CVE-2026-36819: Tenda W20E Buffer Overflow Denial of Service Vulnerability

Technical summary

CVE-2026-36819 is a stack-based buffer overflow (CWE-121) in the fromSetDhcpRules function of Tenda W20E firmware 15.11.0.6. The bindMACAddr parameter fails to validate input length before copying user-supplied data into a fixed-size buffer. An unauthenticated remote attacker can send a crafted HTTP request with an oversized bindMACAddr value, overflowing the buffer and causing the application to crash. The CVSS 3.1 score of 7.5 (HIGH) reflects high availability impact with no confidentiality or integrity exposure.

Business impact

Denial of service against Tenda W20E routers can disrupt network connectivity for affected organizations. In enterprise environments or branch offices relying on these devices, service interruption could block internet access, VoIP, or other critical network functions. Attack frequency could range from sporadic disruption to sustained DoS campaigns if the vulnerability becomes weaponized. Recovery requires manual reboot, creating operational overhead.

Affected systems

Tenda W20E routers running firmware version 15.11.0.6 are confirmed vulnerable. Organizations should verify whether they operate this specific model and firmware version. Tenda has not disclosed whether earlier or later firmware versions are affected; verification against Tenda's official advisory is essential before assuming other versions are safe or patched.

Exploitability

Exploitability is straightforward. The vulnerability requires no authentication, no user interaction, and no special configuration. An attacker needs only network reachability to the router's HTTP interface—typically the management port (usually 80 or 8080 by default). Crafting the malicious request is trivial once the vulnerable parameter and buffer size are known. The attack surface is broad if management interfaces are exposed to untrusted networks.

Remediation

Upgrade firmware to a patched version released by Tenda. Verify the exact patch version number against Tenda's official security advisory, as this summary does not specify which firmware build resolves the issue. If a patch is not yet available, implement network segmentation to restrict access to the router's management interface, limiting exposure to trusted administrative networks only.

Patch guidance

Contact Tenda support or visit their security advisories page to identify and download the fixed firmware version for W20E. Do not assume the next minor or major version is patched without confirmation. Before applying any firmware update, back up your router configuration. Test the update in a non-production environment if feasible. Schedule patching during a maintenance window to minimize user impact.

Detection guidance

Monitor for HTTP requests to your Tenda W20E router management interface containing unusually long or malformed bindMACAddr parameters. Log analysis of router HTTP access logs may reveal attack attempts with oversized parameter values. Network-based detection can flag HTTP requests with suspiciously large payload sizes to router management endpoints. Router crash or reboot without administrative action is a possible indicator of exploitation. Monitor router uptime and reboot logs for unexplained outages.

Why prioritize this

This vulnerability merits prompt attention because it affects network infrastructure, enables unauthenticated remote denial of service, and requires minimal exploit complexity. While not in the CISA KEV catalog, the combination of HIGH severity, remote exploitability, and lack of authentication requirements justifies prioritization above lower-severity flaws. Organizations operating W20E routers should patch on an expedited timeline.

Risk score, explained

The CVSS 3.1 score of 7.5 reflects a vulnerability with high availability impact (A:H) but no confidentiality or integrity compromise (C:N, I:N). The attack vector is network-based (AV:N), attack complexity is low (AC:L), and no privileges or user interaction are required (PR:N, UI:N). The scope is unchanged (S:U), meaning the impact is limited to the vulnerable component. This score appropriately balances the ease of exploitation and service disruption against the absence of data breach or system compromise risk.

Frequently asked questions