CVE-2026-36821 Overview
CVE-2026-36821 is a stack-based buffer overflow [CWE-121] in the Tenda W20E router running firmware version v15.11.0.6. The flaw resides in the formCropAndSetWewifiPic function, where the picCropName parameter is copied into a fixed-size stack buffer without proper length validation. A remote, unauthenticated attacker can send a crafted HTTP request to corrupt the stack and crash the device, producing a Denial of Service (DoS) condition.
Critical Impact
Unauthenticated remote attackers can crash the Tenda W20E router over the network with a single malformed HTTP request, disrupting connectivity for all downstream clients.
Affected Products
- Shenzhen Tenda Technology Co., Ltd. Tenda W20E
- Firmware version v15.11.0.6
- Web management interface exposing the formCropAndSetWewifiPic handler
Discovery Timeline
- 2026-06-09 - CVE-2026-36821 published to NVD
- 2026-06-09 - Last updated in NVD database
Technical Details for CVE-2026-36821
Vulnerability Analysis
The Tenda W20E embedded web server exposes the formCropAndSetWewifiPic handler to process Wi-Fi captive portal image cropping requests. The function reads the user-supplied picCropName HTTP parameter and copies it into a stack-allocated buffer using an unbounded string copy. Because the handler does not verify the input length against the destination buffer size, attacker-controlled bytes overflow into adjacent stack memory, including saved registers and the return address. The result is process termination of httpd, which in turn renders the router unreachable until it is restarted.
This class of flaw is characteristic of consumer SOHO routers, where binary web handlers parse query parameters with strcpy-style primitives. The Exploit Prediction Scoring System (EPSS) ranks public exploitation likelihood for this issue in line with similar Tenda DoS bugs.
Root Cause
The root cause is missing bounds checking on the picCropName parameter inside formCropAndSetWewifiPic. The handler trusts the parameter length supplied by the client and writes directly into a fixed-size local buffer on the stack, satisfying the [CWE-121] stack-based buffer overflow pattern.
Attack Vector
Exploitation requires only network reachability to the router's HTTP management interface. An attacker submits an HTTP request to the vulnerable endpoint with an oversized picCropName value. No authentication, user interaction, or prior access is required, and the request can originate from the LAN side or, where remote management is enabled, from the WAN. A proof-of-concept request layout is documented in the GitHub PoC Repository. The current observed impact is denial of service through httpd crash; reliable code execution has not been demonstrated in public material.
Detection Methods for CVE-2026-36821
Indicators of Compromise
- Repeated crashes or unexpected restarts of the Tenda W20E httpd web management process.
- HTTP POST requests targeting formCropAndSetWewifiPic with abnormally long picCropName values.
- Loss of router management UI availability immediately following inbound HTTP traffic from an unfamiliar source.
Detection Strategies
- Inspect HTTP request bodies destined for the router for picCropName parameter lengths exceeding expected image filename sizes (for example, >64 bytes).
- Alert on bursts of HTTP requests to formCropAndSet* endpoints from a single source, which suggests fuzzing or DoS attempts.
- Correlate router reboots and management-plane outages with preceding HTTP traffic captured at the network edge.
Monitoring Recommendations
- Forward router syslog and SNMP trap data to a centralized log platform for crash and reboot analytics.
- Apply IDS/IPS signatures that flag oversized parameters against known Tenda form handlers.
- Track availability of management interfaces with synthetic HTTP probes to detect repeated outages.
How to Mitigate CVE-2026-36821
Immediate Actions Required
- Restrict access to the router's HTTP management interface to trusted management VLANs and disable WAN-side remote management.
- Place the Tenda W20E behind a network filter that blocks inbound requests to formCropAndSetWewifiPic from untrusted hosts.
- Inventory affected devices running firmware v15.11.0.6 and prioritize replacement or isolation where a vendor patch is unavailable.
Patch Information
No vendor advisory or fixed firmware version has been published for CVE-2026-36821 at the time of NVD publication. Monitor Shenzhen Tenda Technology's support channels for an updated W20E firmware release that addresses the formCropAndSetWewifiPic handler. Until a patch is issued, treat the device as exposed and apply compensating controls.
Workarounds
- Disable remote (WAN) management on the W20E to remove the primary unauthenticated network attack path.
- Segment the router's management interface onto a dedicated administrative network with ACLs that permit only authorized administrator IPs.
- Where feasible, replace the W20E with a supported device that receives current security maintenance.
# Example ACL concept: restrict inbound HTTP to the router management IP
# Adjust to your upstream firewall syntax
deny tcp any host <router_mgmt_ip> eq 80
deny tcp any host <router_mgmt_ip> eq 443
permit tcp <admin_subnet> host <router_mgmt_ip> eq 80
permit tcp <admin_subnet> host <router_mgmt_ip> eq 443
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
