CVE-2026-5355: Trendnet TEW-657BRM RCE Vulnerability

CVE-2026-5355 Overview

A command injection vulnerability has been identified in the Trendnet TEW-657BRM router firmware version 1.00.1. This security flaw exists within the vpn_drop function in the /setup.cgi file, where improper handling of the policy_name argument allows attackers to inject and execute arbitrary operating system commands. The vulnerability can be exploited remotely by authenticated attackers, potentially leading to complete device compromise.

Critical Impact

Remote attackers can execute arbitrary OS commands on affected Trendnet TEW-657BRM routers, potentially gaining full control of the device and using it as a pivot point for further network attacks. Note that this product has been discontinued since June 2011 and no patches will be provided.

Affected Products

• Trendnet TEW-657BRM Firmware Version 1.00.1
• Trendnet TEW-657BRM (End of Life since June 23, 2011)

Discovery Timeline

• April 2, 2026 - CVE-2026-5355 published to NVD
• April 2, 2026 - Last updated in NVD database

Technical Details for CVE-2026-5355

Vulnerability Analysis

This vulnerability falls under CWE-77 (Improper Neutralization of Special Elements used in a Command), commonly known as Command Injection. The flaw resides in the web-based management interface of the Trendnet TEW-657BRM router, specifically within the VPN configuration functionality.

The vpn_drop function processes VPN policy configurations through the /setup.cgi endpoint. When handling the policy_name parameter, the function fails to properly sanitize user-supplied input before passing it to system shell commands. This allows an authenticated attacker to inject shell metacharacters and arbitrary commands that will be executed with the privileges of the web server process, typically root on embedded devices.

The vulnerability requires low privileges (authentication) but can be exploited over the network without user interaction, making it a significant risk for any still-operational devices.

Root Cause

The root cause of this vulnerability is inadequate input validation and sanitization in the vpn_drop function. The policy_name argument is directly incorporated into a system command without proper escaping or filtering of shell metacharacters such as semicolons (;), pipes (|), backticks (`), or command substitution operators ($()). This is a classic command injection pattern commonly found in embedded device firmware where CGI scripts directly invoke shell commands.

Attack Vector

The attack can be carried out remotely over the network. An attacker with valid authentication credentials to the router's web interface can craft a malicious HTTP request to the /setup.cgi endpoint. By manipulating the policy_name parameter to include shell metacharacters followed by arbitrary commands, the attacker can achieve command execution on the underlying operating system.

For example, an attacker could potentially inject commands to:

• Download and execute malicious payloads
• Modify router configurations
• Intercept network traffic
• Use the device as part of a botnet
• Pivot to attack other devices on the network

Technical details and proof-of-concept information are available in the GitHub PoC Repository and the VulDB entry.

Detection Methods for CVE-2026-5355

Indicators of Compromise

• Unusual HTTP POST requests to /setup.cgi containing shell metacharacters in parameters
• Unexpected outbound network connections from the router to unknown IP addresses
• Modified router configurations not performed by administrators
• Presence of unexpected processes or files on the router filesystem
• Anomalous DNS queries or traffic patterns originating from the router

Detection Strategies

• Monitor HTTP traffic to the router's management interface for requests containing shell metacharacters (;, |, `, $()) in the policy_name or similar parameters
• Implement network-based intrusion detection rules to identify command injection patterns in traffic to /setup.cgi
• Deploy SentinelOne Singularity to monitor for anomalous network behavior and lateral movement originating from IoT devices
• Review router logs (if available) for suspicious CGI script executions or authentication events

Monitoring Recommendations

• Isolate the affected device on a separate network segment and closely monitor all traffic to and from it
• Enable verbose logging on network firewalls and analyze traffic patterns from the router
• Implement SentinelOne network detection capabilities to identify exploitation attempts and post-exploitation activity

How to Mitigate CVE-2026-5355

Immediate Actions Required

• Immediately disconnect the Trendnet TEW-657BRM from production networks as this device is end-of-life with no security support
• Replace the affected device with a currently supported router that receives security updates
• If the device must remain operational temporarily, restrict access to the management interface to trusted IP addresses only
• Disable remote management features and ensure the device is not exposed to the internet

Patch Information

No patch is available for this vulnerability. Trendnet has confirmed that the TEW-657BRM was discontinued on June 23, 2011, over 14 years ago, and they no longer provide support or security updates for this product. Trendnet has stated they will announce this vulnerability on their product support page and notify customers who registered their products.

The only effective remediation is to replace the affected device with a currently supported alternative. Additional information can be found in the VulDB submission.

Workarounds

• Replace the vulnerable device with a modern, supported router as the primary mitigation strategy
• If immediate replacement is not possible, place the router behind a firewall that restricts access to the management interface
• Disable WAN-side management access to prevent remote exploitation attempts
• Implement network segmentation to limit the potential impact of device compromise

# Example: Restrict management access at the firewall level
# Block external access to router management ports
iptables -A INPUT -i eth0 -p tcp --dport 80 -s ! 192.168.1.0/24 -j DROP
iptables -A INPUT -i eth0 -p tcp --dport 443 -s ! 192.168.1.0/24 -j DROP