CVE-2026-36777: Tenda W3 Router Stack Overflow DoS Vulnerability

Technical summary

CVE-2026-36777 is a stack-based buffer overflow (CWE-121) located in the formSetCfm function's param_1 parameter within the Tenda W3 router firmware. The vulnerability stems from insufficient bounds checking on user-supplied HTTP input. When oversized data is written to the stack, it overwrites critical return addresses or control structures, triggering a crash. The attack vector is network-adjacent (AV:A), requires no authentication (PR:N), no user interaction (UI:N), and produces high impact to availability (A:H). CVSS v3.1 base score is 6.5 (Medium).

Business impact

Router crashes disrupt internal network connectivity and productivity. Each DoS incident may require manual device restart, consuming IT time and creating user frustration. For organizations relying on Tenda W3 routers for branch office or remote access infrastructure, repeated attacks could degrade service continuity. The impact is primarily operational rather than data-related, but repeated incidents compound support costs.

Affected systems

Tenda W3 wireless routers running firmware version 1.0.0.3 Build 2204 are confirmed vulnerable. Other firmware versions have not been explicitly confirmed as affected or patched in available vendor advisories; verification against Tenda's official documentation is recommended. The vulnerability requires network proximity to the router, limiting exposure to local or adjacent network segments.

Exploitability

The vulnerability is readily exploitable by any attacker with network access to the device. Attack complexity is low (AC:L)—a single HTTP request with an oversized param_1 parameter is sufficient to trigger the crash. No exploit code, special tools, or social engineering are required. However, the attack is limited to adjacent networks (AV:A), not the internet-facing side, reducing external threat likelihood unless the router's management interface is exposed.

Remediation

Contact Tenda Support to obtain firmware updates newer than build 2204 for the W3 router. Implement network segmentation to restrict HTTP access to the router's management interface. Use firewall rules to allow administrative access only from trusted subnets. Consider deploying intrusion prevention signatures to detect oversized HTTP payloads targeting the router. Monitor router uptime and restart logs for signs of repeated crashes.

Patch guidance

Check the Tenda support portal for W3 router firmware updates released after June 2026. Apply any available patches as they become available. During the interim, isolate the router's management interface on a protected management VLAN with strict access controls. Verify patch installation by confirming firmware version in the router's web interface settings. Test connectivity after patching to ensure no regression.

Detection guidance

Monitor network traffic for HTTP POST requests to the router's management interface with unusually large param_1 values or payloads exceeding normal configuration sizes. Set up alerts on router uptime and crash logs—unexpected reboots correlate with successful exploit attempts. Intrusion detection systems should flag payloads exceeding typical parameter boundaries sent to the formSetCfm endpoint. Passive monitoring of console logs and syslog output can reveal stack-related errors preceding crashes.

Why prioritize this

Although categorized as Medium severity, prioritization depends on deployment context. Organizations using Tenda W3 routers in production should prioritize patching because availability disruptions directly impact users. Networks with tight access controls limiting nearby attackers can defer remediation if patches are unavailable. The lack of data breach risk and requirement for network adjacency reduce criticality compared to remote code execution vulnerabilities, but operational impact justifies timely action.

Risk score, explained

CVSS 6.5 reflects a vulnerability with high impact on availability (A:H), balanced by a restricted attack surface (AV:A, adjacent network only). The lack of confidentiality or integrity impact (C:N, I:N) caps the score despite full availability compromise. In isolated or well-segmented networks, actual risk may be lower; in flat networks where many devices sit adjacent to the router, risk increases. This is a DoS-class vulnerability rather than a code execution threat.

Frequently asked questions