CVE-2026-24115 Overview
CVE-2026-24115 is a buffer overflow vulnerability affecting the Tenda W20E router running firmware version V4.0br_V15.11.0.6. The flaw exists in the router's Quality of Service (QoS) handling logic, where the firmware fails to validate the sizes of the gstup and gstdwn parameters before concatenating them into the gstruleQos buffer. An unauthenticated remote attacker can exploit this flaw over the network to corrupt memory, crash the device, or potentially achieve arbitrary code execution. The vulnerability is tracked under CWE-120: Buffer Copy without Checking Size of Input.
Critical Impact
Unauthenticated network attackers can trigger memory corruption on affected Tenda W20E routers, enabling denial of service or potential remote code execution against perimeter network devices.
Affected Products
- Tenda W20E hardware revision 4.0
- Tenda W20E firmware version V4.0br_V15.11.0.6
- Tenda W20E firmware 15.11.0.6
Discovery Timeline
- 2026-03-02 - CVE-2026-24115 published to NVD
- 2026-03-03 - Last updated in NVD database
Technical Details for CVE-2026-24115
Vulnerability Analysis
The vulnerability resides in the QoS rule processing routine of the Tenda W20E firmware. The firmware accepts two attacker-controlled inputs, gstup (upstream bandwidth specification) and gstdwn (downstream bandwidth specification), and concatenates them into a fixed-size destination buffer named gstruleQos. The concatenation occurs without any prior length validation against the destination buffer capacity.
When the combined length of gstup and gstdwn exceeds the allocated size of gstruleQos, adjacent memory is overwritten. This corruption can clobber stack frames, function return addresses, or heap metadata depending on where gstruleQos is allocated. The flaw is exploitable over the network without authentication or user interaction.
Root Cause
The root cause is missing bounds checking before a string concatenation operation, classified as CWE-120: Buffer Copy without Checking Size of Input. The firmware uses an unsafe copy primitive without validating attacker-supplied input length, a common pattern in embedded router firmware written in C.
Attack Vector
An attacker sends a crafted HTTP request to the router's management interface containing oversized gstup and/or gstdwn parameters. The router's web service processes the request and invokes the vulnerable QoS handler, triggering the overflow. Because the management interface is often exposed to the local network and sometimes to the WAN, the attack surface includes both internal threat actors and remote adversaries when remote administration is enabled.
Technical details and proof-of-concept material are documented in the GitHub PoC Repository and the Tenda Security Advisory.
Detection Methods for CVE-2026-24115
Indicators of Compromise
- HTTP POST requests to the Tenda W20E management interface containing abnormally long gstup or gstdwn parameter values
- Unexpected reboots, web service crashes, or watchdog resets on affected W20E devices
- Anomalous outbound traffic from the router following malformed QoS configuration requests
Detection Strategies
- Inspect inbound HTTP traffic to router management interfaces for QoS configuration requests containing parameters exceeding expected byte lengths
- Deploy network intrusion detection signatures that match oversized gstup and gstdwn field values in Tenda administrative traffic
- Correlate router availability events with preceding administrative HTTP requests to identify exploitation attempts
Monitoring Recommendations
- Forward router syslog and management plane logs to a centralized SIEM for retention and analysis
- Alert on repeated connection failures or service restarts on edge network devices
- Monitor for management interface access originating from untrusted network segments or the WAN interface
How to Mitigate CVE-2026-24115
Immediate Actions Required
- Restrict administrative access to the Tenda W20E web interface to trusted management VLANs only
- Disable remote (WAN-side) administration on affected devices until a patched firmware is applied
- Inventory all Tenda W20E devices running firmware V15.11.0.6 and prioritize them for remediation
- Place affected routers behind an upstream filtering device that can drop malformed administrative requests
Patch Information
Consult the Tenda Security Advisory for vendor guidance and firmware updates. At publication time, administrators should apply any firmware release that supersedes V4.0br_V15.11.0.6 and addresses the QoS parameter validation flaw.
Workarounds
- Block external access to the router's HTTP/HTTPS management ports at the network perimeter
- Enforce strong administrative credentials and segment management traffic from user networks
- Replace end-of-life or unpatched Tenda W20E units with supported network hardware where firmware updates are unavailable
# Configuration example: restrict management access via upstream firewall (iptables)
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 -s <trusted_mgmt_subnet> -j ACCEPT
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 -j DROP
iptables -A FORWARD -p tcp -d <router_ip> --dport 443 -s <trusted_mgmt_subnet> -j ACCEPT
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.
