CVE-2026-36820 Overview
CVE-2026-36820 is a stack-based buffer overflow [CWE-121] in the Shenzhen Tenda Technology Tenda W20E router running firmware version v15.11.0.6. The flaw resides in the formAddWebAuthWhiteUser function, which fails to validate the length of the webAuthWhiteUserInfo parameter supplied through HTTP requests. Remote attackers can send a crafted HTTP request to trigger the overflow and cause a Denial of Service (DoS) condition. The vulnerability requires no authentication and no user interaction, making it reachable across the network.
Critical Impact
Unauthenticated remote attackers can crash the Tenda W20E router and disrupt network connectivity by sending a single malformed HTTP request.
Affected Products
- Shenzhen Tenda Technology Co., Ltd Tenda W20E
- Firmware version v15.11.0.6
- formAddWebAuthWhiteUser web management handler
Discovery Timeline
- 2026-06-09 - CVE CVE-2026-36820 published to NVD
- 2026-06-09 - Last updated in NVD database
Technical Details for CVE-2026-36820
Vulnerability Analysis
The vulnerability is a stack-based buffer overflow in the embedded web management interface of the Tenda W20E router. The formAddWebAuthWhiteUser function processes user input from the webAuthWhiteUserInfo HTTP parameter without enforcing length bounds. When the supplied value exceeds the fixed-size destination buffer on the stack, adjacent memory is corrupted and the process aborts. The result is a Denial of Service that interrupts routing, DHCP, and web administration services until the device reboots.
The CWE-121 classification confirms a classic stack-based buffer overflow pattern common in MIPS-based small office/home office (SOHO) routers. Attackers can repeatedly trigger the crash to maintain a persistent denial of service against the device.
Root Cause
The root cause is missing input validation in the formAddWebAuthWhiteUser handler. The function copies attacker-controlled data from webAuthWhiteUserInfo into a fixed-length stack buffer using an unsafe string operation. No bounds check, length truncation, or sanitization is performed before the copy.
Attack Vector
The attack vector is network-based and requires no authentication or user interaction. An attacker sends a single crafted HTTP request to the router's web management interface with an oversized webAuthWhiteUserInfo parameter. The malformed request triggers the overflow, corrupts the stack frame, and crashes the responsible service. Proof-of-concept material is published in the GitHub PoC Repository.
No verified exploit code is reproduced here. Refer to the linked repository for technical details on the parameter layout and crash conditions.
Detection Methods for CVE-2026-36820
Indicators of Compromise
- HTTP POST requests targeting the formAddWebAuthWhiteUser endpoint on the router's management interface.
- Requests containing abnormally long values in the webAuthWhiteUserInfo parameter.
- Unexpected reboots, watchdog resets, or service restarts on the Tenda W20E device.
- Loss of connectivity for clients behind the router coinciding with inbound management traffic.
Detection Strategies
- Inspect HTTP traffic to the router for parameter values exceeding reasonable string lengths, particularly fields ending in UserInfo.
- Deploy network intrusion detection signatures that flag oversized POST bodies directed at the router's web administration port.
- Correlate router crash events with preceding inbound HTTP requests from external or untrusted internal sources.
Monitoring Recommendations
- Forward router syslog and crash logs to a centralized logging platform for review.
- Alert on repeated TCP resets or service unavailability against the management interface.
- Monitor for unsolicited WAN-side connections to the router's HTTP administration port.
How to Mitigate CVE-2026-36820
Immediate Actions Required
- Disable remote WAN-side access to the Tenda W20E web management interface.
- Restrict LAN-side access to the administration interface to a trusted management VLAN or specific host addresses.
- Place the affected device behind a network filter that blocks oversized HTTP parameters until a vendor patch is available.
Patch Information
No vendor advisory or patched firmware release is referenced in the NVD entry at the time of publication. Operators should monitor Shenzhen Tenda Technology security communications for an updated firmware build that addresses the formAddWebAuthWhiteUser input validation defect.
Workarounds
- Block inbound HTTP and HTTPS traffic to the router's management port from untrusted networks.
- Apply an upstream web application firewall rule that drops requests where webAuthWhiteUserInfo exceeds a conservative length threshold.
- Segment the router so only administrative hosts can reach the management URL paths containing formAddWebAuthWhiteUser.
- Replace end-of-life or unpatched W20E units with supported hardware if no firmware fix is issued.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
