CVE-2026-5105: Totolink A3300r Firmware RCE Vulnerability

CVE-2026-5105 Overview

A command injection vulnerability has been identified in Totolink A3300R firmware version 17.0.0cu.557_b20221024. The vulnerability exists in the setVpnPassCfg function within the /cgi-bin/cstecgi.cgi Parameter Handler component. Attackers can exploit this flaw by manipulating the pptpPassThru argument to inject and execute arbitrary system commands on the affected router.

Critical Impact

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

Affected Products

• Totolink A3300R Firmware version 17.0.0cu.557_b20221024
• Totolink A3300R Hardware

Discovery Timeline

• 2026-03-30 - CVE-2026-5105 published to NVD
• 2026-03-30 - Last updated in NVD database

Technical Details for CVE-2026-5105

Vulnerability Analysis

This vulnerability falls under CWE-77 (Command Injection) and CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component). The affected function setVpnPassCfg in the /cgi-bin/cstecgi.cgi CGI handler fails to properly sanitize user-supplied input for the pptpPassThru parameter before passing it to system command execution functions.

The Totolink A3300R router's web interface exposes this CGI endpoint to handle VPN passthrough configuration. When an authenticated user submits configuration changes, the pptpPassThru parameter is processed without adequate input validation, allowing shell metacharacters and command sequences to be injected.

The attack requires network access and low-level authentication to the router's management interface. A proof-of-concept exploit has been made publicly available, increasing the risk of exploitation in the wild.

Root Cause

The root cause of this vulnerability lies in insufficient input sanitization within the setVpnPassCfg function. The firmware fails to implement proper filtering or escaping of special characters in the pptpPassThru argument before using it in command execution contexts. This classic command injection pattern allows attackers to break out of intended parameter boundaries and append or inject arbitrary shell commands.

Attack Vector

The attack is executed remotely over the network through the router's web management interface. An attacker with valid low-privilege credentials can craft a malicious HTTP request to the /cgi-bin/cstecgi.cgi endpoint, manipulating the pptpPassThru parameter to include command injection payloads. The injected commands execute with the privileges of the web server process, which typically runs as root on embedded router devices.

The attack sequence involves:

1. Authentication to the router's web interface with low-privilege credentials
2. Sending a crafted request to the setVpnPassCfg function
3. Including shell metacharacters (such as ;, |, or $()) in the pptpPassThru parameter
4. Arbitrary command execution on the underlying Linux-based operating system

Technical details and proof-of-concept information can be found in the GitHub PoC Repository.

Detection Methods for CVE-2026-5105

Indicators of Compromise

• Unexpected HTTP POST requests to /cgi-bin/cstecgi.cgi containing suspicious characters in the pptpPassThru parameter
• Anomalous process execution or shell spawning from the router's web server process
• Unusual outbound network connections from the router to external IP addresses
• Modified router configuration files or presence of unexpected files in the firmware filesystem

Detection Strategies

• Monitor HTTP traffic to router management interfaces for requests containing command injection payloads such as shell metacharacters (;, |, &&, $(), backticks)
• Deploy network intrusion detection rules to flag suspicious requests targeting /cgi-bin/cstecgi.cgi with abnormal parameter values
• Implement logging on network segments containing Totolink A3300R devices to capture management interface access attempts
• Use SentinelOne Singularity to detect and alert on anomalous network behavior patterns associated with router exploitation

Monitoring Recommendations

• Enable verbose logging on network firewalls and intrusion detection systems for traffic destined to router management ports
• Review router access logs regularly for authentication attempts from unknown IP addresses
• Monitor for DNS queries or network connections from router devices that deviate from normal operational patterns

How to Mitigate CVE-2026-5105

Immediate Actions Required

• Restrict access to the router's web management interface to trusted IP addresses only using firewall rules
• Disable remote management features if not required for operational purposes
• Segment network infrastructure to isolate router management interfaces from general user traffic
• Consider replacing affected devices with alternative hardware if no firmware update is available

Patch Information

At the time of publication, no official patch has been released by Totolink for this vulnerability. Organizations should monitor the Totolink Official Site for security updates and firmware releases. Additional vulnerability details are available through VulDB #354130.

Workarounds

• Implement network access control lists (ACLs) to restrict management interface access to authorized administrator IP addresses only
• Place router management interfaces on isolated VLAN segments with strict access controls
• Use a VPN or jump host architecture to access router management functions rather than exposing interfaces directly
• Deploy web application firewall rules to filter requests containing command injection patterns targeting the vulnerable endpoint

# Example iptables rules to restrict management access (apply on upstream firewall)
# Allow management access only from trusted admin subnet
iptables -A FORWARD -d <ROUTER_IP> -p tcp --dport 80 -s <ADMIN_SUBNET> -j ACCEPT
iptables -A FORWARD -d <ROUTER_IP> -p tcp --dport 443 -s <ADMIN_SUBNET> -j ACCEPT
iptables -A FORWARD -d <ROUTER_IP> -p tcp --dport 80 -j DROP
iptables -A FORWARD -d <ROUTER_IP> -p tcp --dport 443 -j DROP