CVE-2025-5855: Tenda AC6 Buffer Overflow Vulnerability

CVE-2025-5855 Overview

A critical stack-based buffer overflow vulnerability has been identified in Tenda AC6 router firmware version 15.03.05.16. The vulnerability exists in the formSetRebootTimer function located in the /goform/SetRebootTimer endpoint, where improper handling of the rebootTime argument allows attackers to trigger a stack-based buffer overflow condition. This flaw enables remote attackers with low-level privileges to potentially execute arbitrary code on vulnerable devices.

Critical Impact

Remote attackers can exploit this buffer overflow vulnerability to achieve high-impact compromise of device confidentiality, integrity, and availability through network-based attacks against vulnerable Tenda AC6 routers.

Affected Products

• Tenda AC6 Firmware version 15.03.05.16
• Tenda AC6 Hardware version 1.0
• Tenda AC6 routers running vulnerable firmware versions

Discovery Timeline

• June 9, 2025 - CVE-2025-5855 published to NVD
• June 9, 2025 - Last updated in NVD database

Technical Details for CVE-2025-5855

Vulnerability Analysis

This vulnerability is a stack-based buffer overflow (CWE-787, CWE-119) affecting the Tenda AC6 wireless router. The vulnerable code path exists within the web management interface, specifically in the reboot timer configuration functionality. When processing user-supplied input through the rebootTime parameter, the firmware fails to properly validate the length of the input before copying it to a fixed-size stack buffer.

The attack is network-accessible and requires low-privilege authentication to exploit. An attacker who has obtained basic access to the router's administrative interface can craft a malicious request containing an oversized rebootTime value. This causes the function to write beyond the allocated stack buffer boundaries, potentially overwriting critical stack data including return addresses.

Root Cause

The root cause of this vulnerability is improper bounds checking in the formSetRebootTimer function. The function processes the rebootTime argument from HTTP POST requests to /goform/SetRebootTimer without validating that the input length does not exceed the size of the destination stack buffer. This lack of input validation allows attackers to supply arbitrarily long input strings that overflow the allocated buffer space on the stack.

Attack Vector

The vulnerability is exploitable remotely over the network. An authenticated attacker can send a specially crafted HTTP request to the vulnerable endpoint /goform/SetRebootTimer. The attack flow involves:

1. Establishing a connection to the Tenda AC6 web management interface
2. Authenticating with valid credentials (even low-privilege accounts)
3. Sending a POST request to /goform/SetRebootTimer with an oversized rebootTime parameter
4. The malformed input triggers the stack-based buffer overflow in the formSetRebootTimer function
5. Successful exploitation can result in denial of service or potentially arbitrary code execution

The vulnerability mechanism involves the web server processing form data submitted to the /goform/SetRebootTimer endpoint. When the rebootTime parameter exceeds expected bounds, the data overwrites adjacent stack memory. Technical details and documentation about this vulnerability are available through the Notion Documentation for Tenda AC6 and VulDB #311601.

Detection Methods for CVE-2025-5855

Indicators of Compromise

• Unexpected device reboots or crashes following web interface access
• Anomalous HTTP POST requests to /goform/SetRebootTimer with unusually large payloads
• Web server logs showing malformed requests with extended rebootTime parameter values
• Suspicious network traffic patterns targeting the router management interface on port 80/443

Detection Strategies

• Monitor HTTP traffic to Tenda AC6 devices for POST requests to /goform/SetRebootTimer containing abnormally long parameter values
• Implement network intrusion detection signatures to identify buffer overflow exploitation attempts against embedded device web interfaces
• Deploy anomaly detection for unusual request sizes targeting router management endpoints
• Review web server access logs for repeated attempts to access the vulnerable endpoint

Monitoring Recommendations

• Configure alerts for repeated authentication attempts followed by POST requests to /goform/ endpoints
• Implement network segmentation to isolate IoT devices and limit exposure of management interfaces
• Enable logging on network firewalls to capture traffic destined for embedded device management ports
• Monitor for device instability patterns that may indicate exploitation attempts

How to Mitigate CVE-2025-5855

Immediate Actions Required

• Restrict network access to the Tenda AC6 web management interface to trusted IP addresses only
• Disable remote management access if not required for operations
• Place vulnerable devices behind a properly configured firewall
• Implement network segmentation to isolate IoT devices from critical network segments
• Monitor vendor channels for firmware updates addressing this vulnerability

Patch Information

At the time of publication, no official patch has been released by Tenda for this vulnerability. Organizations should monitor the Tenda Official Website for security updates. Until a patch is available, implementing the recommended workarounds and network controls is essential to reduce exposure.

Workarounds

• Disable the web management interface if not actively needed for device administration
• Configure firewall rules to block external access to the router management interface (ports 80/443)
• Use VPN connections for any remote administration requirements rather than exposing the management interface directly
• Consider replacing affected devices with models from vendors with better security update practices

# Example firewall rule to restrict management access (iptables)
# Allow management access only from trusted admin subnet
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP