CVE-2026-36818: Tenda W20E Buffer Overflow DoS Vulnerability

CVE-2026-36818 Overview

CVE-2026-36818 is a buffer overflow vulnerability affecting Shenzhen Tenda Technology Co., Ltd Tenda W20E routers running firmware version v15.11.0.6. The flaw resides in the wewifiWhiteUserInfo parameter handled by the formAddWewifiWhiteUser function. A remote unauthenticated attacker can send a crafted HTTP request to trigger memory corruption. Successful exploitation causes a Denial of Service (DoS) on the affected device. The vulnerability is classified under [CWE-120] Buffer Copy without Checking Size of Input.

Critical Impact

A remote attacker can crash the Tenda W20E router without authentication, disrupting network connectivity for all downstream clients.

Affected Products

• Shenzhen Tenda Technology Co., Ltd Tenda W20E
• Firmware version v15.11.0.6
• Web management interface exposing formAddWewifiWhiteUser

Discovery Timeline

• 2026-06-09 - CVE-2026-36818 published to the National Vulnerability Database
• 2026-06-10 - Last updated in NVD database

Technical Details for CVE-2026-36818

Vulnerability Analysis

The Tenda W20E firmware exposes a web management endpoint that processes the formAddWewifiWhiteUser function. This function accepts the wewifiWhiteUserInfo parameter from HTTP requests submitted to the router's administrative interface. The parameter is copied into a fixed-size stack or heap buffer without sufficient bounds checking. An attacker supplying an oversized value overruns the destination buffer and corrupts adjacent memory. The corruption disrupts the httpd process serving the management interface, causing the router to crash or reboot.

Because the attack vector is network-based and requires no authentication or user interaction, any host able to reach the management interface can trigger the condition. The vulnerability impacts availability only, with no confidentiality or integrity loss reported.

Root Cause

The root cause is unsafe handling of attacker-controlled input in the formAddWewifiWhiteUser handler. The function performs a copy operation on wewifiWhiteUserInfo without validating the input length against the destination buffer size. This is a textbook [CWE-120] classic buffer overflow pattern common in embedded router firmware built around the goahead or similar lightweight web server stacks.

Attack Vector

Exploitation requires sending a single crafted HTTP POST request to the router's web management endpoint containing an oversized wewifiWhiteUserInfo value. No credentials are required. The Exploit Prediction Scoring System (EPSS) score is 0.254% with a percentile of 48.99, indicating low but non-negligible exploitation likelihood.

A proof-of-concept demonstrating the crash condition is published in a public GitHub PoC Repository. The vulnerability mechanism is described in prose because no vendor-validated code example is available. Refer to the linked repository for technical reproduction details.

Detection Methods for CVE-2026-36818

Indicators of Compromise

• Unexpected reboots or crashes of the Tenda W20E router coinciding with HTTP traffic to the management interface
• HTTP POST requests targeting endpoints invoking formAddWewifiWhiteUser with abnormally large wewifiWhiteUserInfo values
• Loss of LAN or WAN connectivity following inbound traffic from untrusted source IP addresses

Detection Strategies

• Inspect HTTP request bodies at the network perimeter for oversized parameter values matching wewifiWhiteUserInfo
• Deploy intrusion detection signatures that flag requests to formAddWewifiWhiteUser exceeding expected length thresholds
• Correlate router availability events with inbound HTTP traffic patterns to identify DoS attempts

Monitoring Recommendations

• Log all access attempts to the router's web administration interface and alert on repeated failures or crashes
• Monitor SNMP or syslog feeds from the router for httpd process restarts
• Track external scanning activity targeting TCP ports used by the Tenda management interface

How to Mitigate CVE-2026-36818

Immediate Actions Required

• Restrict access to the router's web management interface to trusted LAN hosts only and block WAN-side administration
• Place vulnerable devices behind a network segment that filters untrusted HTTP traffic
• Audit inventory for Tenda W20E devices running firmware v15.11.0.6 and prioritize their isolation

Patch Information

No vendor patch is currently referenced in the NVD entry for CVE-2026-36818. Organizations should monitor the Tenda support portal for firmware updates addressing the formAddWewifiWhiteUser buffer overflow. Until a fix is released, compensating controls are the only available remediation.

Workarounds

• Disable remote management on the WAN interface to prevent internet-based exploitation
• Apply access control lists restricting management interface access to specific administrator IP addresses
• Replace end-of-support Tenda W20E devices with currently supported router models if no patch is forthcoming
• Segment the router's management VLAN from general user traffic to reduce internal attack surface

# Configuration example: block external access to router management interface
iptables -A INPUT -i wan0 -p tcp --dport 80 -j DROP
iptables -A INPUT -i wan0 -p tcp --dport 443 -j DROP
iptables -A INPUT -s 192.168.1.0/24 -p tcp --dport 80 -j ACCEPT