CVE-2026-4253: Tenda AC8 RCE Vulnerability

CVE-2026-4253 Overview

A critical OS command injection vulnerability has been discovered in the Tenda AC8 router running firmware version 16.03.50.11. This security flaw affects the route_set_user_policy_rule function within the /cgi-bin/UploadCfg endpoint of the router's Web Interface. By manipulating the wans.policy.list1 argument, remote attackers can inject and execute arbitrary operating system commands on the affected device. The vulnerability is particularly concerning as a proof-of-concept exploit has been publicly released, increasing the risk of active exploitation in the wild.

Critical Impact

Remote attackers with administrative access to the router's web interface can execute arbitrary OS commands, potentially leading to full device compromise, network traffic interception, and lateral movement within the network.

Affected Products

• Tenda AC8 Firmware Version 16.03.50.11
• Tenda AC8 Web Interface (/cgi-bin/UploadCfg endpoint)
• Tenda AC8 route_set_user_policy_rule function

Discovery Timeline

• 2026-03-16 - CVE CVE-2026-4253 published to NVD
• 2026-03-17 - Last updated in NVD database

Technical Details for CVE-2026-4253

Vulnerability Analysis

This vulnerability represents a classic CWE-77 (Command Injection) weakness in the Tenda AC8 router's web interface. The root cause lies in improper neutralization of special elements used in operating system commands within the route_set_user_policy_rule function.

When processing configuration uploads through the /cgi-bin/UploadCfg endpoint, the affected function fails to properly sanitize user-supplied input in the wans.policy.list1 parameter. This allows an attacker to craft malicious input containing shell metacharacters and command sequences that are subsequently executed with the privileges of the web server process—typically root on embedded IoT devices like routers.

The attack requires network access to the router's administrative interface and valid administrative credentials. Once authenticated, an attacker can leverage this vulnerability to gain complete control over the device, potentially using it as a pivot point for further attacks on the internal network.

Root Cause

The vulnerability stems from insufficient input validation in the route_set_user_policy_rule function when handling the wans.policy.list1 parameter. The application passes user-controlled data directly to system shell functions without proper sanitization or escaping of shell metacharacters. This allows command separators (such as ;, |, &&, or backticks) to break out of the intended command context and execute arbitrary commands.

Attack Vector

The attack is conducted remotely over the network by targeting the router's web administration interface. An attacker must first authenticate to the web interface with administrative credentials, then send a specially crafted HTTP request to the /cgi-bin/UploadCfg endpoint. The malicious payload is embedded within the wans.policy.list1 parameter and can include arbitrary shell commands.

A proof-of-concept exploit demonstrating this attack has been publicly released. Technical details and the exploitation script are available in the GitHub PoC Repository. The vulnerability allows remote command execution with the privileges of the web server process, which on embedded router devices typically runs as root.

Detection Methods for CVE-2026-4253

Indicators of Compromise

• Unusual HTTP POST requests to /cgi-bin/UploadCfg containing shell metacharacters (;, |, &&, backticks) in request parameters
• Unexpected outbound network connections originating from the router to external IP addresses
• Presence of unauthorized files or processes running on the router device
• Suspicious log entries showing command execution or configuration changes not initiated by administrators

Detection Strategies

• Monitor network traffic to and from Tenda AC8 routers for anomalous patterns, particularly POST requests to the /cgi-bin/UploadCfg endpoint
• Implement web application firewall (WAF) rules to detect and block command injection attempts targeting the wans.policy.list1 parameter
• Deploy intrusion detection/prevention systems (IDS/IPS) with signatures for common command injection payloads
• Regularly audit router configurations and compare against known-good baselines

Monitoring Recommendations

• Enable and centralize logging from router administrative interfaces for security monitoring
• Set up alerts for multiple failed authentication attempts followed by successful logins to router admin panels
• Monitor for DNS queries or network connections to known malicious infrastructure originating from router IP addresses
• Implement network segmentation to limit exposure of router management interfaces

How to Mitigate CVE-2026-4253

Immediate Actions Required

• Restrict access to the router's web administration interface to trusted IP addresses only
• Ensure strong, unique administrative credentials are in use and change default passwords immediately
• Disable remote administration features if not required for operations
• Place router management interfaces on isolated management VLANs inaccessible from untrusted networks

Patch Information

At the time of publication, no official patch information from Tenda has been identified for this vulnerability. Organizations should monitor the Tenda Official Website for firmware updates addressing this issue. Additional vulnerability details are available through VulDB #351211.

Workarounds

• Implement firewall rules to restrict access to the /cgi-bin/UploadCfg endpoint from untrusted sources
• Deploy a reverse proxy or web application firewall in front of the router's management interface to filter malicious requests
• Consider replacing affected devices with alternative router hardware from vendors with better security track records
• If continued use is necessary, physically disconnect the router's WAN port when remote management is not actively needed

# Example: Restrict access to router management interface using upstream firewall
# Block external access to router admin interface (adjust IP addresses as needed)
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 80 -j DROP
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 443 -j DROP

# Allow management only from trusted admin workstation
iptables -I FORWARD -s 192.168.1.100 -d 192.168.1.1 -p tcp --dport 80 -j ACCEPT
iptables -I FORWARD -s 192.168.1.100 -d 192.168.1.1 -p tcp --dport 443 -j ACCEPT