CVE-2025-67438: Sync-in Server Stored XSS Vulnerability

CVE-2025-67438 Overview

A Stored Cross-Site Scripting (XSS) vulnerability exists in Sync-in Server before version 1.9.3 that allows an authenticated attacker to execute arbitrary JavaScript in a victim's browser. By uploading a crafted SVG file containing a malicious payload, an attacker can access and exfiltrate sensitive information, including the user's session cookies. This vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation).

Critical Impact

Successful exploitation enables attackers to steal session cookies, hijack user accounts, perform actions on behalf of authenticated users, and potentially compromise sensitive data accessed through the Sync-in Server application.

Affected Products

• Sync-in Server versions prior to 1.9.3

Discovery Timeline

• 2026-02-20 - CVE-2025-67438 published to NVD
• 2026-02-23 - Last updated in NVD database

Technical Details for CVE-2025-67438

Vulnerability Analysis

This Stored XSS vulnerability stems from insufficient input validation and sanitization when processing uploaded SVG files. SVG (Scalable Vector Graphics) files are XML-based and can contain embedded JavaScript code within <script> tags or event handlers. When Sync-in Server fails to properly sanitize these uploads, malicious scripts persist in the application's storage and execute whenever a victim views or interacts with the uploaded content.

The attack requires the attacker to be authenticated to the system, providing initial access to the file upload functionality. Once a malicious SVG is uploaded and stored, any user who subsequently views this file will have the embedded JavaScript execute in their browser context. This represents a significant security risk as the malicious payload persists on the server and can affect multiple victims over time.

Root Cause

The root cause of this vulnerability is the absence of proper content validation and sanitization for uploaded SVG files. Sync-in Server prior to version 1.9.3 does not adequately strip or neutralize potentially dangerous elements within SVG uploads, including embedded <script> elements, inline event handlers (such as onload, onclick, onerror), and other JavaScript execution vectors commonly found in malicious SVG payloads.

Attack Vector

The attack is executed over the network and requires user interaction for successful exploitation. An authenticated attacker uploads a specially crafted SVG file containing malicious JavaScript to the Sync-in Server. The server stores this file without adequate sanitization. When another authenticated user views or accesses the malicious SVG file through their browser, the embedded JavaScript executes within that user's session context, enabling cookie theft, session hijacking, or other client-side attacks.

A typical malicious SVG payload might contain embedded JavaScript within SVG event handlers or script elements that automatically execute when the image is rendered. The attacker can leverage this to exfiltrate session cookies to an external server they control, effectively hijacking the victim's authenticated session.

Technical details and proof-of-concept information are available in the GitHub Gist PoC.

Detection Methods for CVE-2025-67438

Indicators of Compromise

• Presence of SVG files containing <script> tags or JavaScript event handlers in upload directories
• Unusual outbound network requests to external domains originating from user browsers after accessing uploaded content
• Server logs showing uploads of SVG files with suspicious content patterns
• User reports of unexpected browser behavior when viewing uploaded files

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block SVG uploads containing embedded scripts
• Monitor file upload endpoints for SVG files with suspicious XML content patterns
• Deploy content security policy (CSP) headers to restrict inline script execution
• Review server access logs for patterns indicating exploitation attempts

Monitoring Recommendations

• Enable detailed logging for all file upload operations and access events
• Monitor for anomalous session activity that could indicate cookie theft or session hijacking
• Set up alerts for uploads containing XML/SVG files with embedded script elements
• Track and investigate any reports of unexpected JavaScript execution in the application

How to Mitigate CVE-2025-67438

Immediate Actions Required

• Upgrade Sync-in Server to version 1.9.3 or later immediately
• Review and audit all previously uploaded SVG files for malicious content
• Consider temporarily disabling SVG file uploads until the patch is applied
• Implement Content Security Policy headers to mitigate the impact of any existing malicious uploads

Patch Information

The vulnerability has been addressed in Sync-in Server version 1.9.3. Organizations should upgrade to this version or later to remediate the vulnerability. Release notes and download information are available at the GitHub Release Notes v1.9.3.

Workarounds

• Disable SVG file uploads entirely if the feature is not business-critical
• Implement server-side SVG sanitization to strip all script elements and event handlers before storage
• Configure the web server to serve SVG files with Content-Type: image/svg+xml and Content-Disposition: attachment headers
• Deploy a Content Security Policy (CSP) that disables inline script execution

# Example Apache configuration to mitigate XSS in SVG files
# Add to .htaccess or virtual host configuration

# Force download of SVG files instead of inline rendering
<FilesMatch "\.svg$">
    Header set Content-Disposition "attachment"
    Header set X-Content-Type-Options "nosniff"
</FilesMatch>

# Add Content Security Policy header
Header always set Content-Security-Policy "default-src 'self'; script-src 'self'; object-src 'none'"