CVE-2026-36821: Tenda W20E Buffer Overflow DoS Vulnerability (CVSS 7.5)

Technical summary

CVE-2026-36821 is a stack-based buffer overflow (CWE-121) in the formCropAndSetWewifiPic function within Tenda W20E firmware v15.11.0.6. The vulnerability exists in improper validation of the picCropName HTTP parameter, which fails to enforce adequate bounds checking before writing to a fixed-size buffer. This memory corruption results in denial of service when the overflow is triggered, causing the affected process to crash or the device to become unstable.

Business impact

Organizations relying on Tenda W20E routers for network access face intermittent connectivity loss if the vulnerability is exploited. Repeated attacks could create operational disruption, particularly in environments where these devices serve as primary or secondary network gateways. While confidentiality and integrity are not compromised, availability impact could affect remote work capabilities, branch office connectivity, or managed service delivery.

Affected systems

Tenda W20E firmware version 15.11.0.6 is confirmed vulnerable. Organizations should verify whether this specific firmware version is deployed in their router fleet. Tenda has not publicly released a comprehensive list of all affected versions, so interim mitigation and monitoring are prudent for any W20E deployment pending vendor guidance.

Exploitability

This vulnerability has a low barrier to exploitation. The attack requires only network access and a properly crafted HTTP request—no authentication, no user interaction, and no special client software. The simplicity of the attack vector (CVSS:3.1/AV:N/AC:L/PR:N/UI:N) means that exploitation can occur at scale if routers are exposed to untrusted networks or connected to the internet without proper access controls.

Remediation

Verify the firmware version on affected Tenda W20E devices and apply any available patches from Tenda. If no patch is available, segment W20E routers from untrusted networks, restrict HTTP access via firewall rules, and disable remote management interfaces. Consider replacing the device with a model from a vendor with more frequent security updates if the risk profile is high.

Patch guidance

Contact Tenda directly or check their support portal for updated firmware releases addressing CVE-2026-36821. Document the current firmware version (v15.11.0.6) on affected devices to track remediation progress. Verify patch availability before rolling out updates in production; test in a non-critical environment first to ensure stability. Until patches are deployed, prioritize devices in critical network positions for mitigation.

Detection guidance

Monitor for HTTP requests with unusually long or malformed picCropName parameter values sent to the formCropAndSetWewifiPic endpoint on Tenda W20E routers. Watch for unexpected device restarts or loss of connectivity from these devices. Implement network-level filtering to block overly long parameter strings. Enable verbose logging on affected routers if supported, and correlate logs with DoS events.

Why prioritize this

With a CVSS score of 7.5 (HIGH) and an unauthenticated, network-accessible attack vector, this vulnerability poses a credible denial-of-service risk to organizations operating Tenda W20E routers. The ease of exploitation and lack of authentication requirements warrant prompt inventory and mitigation planning, particularly for routers exposed to the internet or untrusted networks.

Risk score, explained

The CVSS 3.1 score of 7.5 reflects high severity due to the complete denial of service impact (A:H), combined with a network-based, low-complexity attack requiring no privileges or user interaction (AV:N/AC:L/PR:N/UI:N). The score does not increase to critical because the impact is limited to availability; no confidentiality or integrity breach occurs. Organizations with internet-facing W20E routers should treat this as a near-term remediation priority.

Frequently asked questions