CVE-2025-3691

Executive Summary

CVE-2025-3691 is a Server-Side Request Forgery (SSRF) vulnerability discovered in the Seven Bears Library CMS 2023 by vendor Mirweiye. The flaw resides in the "Add Link Handler" component, allowing an unauthenticated remote attacker to manipulate the server into making arbitrary HTTP requests. This can lead to internal network reconnaissance, access to cloud metadata endpoints, or further exploitation of internal services. The vulnerability has been publicly disclosed with a proof-of-concept (PoC), and no official patch is currently available.

Supply Chain & Package Security

This vulnerability is not related to a package manager (NPM, PyPI, Go, Ruby) but affects a standalone PHP-based CMS. The risk is primarily to organizations that have deployed this CMS internally or exposed it to the internet.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (NVD) – However, based on the SSRF nature, a realistic score is 6.5 (Medium).
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: Low (C:L) – Potential access to internal resources.
• Integrity Impact: Low (I:L) – Possible manipulation of internal requests.
• Availability Impact: None (A:N)

Threat Level: Medium. The exploit is public, but the CMS is not widely deployed. The primary risk is for internal network pivoting.

Deep Dive & Mechanics

The vulnerability is triggered via the "Add Link" functionality in the CMS. The application fails to validate or sanitize user-supplied URLs before making a server-side request (e.g., using curl or file_get_contents). An attacker can supply a URL pointing to internal IP addresses (e.g., http://169.254.169.254/ for cloud metadata) or other internal services.

Root Cause: Lack of input validation and absence of a URL allowlist/blocklist in the link handler.

Attack Flow:

1. Attacker identifies the vulnerable endpoint (e.g., /admin/link/add or similar).
2. Attacker submits a POST request with a malicious URL parameter.
3. The server fetches the URL and returns the response (or performs an action).
4. Attacker exfiltrates internal data or performs further attacks.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual PHP):

<?php
$url = $_POST['link_url'];
$response = file_get_contents($url);
echo $response;
?>

Patched Code (Conceptual):

<?php
$url = $_POST['link_url'];
$allowed_hosts = ['example.com', 'trusted.org'];
$parsed_url = parse_url($url);
if (!in_array($parsed_url['host'], $allowed_hosts)) {
    die('Invalid URL');
}
$response = file_get_contents($url);
echo $response;
?>

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

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Target running Seven Bears Library CMS 2023.
• Network access to the CMS web interface.

Exploit Steps:

1. Identify the vulnerable endpoint (e.g., /admin/link/add).
2. Craft a POST request with a malicious URL.

Example cURL Command:

curl -X POST http://target.com/admin/link/add \
  -d "link_url=http://169.254.169.254/latest/meta-data/"

Expected Output: The server will return the cloud metadata (if hosted on AWS/GCP/Azure).

Advanced Exploitation:

• Use file:///etc/passwd to read local files (if allow_url_fopen is enabled).
• Use gopher:// or dict:// protocols for internal service scanning.

Blue Team (Detection & Defense)

Detection:

• Monitor for outbound HTTP requests to internal IP ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 169.254.0.0/16).
• Alert on URLs containing file://, gopher://, or dict:// in web logs.

Defense:

• Implement a Web Application Firewall (WAF) rule to block requests to internal IPs.
• Disable allow_url_fopen in PHP configuration.
• Apply network segmentation to limit outbound traffic from the web server.

Indicators of Compromise (IOCs)

Network IOCs:

• Outbound HTTP requests to internal IP addresses (e.g., 10.x.x.x, 192.168.x.x).
• Requests to cloud metadata endpoints (169.254.169.254).
• URLs containing file://, gopher://, dict:// in web logs.

File IOCs:

• Unexpected files in /tmp/ or web root (e.g., exfil.txt).
• Modified CMS configuration files.

Log IOCs:

• Web server logs showing POST /admin/link/add with suspicious link_url parameters.

Detection Engineering

YARA Rule

rule CVE_2025_3691_SSRF_SevenBearsCMS {
    meta:
        description = "Detects SSRF exploitation attempts against Seven Bears Library CMS"
        author = "Valters IT"
        date = "2025-04-01"
        reference = "CVE-2025-3691"
    strings:
        $s1 = "link_url=" nocase
        $s2 = "169.254.169.254" nocase
        $s3 = "file://" nocase
        $s4 = "gopher://" nocase
        $s5 = "dict://" nocase
    condition:
        any of ($s1) and any of ($s2, $s3, $s4, $s5)
}

Sigma Rule

title: CVE-2025-3691 SSRF Exploitation Attempt
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects SSRF attempts against Seven Bears Library CMS
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/admin/link/add'
        cs-uri-query|contains: 'link_url='
    filter:
        cs-uri-query|contains:
            - '169.254.169.254'
            - 'file://'
            - 'gopher://'
            - 'dict://'
    condition: selection and filter
falsepositives:
    - Legitimate use of internal URLs (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-3691 SSRF Exploitation Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/admin/link/add"; http_uri;
    content:"link_url="; http_client_body;
    pcre:"/(169\.254\.169\.254|file:\/\/|gopher:\/\/|dict:\/\/)/Ri";
    sid:1000001;
    rev:1;
    reference:cve,2025-3691;
)

Remediation & Mitigation

Immediate Actions:

1. Disable the vulnerable component: Remove or disable the "Add Link" functionality if not required.
2. Apply WAF rules: Block requests containing internal IPs or dangerous URL schemes.
3. Network segmentation: Restrict outbound traffic from the web server to only necessary external services.

Long-term Fix:

• Vendor patch: Await an official patch from Mirweiye. If none is provided, consider migrating to a different CMS.
• Code hardening: Implement URL validation with an allowlist of trusted domains.
• Disable dangerous PHP functions: Set allow_url_fopen = Off and disable curl if not needed.

Workaround:

• Use a reverse proxy to filter outbound requests.
• Monitor logs for SSRF patterns and block offending IPs.

Valters IT
Securing Digital Frontiers