Tenda AC11 Router /goform/setmac Stack Buffer Overflow Remote Code Execution Vulnerability (CVE-2021-31755) Analysis and Mitigation Guide
Executive & SecOps Summary
SECOPS ASSESSMENTTenda AC11 wireless routers running firmware versions up to and including 02.03.01.104_CN
Unauthenticated remote attackers can execute arbitrary system commands with root privileges by sending a specially crafted HTTP POST request
Immediately upgrade to the latest patched firmware version and disable external WAN-side HTTP management interfaces
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Users must download the latest secure firmware version from the official Tenda support portal and perform a manual upgrade. Reboot the device after the upgrade is complete.
2. Temporary Workaround & Hardening Configuration
If patching is not immediately feasible, disable remote WAN-side management and restrict access to the web administration interface to trusted internal IP addresses only.
# Example iptables rule to block external WAN-side access to the web management ports (e.g., 80, 8080)
iptables -A INPUT -i eth0 -p tcp --dport 80 -j DROP
iptables -A INPUT -i eth0 -p tcp --dport 8080 -j DROP3. Log Audit & IoC Verification Commands
Deploy network-level detection rules to identify anomalous POST requests targeting the /goform/setmac endpoint.
# Snort/Suricata IDS rule to detect potential exploitation attempts targeting the setmac endpoint
alert tcp $EXTERNAL_NET any -> $HOME_NET 80 (msg:'COPS_Tenda AC11 setmac Stack Buffer Overflow Attempt'; flow:established,to_server; content:'POST'; http_method; content:'/goform/setmac'; http_uri; fast_pattern; dsize:>500; reference:cve,2021-31755; classtype:web-application-attack; sid:1000001; rev:1;)Stop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
Eliminate public-facing VPN concentrators and firewall management ports with identity-first Zero-Trust Network Access.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2021-31755) is a critical stack-based buffer overflow (CWE-787) located in the /goform/setmac endpoint of the Tenda AC11 web management interface. The root cause lies in the web server's failure to properly validate the length of user-supplied parameters in HTTP POST requests before copying them into a fixed-size stack buffer. This lack of bounds checking allows an attacker to overwrite the stack's return address, hijacking the execution flow of the device.
2. Exploitation Mechanics & Attack Scenario
An unauthenticated attacker can exploit this vulnerability by sending a malicious HTTP POST request targeting the /goform/setmac URI via the LAN or WAN interface. By embedding an excessively long string within the request parameters, the attacker triggers a buffer overflow during processing by the internal CGI binary. This allows the attacker to inject shellcode or construct a Return-Oriented Programming (ROP) chain to gain full control of the device. Due to active exploitation in the wild, this vulnerability has been added to the CISA Known Exploited Vulnerabilities (KEV) catalog.
3. Infrastructure & Business Impact
Since the Tenda AC11 router serves as the gateway for local networks, compromise of this device exposes all internal network traffic. Attackers can enlist compromised routers into botnets for DDoS campaigns, perform DNS hijacking to redirect users to malicious sites, or sniff sensitive transit data, posing a severe threat to business and home network environments.
Official Vendor Advisories & References
An issue was discovered on Tenda AC11 devices with firmware through 02.03.01.104_CN. A stack buffer overflow vulnerability in /goform/setmac allows attackers to execute arbitrary code on the system via a crafted post request.
Frequently Asked SecOps Questions (CVE-2021-31755)
FAQQ1. How critical is CVE-2021-31755 and is it actively exploited in the wild?
CVE-2021-31755 is rated CVSS 9.8 (CRITICAL) with a 30-day EPSS exploitation probability of 86.5%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-31755?
Primary impacted vendors include tenda across ac11. Scope summary: Tenda AC11 routers with firmware versions up to and including 02.03.01.104_CN
Q3. What temporary mitigations or workarounds exist for CVE-2021-31755 before patching?
[Mitigation] Immediately upgrade to the latest patched firmware version and disable external WAN-side HTTP management interfaces Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
Generate an Internal Engineering Notice or CISO Report for CVE-2021-31755
Instantly format this advisory into an executive CISO summary or engineering patch ticket.
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