ExploitPublished 18 May 2025

Zyxel USG FLEX H series uOS 1.31 · Privilege Escalation

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Mentioned vulnerabilities

CVE-2025-1731

Severity

High

Rated by Threat Radar

Data as of 11 Jun 2025, 21:13 UTC

What to do

AI-suggested

Immediate mitigation involves restricting local shell access to trusted users only and monitoring for unusual cron jobs or file changes in `/etc/cron.d/` and `/tmp/`.
Apply strict permissions on the `/tmp` directory by enabling the sticky bit (`chmod +t /tmp`) to prevent unauthorized symbolic link exploitation.
Disable or restrict the use of the vulnerable `/usr/sbin/fermion-wrapper` binary if possible until a vendor patch is available.
Implement host-based intrusion detection systems (HIDS) to alert on creation of suspicious files or SUID binaries like `/tmp/pwned`.
Regularly audit firewall firmware versions and subscribe to Zyxel security advisories for timely patch releases.
Use network segmentation to limit access to firewall management interfaces and reduce the risk of local exploitation.
Employ multi-factor authentication and strong access controls for device management to prevent unauthorized shell access.
If feasible, replace vulnerable devices or firmware versions with updated, patched alternatives.
Monitor logs for unexpected executions of `fermion-wrapper` or cron jobs and conduct periodic integrity checks on critical system files.

Description

Zyxel USG FLEX H series uOS 1.31 - Privilege Escalation

Impact

For European organizations using Zyxel USG FLEX H series firewalls, this vulnerability poses a significant risk. Successful exploitation allows attackers with local access to escalate privileges to root, potentially leading to full device compromise. This can result in unauthorized access to sensitive network traffic, manipulation or disruption of firewall rules, and persistent backdoors within critical network infrastructure. Given that these firewalls often protect enterprise and government networks, exploitation could lead to data breaches, network outages, or lateral movement within corporate environments. The lack of a sticky bit on `/tmp` and the presence of a setuid root binary exacerbate the risk. Although exploitation requires local access, attackers could leverage other vulnerabilities or social engineering to gain initial footholds. The delayed execution via cron job means detection windows exist but also that attackers can maintain stealth. The impact on confidentiality, integrity, and availability is high, as root access on a firewall device compromises the entire network perimeter security. Organizations relying on these devices should consider the risk of insider threats or attackers who have gained initial access to internal systems.

Indicators of compromise

Exploit source code

Exploit code for Zyxel USG FLEX H series uOS 1.31 - Privilege Escalation4,283 characters

# 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 wi…

References

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