CVE-2025-9088: Tenda AC20 Buffer Overflow Vulnerability

CVE-2025-9088 Overview

A stack-based buffer overflow vulnerability has been discovered in Tenda AC20 router firmware version 16.03.08.12. This vulnerability exists within the save_virtualser_data function located in the /goform/formSetVirtualSer endpoint. By manipulating the list argument, an attacker can trigger a stack-based buffer overflow condition that may lead to remote code execution or denial of service on the affected device.

The vulnerability is remotely exploitable and a proof-of-concept exploit has been publicly disclosed, increasing the risk of exploitation in the wild. Tenda AC20 routers are commonly deployed in home and small office networks, making this vulnerability particularly concerning for consumer network security.

Critical Impact

Remote attackers with low privileges can exploit this stack-based buffer overflow to achieve full device compromise, potentially gaining persistent access to network infrastructure and pivoting to other connected devices.

Affected Products

• Tenda AC20 Firmware version 16.03.08.12
• Tenda AC20 Router Hardware

Discovery Timeline

• August 16, 2025 - CVE-2025-9088 published to NVD
• August 21, 2025 - Last updated in NVD database

Technical Details for CVE-2025-9088

Vulnerability Analysis

This vulnerability is classified under CWE-787 (Out-of-bounds Write) and CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw resides in the save_virtualser_data function, which processes user-supplied input from the list parameter without adequate bounds checking.

When processing requests to the /goform/formSetVirtualSer endpoint, the firmware fails to properly validate the length of the list argument before copying it into a fixed-size stack buffer. This allows an attacker to provide an oversized payload that overwrites adjacent stack memory, including the return address and saved registers.

The network-accessible nature of this vulnerability means attackers can exploit it remotely without physical access to the device. The exploit requires only low-level authentication, which in many consumer router deployments may be a default or weak credential.

Root Cause

The root cause is insufficient input validation in the save_virtualser_data function. The code accepts the list parameter from HTTP POST requests without verifying that the input length does not exceed the allocated stack buffer size. This classic stack-based buffer overflow pattern allows attackers to overwrite critical stack data structures and redirect program execution flow.

Attack Vector

The attack is initiated remotely via the network. An authenticated attacker sends a crafted HTTP POST request to the /goform/formSetVirtualSer endpoint with a malicious list parameter containing an oversized payload. The payload overflows the stack buffer, corrupting the return address to redirect execution to attacker-controlled shellcode or return-oriented programming (ROP) gadgets.

The attack mechanism involves sending an HTTP POST request to the vulnerable endpoint with a payload designed to exceed buffer boundaries. The list parameter is processed by the save_virtualser_data function, where the lack of proper length validation allows the overflow to occur. Technical details and a proof-of-concept Python exploit script are available in the GitHub Repository for AC20.

Detection Methods for CVE-2025-9088

Indicators of Compromise

• Unusual HTTP POST requests to /goform/formSetVirtualSer containing abnormally large list parameter values
• Device crashes or unexpected reboots following web management interface access
• Modified device configuration or firmware indicating unauthorized access
• Suspicious outbound network connections from the router to unknown external hosts

Detection Strategies

• Deploy network intrusion detection signatures to identify HTTP requests with oversized POST payloads targeting /goform/formSetVirtualSer
• Monitor router logs for authentication attempts followed by unusual endpoint access patterns
• Implement deep packet inspection to detect buffer overflow payload signatures in HTTP traffic destined for Tenda devices
• Use firmware integrity monitoring to detect unauthorized modifications

Monitoring Recommendations

• Enable logging on the Tenda AC20 web management interface and forward logs to a centralized SIEM
• Monitor network traffic for POST requests exceeding normal parameter sizes targeting Tenda router endpoints
• Establish baseline behavior for router management traffic and alert on anomalies
• Regularly audit device configurations for unauthorized changes

How to Mitigate CVE-2025-9088

Immediate Actions Required

• Restrict access to the router's web management interface to trusted internal networks only
• Disable remote management features if not required for operations
• Change default administrator credentials to strong, unique passwords
• Place the affected router behind a firewall or network segmentation boundary
• Monitor for firmware updates from Tenda and apply patches as soon as available

Patch Information

At the time of publication, no official patch has been released by Tenda for this vulnerability. Users should monitor the Tenda Official Website for security updates and firmware releases. Additional technical information is available through VulDB ID #320356.

Workarounds

• Implement access control lists (ACLs) to restrict management interface access to specific trusted IP addresses
• Deploy a Web Application Firewall (WAF) in front of the management interface to filter malicious requests
• Consider network isolation for vulnerable devices until a patch becomes available
• If possible, disable the Virtual Server configuration functionality that exposes the vulnerable endpoint

# Example: Restrict management access via iptables (upstream firewall)
iptables -A FORWARD -d <router_ip> -p tcp --dport 80 -j DROP
iptables -A FORWARD -d <router_ip> -p tcp --dport 80 -s <trusted_admin_ip> -j ACCEPT