CVE-2025-12611: Tenda AC21 Buffer Overflow Vulnerability

CVE-2025-12611 Overview

A buffer overflow vulnerability has been identified in Tenda AC21 router firmware version 16.03.08.16. This vulnerability exists in the formSetPPTPServer function within the /goform/SetPptpServerCfg endpoint, where improper handling of the startIp argument allows an attacker to trigger a buffer overflow condition. The vulnerability is remotely exploitable and public exploit information is available, making this a significant security concern for organizations and home users utilizing affected Tenda networking equipment.

Critical Impact

Remote attackers with low privileges can exploit this buffer overflow to potentially execute arbitrary code, crash the device, or compromise the network infrastructure protected by the affected router.

Affected Products

• Tenda AC21 Firmware version 16.03.08.16
• Tenda AC21 Hardware version 1.0
• Tenda AC21 routers running vulnerable firmware versions

Discovery Timeline

• 2025-11-03 - CVE-2025-12611 published to NVD
• 2025-11-05 - Last updated in NVD database

Technical Details for CVE-2025-12611

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer) and CWE-120 (Buffer Copy without Checking Size of Input). The formSetPPTPServer function processes user-supplied input through the startIp parameter without adequate bounds checking. When a malicious request is sent to the /goform/SetPptpServerCfg endpoint with a crafted startIp value, the function copies this data into a fixed-size buffer without validating the input length, causing a classic stack or heap-based buffer overflow.

The vulnerability can be exploited remotely over the network by an authenticated attacker with low-level privileges. Once triggered, this overflow can overwrite adjacent memory regions, potentially allowing control-flow hijacking, denial of service, or arbitrary code execution on the embedded device.

Root Cause

The root cause of this vulnerability lies in the unsafe handling of user input within the PPTP server configuration functionality. The formSetPPTPServer function fails to implement proper input validation and size checks before copying the startIp parameter value into a destination buffer. This represents a fundamental secure coding failure where classic buffer overflow protections such as bounds checking, input length validation, and safe string handling functions are not utilized.

Attack Vector

The attack is network-based, targeting the web management interface of the Tenda AC21 router. An attacker must be able to reach the /goform/SetPptpServerCfg endpoint, typically accessible from the local network or potentially exposed if remote management is enabled.

The exploitation flow involves:

1. Sending an HTTP request to /goform/SetPptpServerCfg with a maliciously crafted startIp parameter
2. The oversized input overflows the destination buffer in the formSetPPTPServer function
3. Adjacent memory is corrupted, potentially allowing the attacker to control execution flow
4. Depending on the payload, this can result in denial of service, code execution, or device compromise

Technical details and proof-of-concept information can be found in the GitHub Issue Discussion and VulDB Entry #330906.

Detection Methods for CVE-2025-12611

Indicators of Compromise

• Unusual HTTP POST requests to /goform/SetPptpServerCfg with abnormally long startIp parameter values
• Router crashes, unexpected reboots, or unresponsive web management interface
• Modified PPTP server configurations that were not administratively changed
• Anomalous outbound network traffic from the router indicating potential compromise

Detection Strategies

• Implement network intrusion detection rules to identify HTTP requests to /goform/SetPptpServerCfg containing oversized parameters
• Monitor router logs for repeated configuration change attempts or authentication failures
• Deploy web application firewall rules to block requests with excessively long input values to Tenda router endpoints
• Use network traffic analysis to detect exploitation attempts targeting router management interfaces

Monitoring Recommendations

• Enable and review router access logs for suspicious activity patterns
• Monitor for device reboots or service interruptions that may indicate exploitation attempts
• Implement network segmentation to limit exposure of router management interfaces
• Regularly audit PPTP server configurations for unauthorized modifications

How to Mitigate CVE-2025-12611

Immediate Actions Required

• Restrict access to the router web management interface to trusted networks only
• Disable PPTP server functionality if not required for operations
• Disable remote management features to reduce attack surface
• Implement network segmentation to isolate the router management plane
• Monitor for firmware updates from Tenda and apply patches when available

Patch Information

As of the last NVD update on 2025-11-05, no official patch information is available from Tenda. Users should monitor the Tenda Official Website for security advisories and firmware updates. Given the public availability of exploit information, prioritizing the implementation of compensating controls is recommended until an official fix is released.

Workarounds

• Disable the PPTP VPN server functionality through the router administration interface if not operationally required
• Configure firewall rules to block external access to the router's web management interface on all ports
• Use an upstream firewall or security appliance to filter malicious requests before they reach the router
• Consider replacing the vulnerable device with an alternative that receives regular security updates

# Example: Block external access to router management (on upstream firewall)
# Assuming router management is on 192.168.1.1
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