CVE-2025-10815: Tenda AC20 Buffer Overflow Vulnerability

CVE-2025-10815 Overview

CVE-2025-10815 is a buffer overflow vulnerability affecting Tenda AC20 routers running firmware up to version 16.03.08.12. The flaw resides in the strcpy call within the /goform/SetPptpServerCfg endpoint handled by the HTTP POST Request Handler. Attackers can manipulate the startIp argument to overflow a fixed-size buffer. The issue is remotely exploitable and a proof-of-concept has been published, increasing the likelihood of opportunistic exploitation against exposed devices.

Critical Impact
Remote attackers with low-privilege HTTP access can corrupt memory on Tenda AC20 routers, potentially leading to denial of service or code execution against the device.

Affected Products

Tenda AC20 router (hardware)
Tenda AC20 firmware versions up to and including 16.03.08.12
Devices exposing the /goform/SetPptpServerCfg endpoint over HTTP

Discovery Timeline

2025-09-22 - CVE-2025-10815 published to the National Vulnerability Database (NVD)
2025-09-25 - Last updated in NVD database

Technical Details for CVE-2025-10815

Vulnerability Analysis

The vulnerability is a classic stack-based buffer overflow categorized under [CWE-119], improper restriction of operations within the bounds of a memory buffer. The Tenda AC20 web management interface exposes a /goform/SetPptpServerCfg handler that processes PPTP server configuration POST requests. Within that handler, the attacker-controlled startIp parameter is copied into a fixed-size stack buffer using strcpy without prior length validation.

Because strcpy terminates only on a null byte, an oversized startIp value writes past the destination buffer and corrupts adjacent stack memory, including the saved return address. Successful exploitation can crash the httpd service or redirect control flow, depending on memory layout and any mitigations present on the MIPS-based firmware.

Root Cause

The root cause is the absence of bounds checking on user-supplied HTTP POST data before invoking strcpy. The firmware trusts the length of the startIp field rather than enforcing a maximum size aligned to the destination buffer.

Attack Vector

Exploitation requires network reachability to the router's HTTP management interface and a low-privilege authenticated session. An attacker sends a crafted POST request to /goform/SetPptpServerCfg with an oversized startIp value. The attack does not require user interaction. Devices that expose the management interface to the WAN or that share credentials across deployments are at elevated risk.

// Synthetic code intentionally omitted. See the published proof-of-concept for
// technical reproduction details:
// https://github.com/Juana-2u/Tenda-AC20

Detection Methods for CVE-2025-10815

Indicators of Compromise

HTTP POST requests to /goform/SetPptpServerCfg containing abnormally long startIp parameter values
Unexpected restarts or crashes of the httpd process on Tenda AC20 devices
Outbound connections from the router to unfamiliar hosts following configuration changes

Detection Strategies

Inspect network traffic destined for router management interfaces for malformed PPTP configuration POST bodies
Alert on startIp field lengths exceeding expected IPv4 string bounds (15 characters)
Correlate router log entries showing PPTP configuration changes with administrative session activity

Monitoring Recommendations

Forward router syslog output to a centralized log platform and retain HTTP request metadata
Monitor for repeated POST requests to /goform/* endpoints from a single source within short time windows
Track WAN-side exposure of router administrative ports using external attack surface scans

How to Mitigate CVE-2025-10815

Immediate Actions Required

Restrict access to the router web administration interface to trusted internal management hosts only
Disable WAN-side management of the Tenda AC20 if it is currently enabled
Rotate administrative credentials and disable any default or shared accounts
Audit recent PPTP server configuration changes for signs of tampering

Patch Information

No vendor patch has been referenced in the NVD entry at the time of publication. Refer to the Tenda Official Website for firmware update availability and to the VulDB entry #325173 for tracking advisory updates.

Workarounds

Block external access to the router's HTTP management port at the upstream firewall
Disable the PPTP server feature if it is not required for the deployment
Place the device on an isolated management VLAN with strict access control lists
Consider replacing end-of-life Tenda AC20 hardware where vendor patches are unavailable

bash

# Example: restrict router management access to a single admin host
iptables -A INPUT -p tcp --dport 80 -s 192.0.2.10 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 192.0.2.10 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP