CVE-2026-11523: Tenda W20E Buffer Overflow Vulnerability

CVE-2026-11523 Overview

CVE-2026-11523 is a stack-based buffer overflow vulnerability in the Tenda W20E router running firmware version 15.11.0.6. The flaw resides in the formPortalAuth function within the /goform/PortalAuth endpoint of the Web Management Interface. Attackers manipulate the gotoUrl argument to overflow a fixed-size stack buffer, corrupting adjacent memory and execution flow. The issue is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer). A public exploit has been disclosed, and the attack is remotely launchable over the network.

Critical Impact

Remote attackers with low-privileged access can corrupt router memory through the gotoUrl parameter, leading to denial of service or arbitrary code execution on affected Tenda W20E devices.

Affected Products

• Tenda W20E router
• Firmware version 15.11.0.6
• Web Management Interface component (/goform/PortalAuth)

Discovery Timeline

• 2026-06-08 - CVE-2026-11523 published to NVD
• 2026-06-09 - Last updated in NVD database

Technical Details for CVE-2026-11523

Vulnerability Analysis

The vulnerability exists in the formPortalAuth handler exposed by the embedded HTTP server on the Tenda W20E router. The handler processes the gotoUrl HTTP parameter without enforcing length boundaries before copying it into a fixed-size stack buffer. Once the supplied input exceeds the destination buffer size, adjacent stack memory is overwritten, including saved register values and the return address.

Because the routine runs within the device's web management daemon, successful corruption can crash the service or redirect execution. The exploit has been published, raising the practical risk for unpatched devices exposed on local or wide-area networks. The EPSS probability sits at 0.088% with a percentile of 25.148, reflecting limited observed scanning activity at this time.

Root Cause

The root cause is improper bounds checking when the gotoUrl query argument is copied into a stack-allocated buffer inside formPortalAuth. The code path lacks length validation or use of size-limited copy primitives, a recurring pattern in Tenda CGI handlers parsing user-controlled HTTP arguments.

Attack Vector

Attackers send a crafted HTTP request to /goform/PortalAuth containing an oversized gotoUrl value. The request can originate from any network position with reachability to the router's management interface, including the LAN side by default and the WAN side when remote management is enabled. Authentication requirements are low, consistent with the published CVSS vector reflecting PR:L.

No synthetic exploit code is reproduced here. Refer to the GitHub Security Report and VulDB CVE-2026-11523 for technical proof-of-concept details.

Detection Methods for CVE-2026-11523

Indicators of Compromise

• HTTP POST or GET requests to /goform/PortalAuth containing abnormally long gotoUrl parameter values.
• Unexpected crashes, reboots, or restarts of the httpd web management process on Tenda W20E devices.
• Non-printable or shellcode-like byte sequences within the gotoUrl URL parameter.
• Outbound connections from the router to unknown hosts immediately following management interface requests.

Detection Strategies

• Inspect web access logs and network captures for requests to /goform/PortalAuth with gotoUrl values exceeding typical URL lengths (for example, over 256 bytes).
• Deploy network intrusion detection signatures that flag oversized parameters targeting Tenda goform endpoints.
• Monitor router health telemetry for repeated daemon restarts that correlate with inbound management traffic.

Monitoring Recommendations

• Restrict and log all access to the router's web management interface, alerting on access from non-administrative source addresses.
• Capture full HTTP request bodies at the network perimeter for IoT and SOHO devices to support retrospective analysis.
• Correlate router logs with broader network telemetry to identify post-exploitation lateral movement attempts.

How to Mitigate CVE-2026-11523

Immediate Actions Required

• Disable remote (WAN-side) web management on the Tenda W20E until a fixed firmware is available.
• Restrict LAN-side access to the management interface using ACLs or VLAN segmentation, allowing only trusted administrative hosts.
• Rotate administrator credentials, since the vulnerability requires low-privileged authentication that may be obtained through credential reuse.
• Audit network logs for prior exploitation attempts against /goform/PortalAuth.

Patch Information

At the time of publication, no vendor advisory or patched firmware release has been identified in the referenced sources. Monitor the Tenda Official Website and VulDB Vulnerability #369143 for updates and apply any released firmware as soon as it becomes available.

Workarounds

• Place the router behind a perimeter firewall that blocks inbound HTTP and HTTPS traffic to the device on WAN interfaces.
• Disable the captive portal or portal authentication feature if it is not in active use, reducing exposure of the formPortalAuth code path.
• Replace end-of-life or unsupported Tenda W20E devices with vendor-supported models that receive regular security updates.