects may receive vulnerability reports and apply fixes from time to time. In a way, one benefit of using such open source projects is that developers do not need to fix the bugs found in them, as they can just update that library to the latest version. However, there may be challenges in doing so, either due to worries about compatibility, or not even being aware that a library needs to be updated.
另一个问题来自开源项目的使用。对于开发者来说,导入开源库以节省开发时间是合理且有益的。但是,这些开源项目可能会不时收到漏洞报告并应用修复程序。在某种程度上,使用此类开源项目的一个好处是开发人员不需要修复其中的错误,因为他们只需将该库更新到最新版本即可。但是,这样做可能会遇到挑战,要么是由于担心兼容性,要么是甚至没有意识到库需要更新。

To summarize, aside from fixing vulnerabilities within a device, there are also challenges in ensuring that patches are applied horizontally, that is, across all devices that are affected; as well as vertically, through applying updates performed on the upstream open source projects.
总而言之,除了修复设备内的漏洞外,在确保水平应用补丁方面也存在挑战,即在所有受影响的设备上应用补丁;以及垂直方向,通过应用对上游开源项目执行的更新。

原文始发于Daniel Lim Wee Soong (@daniellimws):Route to Safety: Navigating Router Pitfalls

版权声明:admin 发表于 2024年3月21日 下午11:12。
转载请注明:Route to Safety: Navigating Router Pitfalls | CTF导航

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