CVE-2026-53907: MCO Stored XSS Vulnerability

CVE-2026-53907 Overview

CVE-2026-53907 is a Stored Cross-Site Scripting (XSS) vulnerability in MyComplianceOffice (MCO) affecting the application logo upload functionality. An authenticated attacker with permission to change the application logo can upload a crafted Scalable Vector Graphics (SVG) file containing embedded JavaScript. The malicious script executes in the browser context of any user who renders or opens the logo. The issue is classified under [CWE-79] Improper Neutralization of Input During Web Page Generation.

Because vendor contact attempts were unsuccessful, the vulnerability has only been confirmed in version 25.3.3.1 but may also affect other releases.

Critical Impact
Authenticated high-privilege attackers can inject persistent JavaScript that executes in the session of any user viewing the application logo, enabling session theft and unauthorized actions within MCO.

Affected Products

MyComplianceOffice (MCO) version 25.3.3.1
Other MCO versions are potentially affected but unconfirmed
Deployments allowing user-controlled logo uploads

Discovery Timeline

2026-07-01 - CVE CVE-2026-53907 published to NVD
2026-07-01 - Last updated in NVD database

Technical Details for CVE-2026-53907

Vulnerability Analysis

The vulnerability resides in the MCO application logo upload feature. The application accepts SVG files for use as the branding logo without sanitizing embedded scripting content. SVG is an XML-based image format that supports <script> elements and event handlers such as onload or onclick. When a browser renders the SVG inline or a user opens the image, any embedded JavaScript executes within the origin of the MCO application.

This is a stored XSS variant, meaning the payload persists on the server. Every user who visits pages that render the logo becomes a potential victim. Because the payload runs in the authenticated MCO origin, it inherits access to session cookies, tokens, and API endpoints available to the viewer.

Root Cause

The root cause is missing input validation and output sanitization on uploaded SVG files. The application does not strip active content such as <script> tags, inline event handlers, or javascript: URIs before serving the file to browsers. Serving SVGs with a MIME type such as image/svg+xml from the same origin as the application allows script execution against the MCO domain.

Attack Vector

An attacker requires a high-privilege account with the ability to modify the application logo. The attacker crafts an SVG file that includes a <script> element or an event handler with a malicious payload, then uploads it through the logo configuration interface. The payload activates each time a user's browser renders the logo, delivering the script into their session context.

Refer to the CERT Poland advisory for additional technical context on the vulnerability class and disclosure notes.

Detection Methods for CVE-2026-53907

Indicators of Compromise

SVG files stored as application logos containing <script>, onload, onerror, or javascript: strings
Unexpected outbound HTTP requests from user browsers originating on MCO application pages
Session tokens or cookies appearing in web server access logs from unusual referrers
Recent modifications to the branding or logo configuration made by non-administrative accounts

Detection Strategies

Inspect all uploaded logo files and scan SVG content for scripting elements and event handler attributes
Review audit logs for logo change events and correlate with administrative account activity
Deploy Content Security Policy (CSP) violation reporting to surface inline script execution attempts
Monitor browser console errors reported by users viewing MCO branding assets

Monitoring Recommendations

Alert on any upload with Content-Type: image/svg+xml reaching the logo endpoint
Track privileged configuration changes affecting the logo or branding settings
Log and review outbound requests initiated by MCO web pages to external destinations

How to Mitigate CVE-2026-53907

Immediate Actions Required

Restrict the logo upload capability to a minimal set of trusted administrators
Audit the currently configured application logo and remove any SVG files containing active content
Convert existing SVG logos to raster formats such as PNG or JPEG until a fix is available
Review recent privileged account activity for unauthorized logo modifications

Patch Information

No vendor patch has been published at the time of NVD disclosure. Vendor contact attempts reported in the CERT Poland advisory were unsuccessful. Consult the MyComplianceOffice product page for vendor updates and apply fixes when released.

Workarounds

Disable SVG uploads for the logo field and restrict allowed types to image/png and image/jpeg
Serve uploaded logos from a sandboxed origin distinct from the application domain
Enforce a strict Content Security Policy that disallows inline scripts on pages rendering the logo
Sanitize uploaded SVGs server-side to strip <script>, event handlers, and external references

bash

# Example CSP header to block inline script execution from rendered SVGs
Content-Security-Policy: default-src 'self'; script-src 'self'; img-src 'self' data:; object-src 'none'