CVE-2025-9090: Tenda AC20 Firmware RCE Vulnerability

CVE-2025-9090 Overview

A command injection vulnerability has been identified in the Tenda AC20 wireless router running firmware version 16.03.08.12. The vulnerability exists in the websFormDefine function within the /goform/telnet endpoint of the Telnet Service component. This flaw allows remote attackers with low-level privileges to inject and execute arbitrary operating system commands on the affected device.

The vulnerability enables attackers to manipulate user-supplied input that is passed directly to system command execution functions without proper sanitization. Given the network-accessible nature of this endpoint, attackers can exploit this vulnerability remotely to gain unauthorized control over the router.

Critical Impact
Remote command injection in a router's Telnet Service can lead to complete device compromise, network traffic interception, persistent backdoor installation, and use of the device as a pivot point for further network attacks.

Affected Products

Tenda AC20 Firmware version 16.03.08.12
Tenda AC20 Router Hardware

Discovery Timeline

August 17, 2025 - CVE-2025-9090 published to NVD
August 21, 2025 - Last updated in NVD database

Technical Details for CVE-2025-9090

Vulnerability Analysis

This command injection vulnerability (CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component) affects the Telnet Service functionality in the Tenda AC20 router. The websFormDefine function fails to properly sanitize user-controlled input before incorporating it into system commands, allowing attackers to inject malicious command sequences.

The vulnerability is remotely exploitable over the network without requiring any user interaction. While the attacker needs low-level authentication to access the vulnerable endpoint, the ability to inject arbitrary commands provides significant access to the underlying operating system of the router.

Successful exploitation could allow an attacker to execute arbitrary commands with the privileges of the web server process, potentially leading to complete device compromise, configuration manipulation, credential theft, or installation of persistent malware.

Root Cause

The root cause of this vulnerability is inadequate input validation and sanitization within the websFormDefine function. When processing requests to the /goform/telnet endpoint, the function directly incorporates user-supplied parameters into system command execution without properly escaping or filtering special characters such as shell metacharacters (;, |, &, $, etc.).

This lack of input validation allows attackers to break out of the intended command context and execute additional arbitrary commands on the underlying operating system.

Attack Vector

The attack can be launched remotely over the network by sending specially crafted HTTP requests to the /goform/telnet endpoint. An authenticated attacker with low-level privileges can construct malicious input containing command injection payloads.

The exploitation flow involves:

The attacker authenticates to the router's web interface with minimal credentials
A crafted HTTP request is sent to the /goform/telnet endpoint containing shell metacharacters and malicious commands
The websFormDefine function processes the input without sanitization
The injected commands are executed on the router's operating system
The attacker receives command output or establishes a reverse shell connection

For detailed exploitation techniques, refer to the GitHub PoC Exploit Steps documentation.

Detection Methods for CVE-2025-9090

Indicators of Compromise

Unusual HTTP POST requests to the /goform/telnet endpoint containing shell metacharacters (;, |, &, $, backticks)
Unexpected outbound network connections from the router to external IP addresses
New or modified files in the router's filesystem, particularly in writable directories
Unusual process execution or shell spawning from the web server process
Modified router configuration or DNS settings that were not authorized

Detection Strategies

Monitor HTTP traffic for requests to /goform/telnet containing suspicious payload patterns or shell escape sequences
Implement network-based intrusion detection rules to identify command injection attempts targeting Tenda router endpoints
Deploy behavioral analysis to detect anomalous outbound connections or data exfiltration from router IP addresses
Review router logs for repeated authentication attempts followed by requests to the vulnerable endpoint

Monitoring Recommendations

Enable and centralize logging for all Tenda AC20 router administrative interface access
Configure network monitoring to alert on unexpected traffic patterns from router management interfaces
Implement egress filtering to restrict outbound connections from network infrastructure devices
Establish baseline behavior for router processes and alert on deviations

How to Mitigate CVE-2025-9090

Immediate Actions Required

Disable remote administration access to the router's web interface if not required
Restrict access to the router's management interface to trusted IP addresses only using firewall rules
Disable the Telnet Service functionality if it is not essential for operations
Implement network segmentation to isolate router management interfaces from untrusted networks
Monitor for any signs of compromise using the detection strategies outlined above

Patch Information

At the time of publication, no official patch information is available from Tenda for this vulnerability. Organizations should monitor the Tenda Official Website for security updates and firmware releases addressing this issue.

For additional technical details and vulnerability tracking, refer to the VulDB entry #320358 and the GitHub Project Documentation.

Workarounds

Implement strict access control lists (ACLs) to limit management interface access to specific trusted IP addresses
Place the router's management interface on an isolated management VLAN inaccessible from the general network
Deploy a web application firewall (WAF) or reverse proxy in front of the management interface to filter malicious requests
Consider replacing affected devices with alternative router models that receive regular security updates
Use VPN-only access for remote management rather than exposing the web interface directly

bash

# Example firewall rule to restrict management access (iptables)
# Allow management access only from trusted admin network
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP

# Block access to vulnerable endpoint from external networks
iptables -A INPUT -p tcp --dport 80 -m string --string "/goform/telnet" --algo bm -j DROP