CVE-2026-24110: Tenda W20e Firmware Buffer Overflow Flaw

CVE-2026-24110 Overview

A critical buffer overflow vulnerability has been discovered in Tenda W20E V4.0br_V15.11.0.6 router firmware. The vulnerability exists in the addDhcpRule function where attackers can send overly long addDhcpRules data that, when processed by the sscanf function, leads to buffer overflows due to the lack of proper size validation. This vulnerability allows remote attackers to potentially execute arbitrary code or cause denial of service on affected devices without any authentication.

Critical Impact

Remote attackers can exploit this buffer overflow vulnerability to compromise Tenda W20E routers, potentially gaining full control of the device or disrupting network services without requiring authentication.

Affected Products

• Tenda W20E Firmware version 15.11.0.6
• Tenda W20E Hardware version 4.0
• Tenda W20E V4.0br_V15.11.0.6

Discovery Timeline

• 2026-03-02 - CVE-2026-24110 published to NVD
• 2026-03-03 - Last updated in NVD database

Technical Details for CVE-2026-24110

Vulnerability Analysis

This vulnerability is classified as CWE-120 (Buffer Copy without Checking Size of Input), a classic buffer overflow flaw. The root cause lies in the improper handling of user-supplied input in the DHCP rules processing functionality. When the addDhcpRule function receives data, it uses the sscanf function to parse the input without validating the size of the destination buffers.

The vulnerable code processes DHCP rule data using the format string " %d\t%[^\t]\t%[^\n\r\t]", extracting values into three variables: dhcpsIndex, dhcpsIP, and dhcpsMac. Since no bounds checking is performed before the sscanf operation, an attacker can provide oversized data that overflows the stack-based buffers allocated for these variables.

Root Cause

The vulnerability stems from the use of unsafe string parsing via sscanf without implementing proper input length validation. The function addDhcpRule accepts user-controlled data and directly processes it using:

ret = sscanf(pRule, " %d\t%[^\t]\t%[^\n\r\t]", &dhcpsIndex, dhcpsIP, dhcpsMac);

The %[^\t] and %[^\n\r\t] format specifiers allow unbounded string input to be written into fixed-size buffers (dhcpsIP and dhcpsMac), resulting in a stack buffer overflow when maliciously crafted oversized input is provided.

Attack Vector

The vulnerability is exploitable over the network without authentication. An attacker can craft malicious HTTP requests to the router's web management interface, sending specially crafted addDhcpRules data with oversized values. When the router processes these rules, the buffer overflow occurs, potentially allowing the attacker to overwrite adjacent stack memory, including return addresses, enabling code execution or causing a denial of service condition.

The attack can be carried out by sending oversized strings in the DHCP rule parameters, specifically targeting the IP address and MAC address fields. Since these devices are often exposed on local networks and sometimes inadvertently on the internet, the attack surface is significant. For detailed technical analysis, refer to the GitHub CVE-2026-24110 Report.

Detection Methods for CVE-2026-24110

Indicators of Compromise

• Unexpected device reboots or crashes of Tenda W20E routers
• Anomalous HTTP POST requests to DHCP configuration endpoints containing unusually long strings
• Memory corruption errors or unexpected behavior in router logs
• Unauthorized configuration changes to DHCP settings

Detection Strategies

• Monitor network traffic for HTTP requests to Tenda W20E administrative interfaces containing oversized payloads in DHCP-related parameters
• Implement intrusion detection rules to flag requests with abnormally long strings in addDhcpRules parameters
• Deploy network segmentation to isolate IoT and network infrastructure devices from untrusted networks
• Use anomaly detection to identify unusual patterns of requests targeting router management interfaces

Monitoring Recommendations

• Enable logging on Tenda W20E devices and forward logs to a centralized SIEM solution
• Set up alerts for repeated authentication failures or unusual administrative access patterns
• Monitor for unexpected outbound connections from the router that could indicate compromise
• Regularly audit device configurations for unauthorized changes

How to Mitigate CVE-2026-24110

Immediate Actions Required

• Restrict access to the Tenda W20E web management interface to trusted networks only
• Implement firewall rules to block external access to router administrative interfaces
• Segment network infrastructure devices from user and guest networks
• Monitor for exploitation attempts while awaiting a vendor patch

Patch Information

As of the last update on 2026-03-03, users should check the Tenda Security Material page for firmware updates addressing this vulnerability. Contact Tenda support for the latest security patches for the W20E firmware version 15.11.0.6.

Workarounds

• Disable remote web management if not required
• Use strong, unique credentials for router administration
• Place the router behind an additional firewall that filters malicious requests
• Consider replacing vulnerable devices if no patch becomes available

# Example: Restrict management interface access via firewall rules
# Block external access to Tenda W20E management ports
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP