CVE-2021-31755: Tenda AC11 Router Stack Buffer Overflow Vulnerability

CVE-2021-31755 is a stack buffer overflow in the Tenda AC11 router that can let an attacker run code by sending a crafted POST request to the /goform/setmac endpoint. For IT and security teams, this matters because a compromised edge router can expose the internal network, intercept traffic, or serve as a foothold for further intrusion.

Public detail is limited to the CISA description and the CWE classification; confirm exact impact, fixed builds, and exposure conditions against the vendor advisory before acting.

How it works

The weakness is CWE-787 (out-of-bounds write). On the Tenda AC11, the /goform/setmac handler does not adequately bound data from a crafted POST request before writing it to a stack buffer. An attacker who can reach that interface can overflow the buffer and potentially achieve code execution on the device.

No further exploit mechanics, authentication requirements, or preconditions are stated in the provided facts. Treat any internet-facing or poorly segmented management path as higher risk, and verify the precise attack surface in the vendor advisory.

Am I affected? How to find it in your systems

This issue affects Tenda AC11 routers. These devices commonly appear as small-office or home-office gateways, Wi-Fi access points, or branch routers.

• Inventory: Query asset management, DHCP/DNS records, and network discovery for Tenda AC11 hardware or firmware banners. Check physical labels and the device web UI for model confirmation.
• Version and config: Compare running firmware to the fixed release named by the vendor. Note whether remote management or the setmac-related form is reachable from untrusted networks.
• Telemetry: Look for unexpected POST requests to /goform/setmac, device reboots, or anomalous processes on the router if logging is available. Router logs are often sparse; correlate with upstream firewall or IDS alerts for crafted POSTs toward the device.

If you cannot confirm the model or firmware, assume potential exposure until verified against the vendor advisory.

How to remediate

Patch first. Apply the updates provided by the vendor per their instructions, as required by CISA guidance for this CVE. Confirm the advisory lists the exact fixed firmware for the AC11 and install it through the supported upgrade path.

• After upgrade, re-check firmware version and disable any unnecessary remote administration features.
• Restrict management interfaces to trusted administrative networks only.
• Replace default credentials and ensure unique, strong admin passwords.
• For this class of stack overflow on embedded devices, also review whether the management web service can be bound only to LAN or a dedicated management VLAN.

If you can't patch immediately

Reduce reachability and monitor until the vendor update can be applied.

• Segment the router: Place it behind a firewall or isolate it so that /goform/setmac is not reachable from the internet or untrusted VLANs.
• Block or restrict HTTP/HTTPS management ports at the perimeter; allow admin access only from jump hosts or VPN.
• If a WAF or reverse proxy sits in front of the management interface (uncommon for consumer routers but possible in some deployments), consider virtual patching rules that drop oversized or anomalous POSTs to /goform/setmac—validate any such rule against production traffic.
• Disable remote management and any unused services on the device if the UI permits.
• Increase monitoring: alert on POSTs to the setmac path, sudden configuration changes, or unexpected outbound connections from the router.

These steps only lower risk; they do not replace the vendor patch.

If your data may have been exposed

Actively exploited router vulnerabilities can lead to network compromise and data exposure even when ransomware use is not documented for this CVE. If you suspect exploitation, isolate the device, preserve logs, rotate credentials that traversed the router, and review internal systems for lateral movement. You can run a free exposure scan of your email addresses against known breach data to see whether associated accounts appear in public breach corpora, then prioritize password resets and monitoring for those identities.