CVE-2025-11385: Tenda AC20 Buffer Overflow Vulnerability

CVE-2025-11385 Overview

CVE-2025-11385 is a buffer overflow vulnerability affecting Tenda AC20 routers running firmware up to version 16.03.08.12. The flaw resides in the sscanf function within the /goform/fast_setting_wifi_set endpoint. Attackers can trigger the overflow by manipulating the timeZone argument supplied to this endpoint. The vulnerability is remotely exploitable over the network and a public disclosure has been released. The weakness is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer). Successful exploitation can lead to memory corruption, denial of service, or potential code execution on the affected device.

Critical Impact
Remote attackers with low privileges can exploit the timeZone parameter to corrupt memory on Tenda AC20 routers, threatening device confidentiality, integrity, and availability.

Affected Products

Tenda AC20 router (hardware)
Tenda AC20 firmware versions up to and including 16.03.08.12
Deployments exposing the /goform/fast_setting_wifi_set web management endpoint

Discovery Timeline

2025-10-07 - CVE-2025-11385 published to NVD
2025-10-09 - Last updated in NVD database

Technical Details for CVE-2025-11385

Vulnerability Analysis

The vulnerability exists in the request handler for /goform/fast_setting_wifi_set, part of the router's web administration interface. The handler invokes sscanf to parse the user-supplied timeZone argument into a fixed-size stack buffer. Because no length check constrains the input prior to parsing, an oversized timeZone value overruns the destination buffer. Memory adjacent to the buffer, including saved return addresses and control data, can be overwritten. This memory corruption pattern aligns with [CWE-119] and is consistent with prior Tenda /goform endpoint vulnerabilities. Depending on the firmware build and protections present, an attacker can cause a denial of service through router crash, or potentially achieve arbitrary code execution within the context of the web service running as root.

Root Cause

The root cause is the use of sscanf with an unbounded format specifier writing into a fixed-size local buffer. The handler trusts the length and content of the timeZone parameter received from the HTTP request without sanitization or boundary enforcement. This is a recurring pattern in embedded web administration code that uses C string parsing routines without explicit size checks.

Attack Vector

Exploitation requires network access to the router's web management interface and a valid low-privilege session. An attacker sends a crafted HTTP POST request to /goform/fast_setting_wifi_set containing an oversized timeZone parameter. The malformed value overflows the stack buffer during sscanf processing, corrupting adjacent memory. Where the web management interface is exposed to the WAN, the attack surface extends to internet-based attackers. For technical details and proof-of-concept, see the GitHub CVE Documentation and the VulDB entry #327312.

Detection Methods for CVE-2025-11385

Indicators of Compromise

HTTP POST requests to /goform/fast_setting_wifi_set containing unusually long timeZone parameter values
Unexpected reboots, crashes, or service restarts of the Tenda AC20 web administration daemon
Anomalous outbound connections originating from the router following management interface requests
New or unauthorized configuration changes on the router after suspicious management traffic

Detection Strategies

Inspect HTTP traffic destined for router management interfaces for /goform/fast_setting_wifi_set requests with timeZone values exceeding typical length
Correlate web management requests with subsequent router reboots or service interruptions to identify exploitation attempts
Deploy network IDS signatures matching oversized parameters in Tenda /goform/ endpoint requests

Monitoring Recommendations

Log all HTTP requests reaching router administration endpoints from both LAN and WAN interfaces
Monitor router uptime and syslog output for unexpected restarts indicative of crash-based exploitation
Alert on management interface access from untrusted networks or unfamiliar source addresses

How to Mitigate CVE-2025-11385

Immediate Actions Required

Disable remote (WAN-side) administration of the Tenda AC20 web interface until a vendor patch is available
Restrict access to /goform/fast_setting_wifi_set to trusted management hosts on the LAN only
Rotate router administrative credentials and enforce strong unique passwords to limit low-privilege access required for exploitation
Place vulnerable routers behind segmented network boundaries to reduce attacker reachability

Patch Information

No vendor advisory or firmware patch has been published at the time of disclosure. Affected users should monitor the Tenda official website for firmware updates addressing this vulnerability. Where no patch is available, organizations should evaluate replacement of end-of-life or unsupported hardware.

Workarounds

Disable the router's WAN-facing web administration interface in device settings
Apply firewall rules upstream to block external access to the router management port
Use network segmentation to isolate the router management plane from untrusted user networks
Consider replacing affected Tenda AC20 devices with actively supported hardware if no patch is forthcoming

bash

# Example: block external access to the router management interface
# (run on an upstream firewall, replace ROUTER_IP and MGMT_PORT accordingly)
iptables -A FORWARD -d ROUTER_IP -p tcp --dport MGMT_PORT -i wan0 -j DROP

# Restrict management access to a trusted admin subnet only
iptables -A FORWARD -d ROUTER_IP -p tcp --dport MGMT_PORT -s 192.168.10.0/24 -j ACCEPT
iptables -A FORWARD -d ROUTER_IP -p tcp --dport MGMT_PORT -j DROP