CVE-2025-3820: Tenda W12 Buffer Overflow Vulnerability

CVE-2025-3820 Overview

CVE-2025-3820 is a stack-based buffer overflow vulnerability in Tenda W12 and i24 wireless access points running firmware versions 3.0.0.4(2887) and 3.0.0.5(3644). The flaw resides in the cgiSysUplinkCheckSet function within /bin/httpd. Attackers can trigger the overflow by manipulating the hostIp1 or hostIp2 arguments. The vulnerability is reachable over the network and the exploit has been publicly disclosed.

Critical Impact

Authenticated remote attackers can corrupt the stack of the device web server, potentially achieving arbitrary code execution on the access point and compromising the network perimeter.

Affected Products

• Tenda W12 firmware 3.0.0.4(2887)
• Tenda W12 firmware 3.0.0.5(3644)
• Tenda i24 firmware 3.0.0.4(2887) and 3.0.0.5(3644)

Discovery Timeline

• 2025-04-19 - CVE-2025-3820 published to NVD
• 2025-07-30 - Last updated in NVD database

Technical Details for CVE-2025-3820

Vulnerability Analysis

The vulnerability is classified under [CWE-119] as an improper restriction of operations within the bounds of a memory buffer. The affected function cgiSysUplinkCheckSet is part of the /bin/httpd binary that serves the device administrative web interface. The function processes uplink check configuration requests but fails to validate the length of attacker-supplied input before copying it into a fixed-size stack buffer.

When the hostIp1 or hostIp2 parameters are submitted with oversized values, the unchecked copy operation writes past the buffer boundary. This overwrites adjacent stack data, including the saved return address and potentially function pointers. An attacker controlling these values can redirect execution flow within the httpd process.

Because the httpd process typically runs with elevated privileges on the device, successful exploitation may allow command execution at the operating system level. The attack requires network access to the device management interface and low-privilege authentication.

Root Cause

The root cause is the absence of bounds checking on the hostIp1 and hostIp2 request parameters inside cgiSysUplinkCheckSet. Standard unsafe string operations such as strcpy or sprintf against fixed-size local buffers permit stack corruption when input length exceeds the buffer size.

Attack Vector

The attack vector is network-based and targets the HTTP management service exposed by the access point. An attacker sends a crafted HTTP request containing oversized hostIp1 or hostIp2 values to the uplink check endpoint handled by cgiSysUplinkCheckSet. No user interaction is required. Devices that expose the management interface to untrusted networks face significantly higher exposure.

Technical analysis and proof-of-concept details are published in the GitHub Issue Discussion and the VulDB entry #305726.

Detection Methods for CVE-2025-3820

Indicators of Compromise

• HTTP POST requests to the uplink check endpoint of /bin/httpd containing abnormally long hostIp1 or hostIp2 parameter values
• Repeated crashes or unexpected restarts of the httpd process on Tenda W12 or i24 devices
• Unexpected outbound connections from the access point to unknown remote hosts following management interface activity
• New or modified configurations on the device that were not initiated by an administrator

Detection Strategies

• Inspect HTTP traffic to access point management interfaces for parameter lengths exceeding typical IPv4 string sizes
• Deploy network intrusion detection signatures that flag oversized hostIp1 and hostIp2 values in requests to Tenda CGI endpoints
• Correlate device syslog entries that report httpd crashes or reboots with preceding inbound HTTP traffic

Monitoring Recommendations

• Forward Tenda device logs to a centralized logging or SIEM platform to detect crash patterns and authentication anomalies
• Monitor administrative authentication events to W12 and i24 devices for brute-force or credential stuffing attempts
• Track network flows originating from access points for deviations from baseline behavior

How to Mitigate CVE-2025-3820

Immediate Actions Required

• Restrict access to the device web management interface to trusted management VLANs or jump hosts
• Disable remote WAN-side management on affected W12 and i24 devices
• Rotate administrative credentials and enforce strong password policies to raise the barrier for the required low-privilege authentication
• Inventory affected firmware versions 3.0.0.4(2887) and 3.0.0.5(3644) and prioritize them for replacement or patching

Patch Information

At the time of publication, no vendor patch is referenced in the NVD entry. Administrators should monitor the Tenda Security Information page for firmware updates addressing CVE-2025-3820. Until a fix is available, compensating controls must be applied to limit network exposure of the affected devices.

Workarounds

• Place affected access points behind a firewall that blocks inbound HTTP and HTTPS to the management interface from untrusted segments
• Use access control lists to permit management traffic only from designated administrative IP addresses
• Consider replacing end-of-support Tenda devices in high-risk environments with models that receive active security maintenance

# Example: restrict management access to a single admin subnet using iptables on an upstream gateway
iptables -A FORWARD -p tcp -d <AP_IP> --dport 80 -s 10.10.10.0/24 -j ACCEPT
iptables -A FORWARD -p tcp -d <AP_IP> --dport 80 -j DROP
iptables -A FORWARD -p tcp -d <AP_IP> --dport 443 -s 10.10.10.0/24 -j ACCEPT
iptables -A FORWARD -p tcp -d <AP_IP> --dport 443 -j DROP