CVE-2025-4076 Overview
CVE-2025-4076 is a command injection vulnerability affecting the LB-LINK BL-AC3600 wireless router through firmware version 1.0.22. The flaw resides in the easy_uci_set_option_string_0 function within /cgi-bin/lighttpd.cgi, part of the device's Password Handler component. Attackers can manipulate the routepwd argument to inject arbitrary operating system commands. The vulnerability is remotely exploitable over the network and requires low privileges. A public proof-of-concept exploit is available on GitHub. The vendor was contacted prior to disclosure but did not respond.
Critical Impact
Authenticated remote attackers can execute arbitrary commands on affected LB-LINK BL-AC3600 routers by injecting shell metacharacters into the routepwd parameter, leading to full device compromise.
Affected Products
- LB-LINK BL-AC3600 router (firmware versions up to and including 1.0.22)
- /cgi-bin/lighttpd.cgi web management interface
- Password Handler component using easy_uci_set_option_string_0
Discovery Timeline
- 2025-04-29 - CVE-2025-4076 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-4076
Vulnerability Analysis
The vulnerability is classified under [CWE-74] Improper Neutralization of Special Elements in Output Used by a Downstream Component (Injection). It affects the router's web-based configuration interface served by lighttpd. When a user submits a password change request, the routepwd parameter is passed to the easy_uci_set_option_string_0 function. This function forwards the value into an underlying shell context without adequate sanitization or neutralization of shell metacharacters. Attackers who supply crafted input containing characters such as backticks, semicolons, or $() cause the router firmware to execute injected commands with the privileges of the web server process, typically root on embedded Linux routers.
Root Cause
The root cause is unsafe construction of a shell command string using attacker-controlled input inside easy_uci_set_option_string_0. The function invokes UCI (Unified Configuration Interface) operations by concatenating user input directly into a command line rather than passing arguments through a safe API. This design pattern is common in OpenWrt-derived firmware and repeatedly leads to command injection when input validation is missing.
Attack Vector
Exploitation requires network access to the router's management interface and low-privileged credentials. An attacker sends an HTTP request to /cgi-bin/lighttpd.cgi targeting the password-change functionality, embedding shell metacharacters within the routepwd field. The public proof-of-concept published at the GrayLxton BLink_poc repository demonstrates the request structure and payload placement. See the PoC script for full reproduction details. Successful exploitation yields arbitrary command execution on the router, enabling persistence, traffic interception, lateral movement into the internal network, and inclusion of the device in botnets.
Detection Methods for CVE-2025-4076
Indicators of Compromise
- HTTP POST requests to /cgi-bin/lighttpd.cgi containing shell metacharacters (;, |, `, $(), &&) inside the routepwd parameter.
- Unexpected outbound connections initiated by the router to unfamiliar IP addresses following password-change requests.
- New or modified files in router-writable paths such as /tmp, /etc/config, or cron directories.
- Unauthorized changes to DNS settings, routing tables, or firewall rules on the BL-AC3600.
Detection Strategies
- Deploy inline HTTP inspection at the network edge to identify shell metacharacter patterns in requests bound for router management interfaces.
- Correlate authentication events with configuration changes to flag password-change requests immediately followed by outbound connections.
- Baseline expected traffic from consumer/SMB routers and alert on deviations such as SSH, IRC, or high-volume outbound flows.
Monitoring Recommendations
- Restrict router management interface access to a dedicated management VLAN and log all HTTP requests to /cgi-bin/lighttpd.cgi.
- Forward router syslog data to a centralized log platform and alert on lighttpd errors or spawned child processes.
- Monitor for new administrative sessions originating from non-management network segments.
How to Mitigate CVE-2025-4076
Immediate Actions Required
- Disable remote (WAN-side) administration of the BL-AC3600 immediately and restrict LAN-side management access to trusted hosts.
- Rotate all router administrative credentials and enforce strong, unique passwords to reduce the low-privilege access required for exploitation.
- Isolate affected routers behind a segmentation boundary until a vendor fix is available.
- Audit router configuration for unauthorized DNS servers, port forwards, firewall rules, and administrative accounts.
Patch Information
No vendor patch is currently available. According to the disclosure, LB-LINK was contacted prior to public disclosure but did not respond. Consult the VulDB entry #306513 for updates on remediation status.
Workarounds
- Block external access to TCP ports serving the router web interface at the perimeter firewall or upstream ISP device.
- Place the router management interface on a dedicated administrative network reachable only through a jump host or VPN.
- Consider replacing the device with hardware from a vendor with an active security response process if no patch is issued.
- Where feasible, replace stock firmware with a maintained third-party OpenWrt build that does not include the vulnerable handler.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
