CVE-2025-1731: Zyxel USG FLEX Privilege Escalation Flaw

CVE-2025-1731 Overview

CVE-2025-1731 is a privilege escalation vulnerability in the Zyxel USG FLEX H series firewalls running uOS firmware. The flaw stems from an incorrect permission assignment [CWE-732] in the PostgreSQL commands exposed by the device. An authenticated local attacker with low privileges can leverage this weakness to reach the underlying Linux shell and escalate to administrator-level access. Exploitation requires a valid, unexpired administrator token, which the attacker can misuse to craft malicious scripts or modify system configurations. The vulnerability affects uOS firmware versions V1.20 through V1.31 across the USG FLEX 50H, 100H, 200H, 500H, and 700H product lines.

Critical Impact
A low-privileged local user can escalate to administrator access on the firewall, gaining shell-level control over a security-critical network appliance.

Affected Products

Zyxel uOS firmware versions V1.20 through V1.31
Zyxel USG FLEX 50H / 50HP, 100H / 100HP, 200H / 200HP
Zyxel USG FLEX 500H and 700H

Discovery Timeline

2025-04-22 - CVE-2025-1731 published to NVD
2025-04-22 - Zyxel publishes security advisory for USG FLEX H series firewalls
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-1731

Vulnerability Analysis

The vulnerability resides in how the uOS firmware assigns permissions to PostgreSQL command interfaces available to authenticated users. Low-privileged accounts can invoke PostgreSQL operations that were never intended to be reachable from their permission tier. Through these operations, an attacker can pivot into the Linux shell that underlies the firewall's management stack.

Once shell access is obtained, the attacker can execute arbitrary commands in the operating system context, bypassing the constrained CLI that Zyxel exposes for non-administrator users. If a valid administrator token remains active in the session store, the attacker can reuse that token to perform configuration changes reserved for administrators.

The issue is tracked under CWE-732: Incorrect Permission Assignment for Critical Resource. Full details are available in the Zyxel Security Advisory.

Root Cause

PostgreSQL commands on the affected devices were granted overly permissive execution rights. Low-privileged users can invoke database-adjacent functionality that reaches into the host operating system, producing an unintended path from restricted user context to interactive shell execution.

Attack Vector

Exploitation requires local, authenticated access with low privileges, typically shell or web management access to the firewall as a non-administrator user. The attacker executes crafted PostgreSQL commands to obtain a Linux shell. To escalate to administrator-level configuration changes, the attacker must also capture a valid administrator token, which is only available while an administrator session remains active and has not been logged out.

See the Full Disclosure mailing list post for additional exploitation context. No public exploit code is currently listed in ExploitDB against a specific proof-of-concept URL.

Detection Methods for CVE-2025-1731

Indicators of Compromise

Unexpected psql or PostgreSQL client process invocations spawned by low-privileged Zyxel user accounts
Shell processes such as /bin/sh or /bin/bash launched as children of the PostgreSQL service on USG FLEX H devices
Administrator-level configuration changes originating from user sessions that were not authenticated as administrators
New or modified files in system script paths outside routine firmware update windows

Detection Strategies

Monitor Zyxel USG FLEX H management and audit logs for PostgreSQL command execution by non-administrator users
Correlate low-privileged user sessions with subsequent administrator token reuse events
Alert on any shell activity on firewall appliances outside of vendor-authorized maintenance windows

Monitoring Recommendations

Forward Zyxel appliance syslog data to a centralized SIEM and retain audit trails for administrator token issuance and reuse
Track administrator logout events and flag configuration changes that occur under a token issued to a session whose owner is no longer active
Baseline expected CLI and API command patterns per user role and alert on deviations that reference database utilities

How to Mitigate CVE-2025-1731

Immediate Actions Required

Upgrade uOS firmware on all USG FLEX H series appliances to the fixed version listed in the Zyxel Security Advisory
Audit all local user accounts on affected firewalls and remove or disable accounts that do not require management access
Force administrator logout after each session and shorten session timeout windows to invalidate tokens promptly

Patch Information

Zyxel released fixed uOS firmware addressing CVE-2025-1731 on 2025-04-22. Administrators should consult the vendor advisory for the exact patched version per model and apply it across the USG FLEX 50H, 100H, 200H, 500H, and 700H product families. Firmware versions V1.20 through V1.31 are vulnerable and must be upgraded.

Workarounds

Restrict management-plane access to trusted administrative networks using ACLs on the firewall
Limit the creation of local low-privileged accounts on the appliance until firmware upgrades are completed
Require administrators to explicitly log out of the management interface rather than relying on browser closure, invalidating any active tokens

bash

# Example: restrict firewall management access to an admin jump host
# (adjust interface, subnet, and rule identifiers to match your deployment)
configure terminal
access-list MGMT_ACL permit ip host 10.10.0.5 any
access-list MGMT_ACL deny ip any any
interface mgmt0
  ip access-group MGMT_ACL in
end
write memory