CVE-2021-35029: Zyxel USG1900 Auth Bypass Vulnerability

CVE-2021-35029 Overview

CVE-2021-35029 is an authentication bypass vulnerability affecting the web-based management interface of multiple Zyxel security appliance product lines. This vulnerability exists in Zyxel USG/ZyWALL series firmware versions 4.35 through 4.64 and USG Flex, ATP, and VPN series firmware versions 4.35 through 5.01. A remote attacker can exploit this flaw to bypass authentication mechanisms and execute arbitrary commands on affected devices without valid credentials.

Critical Impact

Remote attackers can gain unauthorized administrative access to network security appliances and execute arbitrary commands, potentially compromising entire network perimeters.

Affected Products

• Zyxel USG/ZyWALL Series (USG20, USG40, USG60, USG110, USG210, USG310, USG1100, USG1900, ZyWALL 110, ZyWALL 310, ZyWALL 1100) - Firmware versions 4.35 through 4.64
• Zyxel USG Flex Series (USG Flex 100, USG Flex 100W, USG Flex 200, USG Flex 500, USG Flex 700) - Firmware versions 4.35 through 5.01
• Zyxel ATP Series (ATP100, ATP100W, ATP200, ATP500, ATP700, ATP800) - Firmware versions 4.35 through 5.01
• Zyxel VPN Series (VPN50, VPN100, VPN300, USG20-VPN, USG20W-VPN, USG2200-VPN) - Firmware versions 4.35 through 5.01

Discovery Timeline

• July 2, 2021 - CVE-2021-35029 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-35029

Vulnerability Analysis

This authentication bypass vulnerability (CWE-287: Improper Authentication) resides in the web-based management interface of affected Zyxel security appliances. The flaw allows remote attackers to circumvent the authentication controls that normally protect administrative functions on the device's management interface. Once authentication is bypassed, attackers can execute arbitrary commands with elevated privileges on the affected device.

The vulnerability is particularly concerning because Zyxel security appliances are typically deployed at network perimeters as firewalls, VPN concentrators, and unified threat management devices. Successful exploitation grants attackers control over these critical network infrastructure components, potentially allowing them to modify security policies, intercept traffic, establish persistent backdoor access, or pivot deeper into protected networks.

Root Cause

The root cause is improper authentication validation in the web-based management interface. The authentication mechanism fails to properly verify user credentials or session tokens under certain conditions, allowing unauthenticated requests to be processed as if they originated from an authenticated administrative user. This represents a fundamental breakdown in the access control logic protecting the device's management functions.

Attack Vector

The attack can be executed remotely over the network without requiring prior authentication or user interaction. An attacker needs only network access to the management interface of a vulnerable device, which may be exposed to the internet or accessible from internal network segments. The attack complexity is low, making this vulnerability accessible to attackers with limited technical sophistication.

Exploitation involves sending specially crafted HTTP requests to the web management interface that exploit the authentication bypass condition. Once the attacker bypasses authentication, they can leverage the command execution capability to run arbitrary system commands on the underlying operating system of the security appliance.

Detection Methods for CVE-2021-35029

Indicators of Compromise

• Unexpected administrative sessions or login events on Zyxel appliances without corresponding legitimate user activity
• Unusual command execution or configuration changes on affected devices appearing in system logs
• Suspicious HTTP requests to the web management interface containing potential bypass attempts or exploitation patterns
• New user accounts, modified firewall rules, or VPN configurations created without authorization

Detection Strategies

• Monitor web management interface access logs for anomalous authentication patterns or requests that result in administrative access without proper credential validation
• Implement network-based intrusion detection rules to identify exploitation attempts targeting the Zyxel management interface
• Deploy SentinelOne Singularity to monitor for post-exploitation activity and command execution patterns on network segments where Zyxel appliances reside

Monitoring Recommendations

• Enable comprehensive logging on all Zyxel appliances and forward logs to a centralized SIEM for correlation and analysis
• Configure alerts for configuration changes on security appliances, particularly changes to administrative accounts, firewall rules, or VPN settings
• Regularly audit management interface access and compare against authorized administrator activity

How to Mitigate CVE-2021-35029

Immediate Actions Required

• Update all affected Zyxel appliances to the latest firmware version that addresses this vulnerability as indicated in the Zyxel Security Advisory
• Restrict management interface access to trusted networks only and disable WAN-side management access if not required
• Review appliance configurations for any signs of compromise including unauthorized accounts, modified policies, or suspicious VPN configurations
• Implement network segmentation to limit access to management interfaces from general user networks

Patch Information

Zyxel has released firmware updates to address this vulnerability. Organizations should update USG/ZyWALL series devices to firmware versions newer than 4.64, and USG Flex, ATP, and VPN series devices to firmware versions newer than 5.01. The official security advisory and firmware downloads are available from the Zyxel Security Advisory.

Workarounds

• Disable remote management access from the WAN interface and restrict management access to internal trusted networks only
• Implement IP-based access control lists (ACLs) to limit which source addresses can access the management interface
• Place management interfaces on a dedicated management VLAN with strict access controls
• Deploy a VPN requirement for all administrative access if remote management is necessary

# Example: Restrict management access to specific trusted networks via ACL
# Access the Zyxel CLI and configure management access restrictions
configure terminal
access-list management permit 10.0.0.0/24
access-list management permit 192.168.1.0/24
access-list management deny any
interface mgmt
  ip access-group management in