CVE-2025-4999: Linksys FGW3000 Firmware RCE Vulnerability

CVE-2025-4999 Overview

A critical command injection vulnerability has been identified in Linksys FGW3000-AH and FGW3000-HK routers running firmware versions up to 1.0.17.000000. The vulnerability exists within the sub_4153FC function in the /cgi-bin/sysconf.cgi component, which is part of the HTTP POST Request Handler. Attackers can exploit improper handling of the supplicant_rnd_id_en argument to inject and execute arbitrary commands on the affected device.

This vulnerability is particularly concerning as it can be exploited remotely over the network, allowing attackers to potentially gain unauthorized access to the router and underlying network infrastructure. The exploit has been publicly disclosed, and the vendor was contacted but did not respond to the disclosure.

Critical Impact

Remote attackers can inject arbitrary commands through the supplicant_rnd_id_en parameter in HTTP POST requests, potentially leading to complete device compromise and network intrusion.

Affected Products

• Linksys FGW3000-AH Firmware (up to version 1.0.17.000000)
• Linksys FGW3000-AH Hardware
• Linksys FGW3000-HK Firmware (up to version 1.0.17.000000)
• Linksys FGW3000-HK Hardware

Discovery Timeline

• 2025-05-20 - CVE-2025-4999 published to NVD
• 2025-06-12 - Last updated in NVD database

Technical Details for CVE-2025-4999

Vulnerability Analysis

This command injection vulnerability (CWE-77) stems from insufficient input validation in the router's web management interface. The sub_4153FC function within /cgi-bin/sysconf.cgi fails to properly sanitize user-supplied input passed through the supplicant_rnd_id_en parameter during HTTP POST request handling. This allows an authenticated attacker to inject shell metacharacters and arbitrary commands that are subsequently executed by the underlying operating system with elevated privileges.

The vulnerability falls under both CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component) and CWE-77 (Command Injection), indicating a fundamental failure in input validation and output encoding practices within the firmware's CGI handler components.

Root Cause

The root cause lies in the improper neutralization of special elements within the supplicant_rnd_id_en parameter. When processing HTTP POST requests to /cgi-bin/sysconf.cgi, the sub_4153FC function directly incorporates user-supplied data into system command execution without adequate sanitization or escaping. This allows shell metacharacters such as semicolons, backticks, and pipe operators to break out of the intended command context and execute attacker-controlled commands.

Attack Vector

The attack can be launched remotely over the network by sending a specially crafted HTTP POST request to the vulnerable CGI endpoint. An attacker with low-privilege access to the router's web interface can manipulate the supplicant_rnd_id_en argument to inject malicious commands. The attack requires network access to the router's management interface but does not require user interaction beyond initial authentication.

The vulnerability manifests when the router processes configuration requests through the sysconf.cgi handler. Attackers can embed shell commands within the supplicant_rnd_id_en parameter value, which are then passed unsanitized to system command execution functions. For detailed technical analysis, refer to the GitHub Resource Guide and VulDB Advisory #309650.

Detection Methods for CVE-2025-4999

Indicators of Compromise

• Unusual HTTP POST requests to /cgi-bin/sysconf.cgi containing shell metacharacters (;, |, `, $()) in the supplicant_rnd_id_en parameter
• Unexpected outbound network connections originating from the router device
• Modified system files or unauthorized processes running on the router
• Authentication logs showing repeated access attempts to the router's web management interface

Detection Strategies

• Implement network intrusion detection rules to identify malicious POST requests targeting /cgi-bin/sysconf.cgi with command injection patterns
• Monitor router logs for unusual CGI handler activity or error messages related to command execution
• Deploy web application firewall (WAF) rules to block requests containing shell metacharacters in form parameters
• Perform regular firmware integrity checks to detect unauthorized modifications

Monitoring Recommendations

• Enable verbose logging on the router's web management interface if supported
• Configure network monitoring to alert on traffic patterns consistent with command injection exploitation
• Implement periodic vulnerability scanning against router management interfaces
• Monitor for DNS queries or network connections to suspicious external hosts from the router's IP address

How to Mitigate CVE-2025-4999

Immediate Actions Required

• Restrict access to the router's web management interface to trusted IP addresses only using firewall rules
• Disable remote management access if not required for operations
• Implement network segmentation to isolate the router's management interface from untrusted networks
• Monitor for exploitation attempts while awaiting a vendor patch

Patch Information

As of the last update, Linksys has not released a security patch addressing this vulnerability. The vendor was contacted during the disclosure process but did not respond. Organizations should monitor Linksys Security Resources for future firmware updates that may address this issue.

Until a patch is available, implementing the recommended workarounds and mitigations is critical for reducing exposure to this vulnerability.

Workarounds

• Disable the web management interface entirely and manage the device via physical console access where possible
• Place the router's management interface on an isolated VLAN accessible only to authorized administrators
• Implement strong access controls and multi-factor authentication for any remaining web management access
• Consider deploying an upstream firewall or IPS to filter malicious requests before they reach the vulnerable device

# Example: Restrict management access via iptables on upstream firewall
# Block external access to router management interface
iptables -A FORWARD -d <ROUTER_IP> -p tcp --dport 80 -j DROP
iptables -A FORWARD -d <ROUTER_IP> -p tcp --dport 443 -j DROP

# Allow only trusted admin subnet
iptables -I FORWARD -s 192.168.100.0/24 -d <ROUTER_IP> -p tcp --dport 443 -j ACCEPT