CVE-2025-13446: Tenda AC21 Buffer Overflow Vulnerability

CVE-2025-13446 Overview

A stack-based buffer overflow vulnerability has been discovered in the Tenda AC21 wireless router running firmware version 16.03.08.16. This vulnerability exists in the /goform/SetSysTimeCfg endpoint, where improper handling of the timeZone and time parameters allows an attacker to overflow the stack buffer. As a network-accessible vulnerability requiring only low privileges, this flaw enables remote attackers to potentially execute arbitrary code or cause a denial of service condition on affected devices.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow to compromise Tenda AC21 routers, potentially gaining full control over the device and pivoting to attack other network resources.

Affected Products

• Tenda AC21 Firmware version 16.03.08.16
• Tenda AC21 Hardware

Discovery Timeline

• 2025-11-20 - CVE-2025-13446 published to NVD
• 2025-11-21 - Last updated in NVD database

Technical Details for CVE-2025-13446

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The affected code resides in the system time configuration handler at /goform/SetSysTimeCfg, which processes user-supplied input for the timeZone and time arguments without adequate bounds checking.

When a request is made to configure the system time, the firmware copies user-supplied data into fixed-size stack buffers. Due to missing input validation and boundary checks, an attacker can supply overly long strings that exceed the allocated buffer space, corrupting adjacent stack memory including saved return addresses and other critical data structures.

The network-accessible nature of this vulnerability is particularly concerning for consumer routers, as these devices are often directly exposed to the internet or accessible from local networks with minimal authentication requirements.

Root Cause

The root cause is insufficient input validation in the firmware's web interface handler. The SetSysTimeCfg function fails to verify the length of the timeZone and time parameters before copying them to stack-allocated buffers. This classic programming error allows attackers to write beyond the intended buffer boundaries, overwriting adjacent memory on the stack.

Attack Vector

The vulnerability can be exploited remotely over the network. An attacker with low-level authentication (such as basic router access) can craft malicious HTTP POST requests to the /goform/SetSysTimeCfg endpoint containing oversized values for the timeZone or time parameters. The attack does not require user interaction and can be executed against any vulnerable Tenda AC21 router accessible on the network.

The exploitation process involves:

1. Authenticating to the router's web interface with minimal credentials
2. Sending a crafted POST request to /goform/SetSysTimeCfg
3. Including malicious payload in the timeZone or time parameters that exceeds buffer boundaries
4. The overflow overwrites return addresses or function pointers on the stack
5. Attacker-controlled code execution or device crash occurs

Technical details and proof-of-concept information have been publicly disclosed. See the GitHub Vulnerability Overview and additional vulnerability details for more information.

Detection Methods for CVE-2025-13446

Indicators of Compromise

• Unexpected HTTP POST requests to /goform/SetSysTimeCfg with abnormally large timeZone or time parameter values
• Router crashes, reboots, or unresponsive behavior following web interface access
• Anomalous outbound network connections from the router to unknown destinations
• Modified router configurations or unauthorized administrative access

Detection Strategies

• Monitor HTTP traffic for requests to /goform/SetSysTimeCfg containing payloads exceeding normal parameter lengths (typically greater than 128 bytes)
• Implement intrusion detection rules to flag POST requests with suspicious patterns targeting Tenda router form endpoints
• Deploy network anomaly detection to identify unusual traffic patterns originating from or directed at router management interfaces

Monitoring Recommendations

• Enable logging on network firewalls to capture traffic to router management interfaces
• Implement network segmentation to restrict access to router administration from untrusted networks
• Use SentinelOne Singularity for IoT/OT to monitor firmware integrity and detect exploitation attempts on network devices

How to Mitigate CVE-2025-13446

Immediate Actions Required

• Restrict network access to the router's web management interface to trusted IP addresses only
• Disable remote management features if not required
• Implement firewall rules to block external access to router administration ports
• Monitor for firmware updates from Tenda and apply patches immediately when available

Patch Information

As of the last update on 2025-11-21, no official patch information has been published by Tenda. Users should monitor the Tenda Official Website for security advisories and firmware updates. Additional vulnerability tracking information is available at VulDB #333018.

Workarounds

• Configure access control lists (ACLs) to limit web interface access to specific management workstations
• Place the router behind a separate firewall that can filter and inspect HTTP traffic
• If feasible, consider replacing the vulnerable device with an alternative router model until a patch is available
• Disable unnecessary services and close unused ports on the router

# Example: Restrict management interface access via iptables on upstream firewall
# Block external access to router management port
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 only trusted management workstation
iptables -I FORWARD -s 192.168.1.100 -d 192.168.1.1 -p tcp --dport 80 -j ACCEPT