CVE-2026-2872: Tenda A21 Buffer Overflow Vulnerability

CVE-2026-2872 Overview

CVE-2026-2872 is a stack-based buffer overflow in the Tenda A21 router running firmware version 1.0.0.0. The flaw resides in the set_device_name function reachable through the /goform/setBlackRule endpoint, which handles MAC Filtering configuration. Attackers manipulate the devName or mac arguments to overflow a fixed-size stack buffer. The vulnerability is exploitable over the network and has been publicly disclosed, increasing the risk of opportunistic attacks against exposed devices.

Critical Impact
Remote attackers with low-privilege access can corrupt stack memory on the Tenda A21, enabling denial of service and potential arbitrary code execution on the router.

Affected Products

Tenda A21 router (hardware)
Tenda A21 firmware version 1.0.0.0
Deployments exposing the /goform/setBlackRule MAC Filtering endpoint

Discovery Timeline

2026-02-21 - CVE-2026-2872 published to the National Vulnerability Database
2026-02-23 - Last updated in the NVD database

Technical Details for CVE-2026-2872

Vulnerability Analysis

The vulnerability is a classic stack-based buffer overflow [CWE-119] in the web management interface of the Tenda A21. When a user submits a MAC filtering rule, the firmware invokes the set_device_name handler bound to /goform/setBlackRule. The handler reads the devName and mac parameters from the HTTP request and copies them into fixed-size stack buffers without enforcing length validation. Oversized input overwrites adjacent stack memory, including saved return addresses and frame pointers.

Successful exploitation can crash the httpd process and reboot the device, producing a denial of service. On embedded MIPS or ARM targets that lack stack canaries and address space layout randomization, attackers can also redirect control flow to attacker-controlled shellcode or ROP gadgets, yielding code execution in the context of the web server.

Root Cause

The handler trusts client-supplied string lengths and uses unbounded copy operations such as strcpy or sprintf against stack buffers. The firmware does not sanitize the MAC format or enforce a maximum device name length before the copy.

Attack Vector

Exploitation requires network reachability to the device administration interface and a low-privilege authenticated session. An attacker sends a crafted POST request to /goform/setBlackRule with an oversized devName or mac value. The exploit has been disclosed publicly through VulDB and a GitHub issue, so weaponization is feasible.

No verified proof-of-concept code is available in trusted repositories. Refer to the GitHub Issue Discussion and VulDB #347109 for technical specifics.

Detection Methods for CVE-2026-2872

Indicators of Compromise

Unexpected reboots or crashes of the httpd process on Tenda A21 devices
HTTP POST requests to /goform/setBlackRule containing abnormally long devName or mac parameter values
Administrative sessions originating from unfamiliar source IP addresses or geographies
Modifications to MAC filtering rules that were not initiated by an authorized administrator

Detection Strategies

Inspect HTTP traffic to the router web interface for parameter lengths exceeding typical MAC address (17 characters) or device name (32 characters) sizes.
Apply intrusion detection signatures that flag oversized payloads targeting /goform/setBlackRule.
Correlate router crash and watchdog reboot events with prior authenticated web sessions.

Monitoring Recommendations

Forward router syslog and web access logs to a centralized log platform for retention and analysis.
Alert on repeated authentication attempts and configuration changes targeting the MAC filtering subsystem.
Monitor for new or unusual MAC filtering rules appearing in the device configuration.

How to Mitigate CVE-2026-2872

Immediate Actions Required

Restrict access to the Tenda A21 web administration interface to trusted management VLANs and disable WAN-side administration.
Rotate the administrator credentials and enforce strong, unique passwords on every Tenda A21 device.
Audit existing MAC filtering rules and remove any entries that were not added by authorized personnel.
Segment vulnerable routers away from sensitive internal systems until a firmware fix is available.

Patch Information

At the time of publication, Tenda has not released a fixed firmware version addressing CVE-2026-2872. Monitor the Tenda Official Website and the GitHub Issue Discussion for vendor updates and apply firmware patches promptly when released.

Workarounds

Disable remote management on the WAN interface and limit administrative access to a single management host.
Place the router behind a reverse proxy or network ACL that enforces parameter length limits on requests to /goform/setBlackRule.
Replace end-of-support or unpatched Tenda A21 units with a supported router model in high-risk environments.

bash

# Example ACL to restrict router admin access to a single management host
iptables -A INPUT -i wan0 -p tcp --dport 80 -j DROP
iptables -A INPUT -i wan0 -p tcp --dport 443 -j DROP
iptables -A INPUT -s 10.10.10.5 -p tcp --dport 80 -j ACCEPT