CVE-2026-5102: Totolink A3300r Firmware RCE Vulnerability

CVE-2026-5102 Overview

A command injection vulnerability has been discovered in Totolink A3300R firmware version 17.0.0cu.557_b20221024. This vulnerability affects the setSmartQosCfg function within the /cgi-bin/cstecgi.cgi component (Parameter Handler). By manipulating the qos_up_bw argument, an attacker can inject arbitrary commands that execute on the underlying operating system. The attack can be executed remotely by authenticated users, and public exploit information has been disclosed.

Critical Impact

Remote attackers with low privileges can execute arbitrary commands on affected Totolink A3300R routers, potentially leading to complete device compromise, network reconnaissance, and lateral movement within the network.

Affected Products

• Totolink A3300R Firmware version 17.0.0cu.557_b20221024
• Totolink A3300R hardware device

Discovery Timeline

• 2026-03-30 - CVE-2026-5102 published to NVD
• 2026-03-30 - Last updated in NVD database

Technical Details for CVE-2026-5102

Vulnerability Analysis

This command injection vulnerability (CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component) exists in the web management interface of the Totolink A3300R router. The vulnerable endpoint /cgi-bin/cstecgi.cgi processes QoS (Quality of Service) configuration requests through the setSmartQosCfg function without properly sanitizing user-supplied input.

When the qos_up_bw parameter is processed, the application fails to validate or escape special characters before passing the value to system-level commands. This allows an attacker to break out of the intended command context and inject additional commands that execute with the privileges of the web server process, typically running as root on embedded devices.

The vulnerability can be exploited remotely over the network, requires low-level authentication (standard user access), and does not require user interaction. Successful exploitation can result in limited confidentiality, integrity, and availability impacts on the device.

Root Cause

The root cause of this vulnerability is insufficient input validation in the setSmartQosCfg function. The qos_up_bw parameter accepts user-controlled data that is directly concatenated into shell commands without proper sanitization or escaping. This classic command injection pattern allows metacharacters such as semicolons, pipes, and backticks to be interpreted by the shell, enabling arbitrary command execution.

Attack Vector

The attack is network-based and targets the router's web management interface at /cgi-bin/cstecgi.cgi. An authenticated attacker sends a crafted HTTP request to the setSmartQosCfg endpoint with a malicious payload in the qos_up_bw parameter. The injected commands execute in the context of the web server process, which typically has elevated privileges on embedded router devices.

For technical details and proof-of-concept information, refer to the GitHub Vulnerability Database Entry and the VulDB #354127 advisory.

Detection Methods for CVE-2026-5102

Indicators of Compromise

• Unusual HTTP POST requests to /cgi-bin/cstecgi.cgi containing shell metacharacters (;, |, `, $()) in the qos_up_bw parameter
• Unexpected outbound network connections from the router to unknown external hosts
• Modified system files or configuration changes on the router that were not performed by administrators
• Presence of unauthorized processes or unexpected command execution in router logs

Detection Strategies

• Implement network intrusion detection rules to identify HTTP requests containing command injection payloads targeting /cgi-bin/cstecgi.cgi
• Monitor router management interface access logs for requests with suspicious parameter values
• Deploy web application firewall (WAF) rules to block requests containing shell metacharacters in QoS configuration parameters
• Analyze network traffic for signs of command-and-control communication originating from router IP addresses

Monitoring Recommendations

• Enable and centralize logging from all Totolink router management interfaces
• Configure alerts for failed and successful authentication attempts to the router web interface
• Monitor for unusual DNS queries or network traffic patterns originating from router devices
• Implement regular configuration integrity checks on affected devices

How to Mitigate CVE-2026-5102

Immediate Actions Required

• Restrict access to the router's web management interface to trusted internal networks only
• Disable remote management if not absolutely required for operations
• Implement strong access controls and authentication for all users with management access
• Place affected routers behind a firewall that blocks external access to the management interface
• Review router logs for evidence of exploitation attempts

Patch Information

At the time of publication, no vendor patch information is available from Totolink. Monitor the Totolink Security Resources page for firmware updates. Organizations should contact Totolink support directly for remediation guidance or consider replacing affected devices with supported alternatives if no patch is forthcoming.

Workarounds

• Restrict management interface access by binding it to trusted internal network interfaces only
• Implement network segmentation to isolate router management traffic from general user traffic
• Use firewall rules to limit which IP addresses can access the /cgi-bin/cstecgi.cgi endpoint
• Consider deploying an additional security layer such as a reverse proxy with input validation in front of the management interface
• If the device supports it, disable the Smart QoS feature entirely until a patch is available

# Example firewall rule to restrict management access (adjust for your environment)
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP