CVE-2026-36823: Tenda W20E Buffer Overflow Denial of Service Vulnerability

Technical summary

CVE-2026-36823 is a stack-based buffer overflow (CWE-121) in the formAddWebAuthUser function of Tenda W20E v15.11.0.6. The webAuthUserInfo parameter fails to validate input length before copying data into a fixed-size buffer. An attacker can construct an HTTP request with an oversized webAuthUserInfo value to overwrite stack memory, triggering a crash and service denial. The attack is network-accessible, requires no credentials, and has no user interaction requirement. While the CVSS vector indicates no confidentiality or integrity impact, the stack corruption could theoretically enable code execution depending on memory layout and compiler specifics—however, the disclosed impact is limited to availability.

Business impact

Affected routers will become unavailable during and after the attack, disrupting network connectivity for all users relying on the device. For small office or home office environments, this can cause significant downtime. For managed service providers or enterprises with these devices in their infrastructure, a coordinated attack could affect multiple customer sites simultaneously. Recovery requires manual intervention (reboot), and repeated attacks could create operational burden and potential SLA violations.

Affected systems

The vulnerability affects Tenda W20E routers running firmware version 15.11.0.6. Verify your installed firmware version via the router's administration interface or check device labels. The W20E is a cost-effective dual-band router commonly deployed in SMB and residential networks. Tenda has not publicly disclosed whether earlier or later firmware versions are affected; check Tenda's security advisory for the complete scope and patch availability.

Exploitability

This vulnerability is highly exploitable. No authentication, special privileges, or user interaction is required—any attacker on the network or internet can trigger it by sending a single malformed HTTP request to the router's web interface (typically port 80 or 8080). Exploit development is straightforward for someone with basic HTTP fuzzing knowledge. The lack of authentication and simple trigger mechanism mean this vulnerability poses an immediate risk and should be treated as remotely exploitable without prerequisites.

Remediation

Firmware update is the primary remediation. Contact Tenda or check their support portal for a patched firmware version addressing CVE-2026-36823. In the interim, restrict access to the router's web interface using firewall rules—block HTTP/HTTPS traffic to the router from untrusted networks, and allow access only from trusted administrative sources. If the device supports it, disable remote management features that expose the web interface to the internet.

Patch guidance

Check Tenda's official security advisory and firmware download page for a patched version of W20E firmware. Download the correct file for your hardware revision and follow Tenda's documented upgrade procedure (usually via the router's web admin panel). Test the patch in a non-production environment if possible. After upgrading, verify the new firmware version and confirm the router remains accessible to authorized users. Document the patch date for compliance records.

Detection guidance

Monitor your network for HTTP requests to the router's web interface containing unusually long or malformed webAuthUserInfo parameter values. Watch for sudden restarts or loss of connectivity to W20E devices, which may indicate a crash. Enable logging on the router if available. Implement network intrusion detection signatures that flag oversized parameters in formAddWebAuthUser requests. Check router uptime and log files for unexpected resets correlated with external traffic spikes.

Why prioritize this

This vulnerability merits immediate attention due to its combination of high CVSS score (7.5), unauthenticated remote exploitability, and straightforward attack mechanism. Although impact is limited to denial of service rather than data breach, the ease of exploitation and broad network access requirements mean attackers can disrupt operations at scale with minimal effort. Organizations should prioritize patching within the same business cycle or accelerate if these routers are internet-exposed or in critical network paths.

Risk score, explained

The CVSS 3.1 score of 7.5 (HIGH) reflects a network-accessible, unauthenticated, low-complexity attack (AV:N/AC:L/PR:N/UI:N) with complete availability impact (A:H) but no confidentiality or integrity compromise. This is appropriate for a remotely triggerable denial-of-service flaw with no prerequisites. The absence from the CISA Known Exploited Vulnerabilities (KEV) catalog does not imply lower risk—it indicates the vulnerability has not yet been observed in active, widespread exploitation, but given its simplicity and network exposure, it should be monitored closely.

Frequently asked questions