CVE-2026-4976: Totolink LR350 Buffer Overflow Vulnerability

CVE-2026-4976 Overview

A buffer overflow vulnerability has been identified in Totolink LR350 firmware version 9.3.5u.6369_B20220309. This vulnerability exists in the setWiFiGuestCfg function located within the /cgi-bin/cstecgi.cgi file. By manipulating the ssid argument, an attacker can trigger a buffer overflow condition that may lead to arbitrary code execution or denial of service on the affected device.

Critical Impact

Remote attackers with low-privilege access can exploit this buffer overflow vulnerability to compromise the confidentiality, integrity, and availability of the affected Totolink LR350 router, potentially gaining full control of the device.

Affected Products

• Totolink LR350 firmware version 9.3.5u.6369_B20220309
• Potentially other Totolink LR350 firmware versions with the vulnerable setWiFiGuestCfg function

Discovery Timeline

• 2026-03-27 - CVE-2026-4976 published to NVD
• 2026-03-30 - Last updated in NVD database

Technical Details for CVE-2026-4976

Vulnerability Analysis

This vulnerability (CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer) affects the WiFi guest configuration functionality on Totolink LR350 routers. The setWiFiGuestCfg function fails to properly validate the length of user-supplied input in the ssid parameter before copying it into a fixed-size buffer. This classic buffer overflow condition allows an attacker to overwrite adjacent memory locations, potentially corrupting program state, injecting malicious code, or causing the device to crash.

The vulnerability is particularly concerning because it can be exploited remotely over the network. An authenticated attacker with low-level privileges can craft a malicious HTTP request to the /cgi-bin/cstecgi.cgi endpoint with an oversized ssid value, triggering the overflow condition.

Root Cause

The root cause of this vulnerability lies in the improper handling of user-supplied input in the setWiFiGuestCfg function. The function does not perform adequate bounds checking on the ssid argument before processing it, allowing data to exceed the allocated buffer space. This is a common programming error in embedded device firmware where memory constraints often lead to fixed-size buffers without corresponding input validation.

Attack Vector

The attack can be performed remotely over the network by an authenticated user with low privileges. The attacker sends a crafted HTTP request to the CGI endpoint /cgi-bin/cstecgi.cgi, invoking the setWiFiGuestCfg function with a maliciously crafted ssid parameter containing more data than the destination buffer can accommodate. This causes memory corruption that could lead to arbitrary code execution, privilege escalation, or denial of service.

The vulnerability requires no user interaction and has low attack complexity, making it a significant threat in environments where the router's management interface is accessible. For detailed technical analysis, refer to the Notion Configuration Guide documenting this vulnerability.

Detection Methods for CVE-2026-4976

Indicators of Compromise

• Unexpected HTTP POST requests to /cgi-bin/cstecgi.cgi containing unusually long ssid parameter values
• Router crashes or spontaneous reboots that may indicate exploitation attempts
• Unusual network traffic patterns originating from or directed at the Totolink LR350 device
• Modified WiFi guest configuration settings that were not authorized by administrators

Detection Strategies

• Implement network intrusion detection rules to identify HTTP requests to /cgi-bin/cstecgi.cgi with ssid parameters exceeding normal length thresholds
• Monitor router system logs for signs of memory corruption, crashes, or unexpected process terminations
• Deploy web application firewall (WAF) rules to inspect and filter requests targeting the vulnerable CGI endpoint
• Use SentinelOne Singularity to monitor network traffic and detect anomalous behavior patterns associated with router exploitation

Monitoring Recommendations

• Enable verbose logging on the Totolink LR350 if available and review logs regularly for suspicious activity
• Segment the network to limit exposure of the router's management interface
• Implement baseline monitoring for normal router behavior to quickly identify deviations that may indicate compromise

How to Mitigate CVE-2026-4976

Immediate Actions Required

• Restrict network access to the router's management interface to trusted IP addresses only
• Disable remote management functionality if it is not required for operations
• Place the Totolink LR350 behind a properly configured firewall that filters malicious requests
• Monitor the TOTOLINK Official Website for firmware updates addressing this vulnerability

Patch Information

At the time of publication, no official patch has been confirmed by the vendor. Organizations should monitor Totolink's official channels for security updates. In the interim, apply the workarounds and compensating controls described below to reduce risk exposure.

Additional vulnerability details and tracking information can be found at VulDB ID #353863.

Workarounds

• Restrict access to the CGI interface by implementing IP-based access control lists (ACLs) on the router or upstream firewall
• Disable the WiFi guest network feature if it is not operationally required
• Consider replacing the affected device with a router from a vendor with a more responsive security patch cycle
• Implement network segmentation to isolate the vulnerable device from critical infrastructure

# Example firewall rule to restrict access to management interface
# Restrict access to Totolink LR350 management interface
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