CVE-2026-36823 Overview
CVE-2026-36823 is a stack-based buffer overflow [CWE-121] in the Tenda W20E router firmware version v15.11.0.6. The flaw resides in the formAddWebAuthUser function, which processes the webAuthUserInfo parameter without sufficient bounds checking. An unauthenticated remote attacker can send a crafted HTTP request to trigger the overflow and cause a denial of service condition on the device.
Critical Impact
Remote, unauthenticated attackers can crash the Tenda W20E router by sending a single malformed HTTP request, disrupting network availability for all connected clients.
Affected Products
- Shenzhen Tenda Technology Co., Ltd Tenda W20E
- Firmware version v15.11.0.6
- Web management interface exposing formAddWebAuthUser
Discovery Timeline
- 2026-06-09 - CVE-2026-36823 published to NVD
- 2026-06-09 - Last updated in NVD database
Technical Details for CVE-2026-36823
Vulnerability Analysis
The vulnerability exists within the formAddWebAuthUser handler in the Tenda W20E web management binary. This handler is invoked when an HTTP request is made to add a web authentication user to the router's portal configuration. The function reads the webAuthUserInfo parameter from the request body and copies it into a fixed-size stack buffer.
Because the copy operation does not validate the length of the attacker-controlled input, oversized data overwrites adjacent stack memory. The overflow corrupts the saved return address and surrounding stack frame, causing the HTTPD process to crash. The router then enters a degraded state or reboots, breaking management and routed connectivity until service is restored.
The issue is categorized under [CWE-121] Stack-based Buffer Overflow. The current EPSS probability stands at 0.254%, placing the CVE in the 48.96th percentile for likelihood of near-term exploitation activity.
Root Cause
The root cause is missing input length validation in the formAddWebAuthUser function. The handler trusts the size of the webAuthUserInfo form parameter supplied through the HTTP interface. No strncpy, length check, or boundary enforcement guards the destination buffer on the stack.
Attack Vector
Exploitation requires only network reachability to the router's web management interface. The attacker issues a single HTTP POST containing an oversized webAuthUserInfo value to the endpoint serviced by formAddWebAuthUser. No authentication, user interaction, or elevated privileges are required.
A proof-of-concept demonstrating the crash is published in the SemVulLLM GitHub repository. The PoC submits a long string in the vulnerable parameter, which is sufficient to terminate the HTTPD process and produce a denial of service.
Detection Methods for CVE-2026-36823
Indicators of Compromise
- HTTP POST requests targeting the Tenda W20E management interface containing abnormally large webAuthUserInfo parameter values.
- Unexpected HTTPD process crashes, watchdog-triggered reboots, or loss of management plane reachability on Tenda W20E devices.
- Repeated short-lived TCP connections to the router's management port followed by service unavailability.
Detection Strategies
- Inspect HTTP request bodies destined for Tenda W20E routers and alert when the webAuthUserInfo parameter exceeds a reasonable length threshold.
- Correlate router availability loss with preceding inbound HTTP traffic to identify externally driven DoS attempts.
- Apply signatures matching the PoC payload pattern from the public repository against perimeter and east-west traffic.
Monitoring Recommendations
- Enable SNMP or syslog forwarding from Tenda devices to a central log platform and alert on repeated HTTPD restarts.
- Track inbound connections to router management ports from non-administrative source IPs and flag anomalies.
- Baseline normal request sizes against the router web interface and alert on statistical deviations.
How to Mitigate CVE-2026-36823
Immediate Actions Required
- Restrict access to the Tenda W20E web management interface to trusted management VLANs or hosts using ACLs.
- Disable remote WAN-side management on affected routers until a vendor fix is available.
- Inventory all Tenda W20E devices running firmware v15.11.0.6 and prioritize them for replacement or isolation.
Patch Information
No vendor patch is referenced in the published CVE data at this time. Monitor the Shenzhen Tenda Technology security advisories for firmware updates addressing the formAddWebAuthUser function. Until a fix is released, treat the device as exposed to unauthenticated network DoS.
Workarounds
- Place the router's management interface behind a firewall rule that permits only specific administrator source addresses.
- Deploy an intermediate web application firewall or reverse proxy that enforces maximum length limits on POST parameters reaching the device.
- Where feasible, replace the Tenda W20E with a supported device that receives active security maintenance.
# Example ACL restricting management access to a trusted host
# (apply on upstream firewall, not on the vulnerable device itself)
iptables -A FORWARD -p tcp -d <ROUTER_IP> --dport 80 -s <ADMIN_IP> -j ACCEPT
iptables -A FORWARD -p tcp -d <ROUTER_IP> --dport 443 -s <ADMIN_IP> -j ACCEPT
iptables -A FORWARD -p tcp -d <ROUTER_IP> --dport 80 -j DROP
iptables -A FORWARD -p tcp -d <ROUTER_IP> --dport 443 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
