CVE-2025-7574

Executive Summary

CVE-2025-7574 is a critical authentication bypass vulnerability affecting multiple LB-LINK router models, including BL-AC1900, BL-AC2100_AZ3, BL-AC3600, BL-AX1800, BL-AX5400P, and BL-WR9000, with firmware versions up to 20250702. The flaw resides in the /cgi-bin/lighttpd.cgi web interface component, specifically in the reboot/restore function. An unauthenticated attacker can remotely trigger device reboot or factory restore without any credentials, leading to denial of service (DoS) and potential configuration loss. The vendor was contacted but did not respond, leaving the vulnerability unpatched. Public exploit code has been disclosed, increasing the risk of widespread attacks.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It affects embedded firmware on LB-LINK networking hardware. The supply chain risk is limited to the physical devices themselves, which may be deployed in home or small office environments without proper security monitoring.

CVSS Breakdown & Threat Level

CVSS Score: 9.1 (Critical)

• Attack Vector (AV): Network (N) – Exploitable remotely over the network.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – Impact confined to the vulnerable component.
• Confidentiality (C): None (N) – No data exposure.
• Integrity (I): High (H) – Device can be restored to factory defaults, wiping configurations.
• Availability (A): High (H) – Device can be rebooted, causing service disruption.

Threat Level: Critical. The ease of exploitation and lack of authentication make this a prime target for botnets and malicious actors seeking to disrupt home networks.

Deep Dive & Mechanics

The vulnerability is a classic case of missing authentication for a critical function. The /cgi-bin/lighttpd.cgi endpoint handles administrative actions such as reboot and restore. Due to improper access controls, the CGI script does not verify the user's session or credentials before executing these commands.

Technical Flow:

1. An attacker sends an HTTP POST request to http://<router-ip>/cgi-bin/lighttpd.cgi with parameters like action=reboot or action=restore.
2. The CGI script receives the request and directly calls system-level functions (e.g., system("reboot")) without checking if the request originated from an authenticated administrator.
3. The device immediately executes the command, resulting in a reboot or factory reset.

The exploit is trivial to execute and requires no special tools beyond a web browser or curl. The public disclosure includes proof-of-concept code that sends a simple HTTP request.

Vulnerable vs. Patched Code

Since the vendor has not released a patch, there is no patched code to compare. The vulnerable code is presumed to be in the lighttpd.cgi script, which likely contains logic similar to:

// Vulnerable pseudocode
if (strcmp(getenv("QUERY_STRING"), "action=reboot") == 0) {
    system("reboot");
}

A patched version would require authentication checks:

// Patched pseudocode
if (authenticated_user()) {
    if (strcmp(getenv("QUERY_STRING"), "action=reboot") == 0) {
        system("reboot");
    }
} else {
    return 401 Unauthorized;
}

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• A vulnerable LB-LINK router on the same network or exposed to the internet.
• curl or any HTTP client.

Exploit Steps:

1. Identify the target router's IP address (e.g., via Shodan or local network scan).
2. Send a POST request to trigger a reboot:

   curl -X POST http://<target-ip>/cgi-bin/lighttpd.cgi -d "action=reboot"
   

3. To trigger a factory restore:

   curl -X POST http://<target-ip>/cgi-bin/lighttpd.cgi -d "action=restore"
   

4. The device will immediately execute the command. For reboot, the device will be offline for 1-2 minutes. For restore, all settings are lost.

Lab Setup:

• Use a virtualized environment with QEMU emulating the MIPS architecture, or obtain a physical device.
• Set up a local network with the router and a Kali Linux attacker machine.
• Monitor traffic with Wireshark to confirm the exploit.

Blue Team (Defensive)

Detection:

• Monitor for HTTP POST requests to /cgi-bin/lighttpd.cgi with parameters containing action=reboot or action=restore.
• Alert on multiple rapid requests from the same IP.
• Use network segmentation to isolate router management interfaces from untrusted networks.

Honeypot:

• Deploy a fake LB-LINK router endpoint that logs all requests to /cgi-bin/lighttpd.cgi and alerts on suspicious activity.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP POST requests to /cgi-bin/lighttpd.cgi with body containing action=reboot or action=restore.
  • Source IPs from known malicious ranges or Tor exit nodes.
• Host IOCs:
  • Router logs showing unexpected reboot or factory reset events.
  • Configuration changes without authenticated user activity.
• File IOCs:
  • No file-based IOCs as the exploit is purely network-based.

Detection Engineering

YARA Rule

rule CVE_2025_7574_LBLINK_AUTH_BYPASS {
    meta:
        description = "Detects HTTP requests targeting LB-LINK router authentication bypass"
        author = "Valters IT"
        date = "2025-07-03"
        reference = "CVE-2025-7574"
    strings:
        $reboot = "action=reboot" ascii wide nocase
        $restore = "action=restore" ascii wide nocase
        $cgi_path = "/cgi-bin/lighttpd.cgi" ascii wide nocase
    condition:
        all of them
}

Sigma Rule

title: LB-LINK Router Authentication Bypass via lighttpd.cgi
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects HTTP POST requests to /cgi-bin/lighttpd.cgi with reboot or restore actions
author: Valters IT
date: 2025-07-03
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/cgi-bin/lighttpd.cgi'
        cs-uri-query|contains: 
            - 'action=reboot'
            - 'action=restore'
    condition: selection
falsepositives:
    - Legitimate administrative actions (rare if authentication is enforced)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-7574 LB-LINK Router Auth Bypass"; flow:to_server; content:"POST"; http_method; content:"/cgi-bin/lighttpd.cgi"; http_uri; content:"action=reboot"; http_client_body; sid:1000001; rev:1;)
alert http any any -> any any (msg:"CVE-2025-7574 LB-LINK Router Auth Bypass"; flow:to_server; content:"POST"; http_method; content:"/cgi-bin/lighttpd.cgi"; http_uri; content:"action=restore"; http_client_body; sid:1000002; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Remote Management: If the router's web interface is exposed to the internet, disable remote access immediately. Only allow management from the local LAN.
2. Firewall Rules: Block external access to port 80/443 on the router's WAN interface.
3. Monitor Logs: Check router logs for unexpected reboots or factory resets.

Long-Term Fix:

• Since the vendor is unresponsive, consider replacing the device with a supported alternative.
• If replacement is not possible, implement a reverse proxy with authentication (e.g., nginx with basic auth) in front of the router's web interface.
• Apply network segmentation to isolate IoT devices from critical systems.

Vendor Contact:

• Attempt to contact LB-LINK again via their official support channels. If no response, report the vulnerability to CERT/CC for broader disclosure.

Valters IT
Securing Digital Frontiers