CVE-2026-36820: Tenda W20E Buffer Overflow Denial of Service (HIGH)

Technical summary

CVE-2026-36820 is a stack-based buffer overflow (CWE-121) in the formAddWebAuthWhiteUser function of Tenda W20E v15.11.0.6. The webAuthWhiteUserInfo parameter fails to properly validate input length before writing to a fixed-size buffer, allowing unauthenticated remote attackers to trigger a denial of service condition. The attack surface is the device's HTTP interface, which is typically exposed on the LAN.

Business impact

Affected Tenda W20E devices will become temporarily unresponsive when exploited, disrupting network access for all users relying on the router. In environments where this model serves as the primary internet gateway or guest network gateway, the impact cascades to business continuity. Organizations cannot route traffic, access cloud services, or maintain VPN connections while the router is down. Recovery requires manual device reboot.

Affected systems

Tenda W20E routers running firmware version 15.11.0.6 are confirmed vulnerable. Organizations should verify whether earlier or later firmware versions are affected by reviewing Tenda's official advisory, as the vulnerability report identifies only this specific version as tested.

Exploitability

This vulnerability is readily exploitable. No authentication, user interaction, or special network position is required—an attacker need only send an HTTP request over the network. The attack is reliable and repeatable. The CVSS score of 7.5 (HIGH) reflects the ease of exploitation and availability impact, though confidentiality and integrity are not compromised.

Remediation

Organizations must upgrade Tenda W20E devices to a patched firmware version released by Shenzhen Tenda Technology Co., Ltd. Consult Tenda's security advisory for the exact fixed version number and upgrade procedure. As an interim measure, restrict network access to the router's web interface using firewall rules if the device is internet-facing, or disable web-based administration if not required.

Patch guidance

Contact Tenda support or visit their official firmware download portal to obtain and install the latest firmware build for the W20E model. Verify the build number against Tenda's published advisory before deployment. Firmware updates typically require brief downtime; schedule updates during maintenance windows. If an automatic update feature is available in the web interface, enable it after patching to prevent regression.

Detection guidance

Monitor router access logs for unusual HTTP requests to the formAddWebAuthWhiteUser endpoint with abnormally long parameter values. Watch for router restarts or service disruptions that coincide with external network traffic. Network-based detection can identify malformed webAuthWhiteUserInfo payloads that exceed expected length boundaries. Establish baseline router availability metrics to trigger alerts on unexpected downtime.

Why prioritize this

Despite the lack of KEV status, this vulnerability merits immediate priority due to its high CVSS score, complete lack of authentication barrier, and direct impact on network availability. Router compromise—even denial of service—can facilitate broader attacks if the device is also targeted for persistence or reconnaissance. Organizations using W20E as a critical path device should patch first.

Risk score, explained

The CVSS 3.1 score of 7.5 reflects a network-accessible buffer overflow with no authentication required (AV:N, PR:N) and high availability impact (A:H). Attack complexity is low (AC:L), and the attack does not require user interaction (UI:N). While no data confidentiality or integrity loss occurs, the denial of service impact and ease of exploitation elevate risk to HIGH. Scoring does not account for network perimeter protections that may limit attacker reach.

Frequently asked questions