CVE-2026-90688 Overview
CVE-2026-90688 is a stack-based buffer overflow vulnerability in the Tenda W20E router running firmware version 15.11.0.61068_1546_841_CN_TDC. The flaw resides in the formIPMacBindAdd function within the HTTP request handler. Attackers can trigger the overflow by manipulating the IPMacBindRule argument over the network. The vulnerability is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer) and can be exploited remotely with low privileges. Successful exploitation impacts device availability and may enable further compromise of the router.
Critical Impact
Remote attackers with low privileges can overflow a stack buffer in the Tenda W20E HTTP handler, causing denial of service and potentially enabling arbitrary code execution on the router.
Affected Products
- Tenda W20E router
- Firmware version 15.11.0.61068_1546_841_CN_TDC
- HTTP Handler component exposing the formIPMacBindAdd endpoint
Discovery Timeline
- 2026-09-14 - CVE-2026-90688 published to NVD
- 2026-09-15 - Last updated in NVD database
Technical Details for CVE-2026-90688
Vulnerability Analysis
The vulnerability affects the formIPMacBindAdd function inside the Tenda W20E HTTP handler. This function processes IP-to-MAC binding rules submitted through the router's web management interface. The handler reads the IPMacBindRule parameter from an HTTP request and copies it into a fixed-size stack buffer without validating its length. An attacker on the network who can reach the management interface can submit an oversized IPMacBindRule value, corrupting adjacent stack memory including saved registers and return addresses. The condition is remotely triggerable and requires only low-privilege access to the web interface.
Root Cause
The root cause is the absence of bounds checking on user-supplied input before it is copied into a stack-allocated buffer. This maps to [CWE-119]: Improper Restriction of Operations within the Bounds of a Memory Buffer. Embedded router firmware commonly uses fixed-size buffers with unsafe string operations such as strcpy or sprintf, and the formIPMacBindAdd handler follows this pattern.
Attack Vector
Exploitation requires network access to the router's HTTP management interface and a low-privileged session. The attacker submits a crafted POST request to the endpoint invoking formIPMacBindAdd, placing an oversized string in the IPMacBindRule parameter. The overflow first causes the HTTP daemon to crash, resulting in denial of service on the management plane. On MIPS-based Tenda firmware without stack protections, a carefully constructed payload may overwrite the return address and lead to code execution in the context of the web server process.
A proof-of-concept and technical writeup are documented in the GitHub advisory for Tenda W20E formIPMacBindAdd and the VulDB entry for CVE-2026-90688.
Detection Methods for CVE-2026-90688
Indicators of Compromise
- Unexpected crashes or restarts of the Tenda W20E HTTP daemon (httpd) on the router
- HTTP POST requests to management endpoints containing abnormally long IPMacBindRule parameter values
- Loss of connectivity to the router's web management interface following configuration requests
- Unrecognized IP-MAC binding entries appearing in the router configuration
Detection Strategies
- Inspect network traffic to router management interfaces for POST requests carrying IPMacBindRule payloads that exceed expected length limits
- Deploy intrusion detection signatures that flag requests to formIPMacBindAdd containing oversized argument values
- Correlate router reboots or web interface outages with preceding administrative HTTP requests
Monitoring Recommendations
- Log all administrative access attempts to router web interfaces and alert on failed or anomalous authentication events
- Restrict router management interface exposure to trusted management VLANs and monitor for lateral access attempts
- Track firmware version inventory across deployed Tenda W20E devices to identify unpatched units
How to Mitigate CVE-2026-90688
Immediate Actions Required
- Disable remote WAN-side administration on affected Tenda W20E routers and restrict LAN-side management to trusted hosts
- Change default and low-privilege web interface credentials to reduce the exploitable authenticated attack surface
- Segment the router management interface onto an isolated administrative network
- Monitor the Tenda official website for firmware updates addressing this issue
Patch Information
At the time of NVD publication on 2026-09-14, no vendor patch is referenced for firmware 15.11.0.61068_1546_841_CN_TDC. Administrators should track the VulDB vulnerability record #403221 and the Tenda official website for firmware releases. If a fixed firmware version becomes available, apply it to all affected W20E devices as the primary remediation.
Workarounds
- Block external access to the router's HTTP management port using upstream firewall rules
- Enforce access control lists that permit management requests only from designated administrator IP addresses
- Where feasible, replace end-of-support consumer routers with hardware receiving active security maintenance
- Deploy network-level filtering to drop HTTP requests containing oversized parameter values destined for router management interfaces
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
