CVE-2025-59903: Kubysoft Stored XSS Vulnerability

CVE-2025-59903 Overview

A Stored Cross-Site Scripting (XSS) vulnerability has been identified in Kubysoft, stemming from improper sanitization of uploaded SVG images. This vulnerability allows attackers to embed malicious scripts within SVG files, which are subsequently stored on the server and executed in the context of any user who accesses the compromised resource.

Critical Impact

Attackers can embed persistent malicious scripts in SVG uploads that execute in victims' browsers, potentially leading to session hijacking, credential theft, and unauthorized actions on behalf of authenticated users.

Affected Products

• Kubysoft (specific versions not disclosed)

Discovery Timeline

• 2026-02-16 - CVE CVE-2025-59903 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2025-59903

Vulnerability Analysis

This vulnerability falls under CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The flaw exists in the file upload functionality of Kubysoft, where SVG images are accepted and stored without adequate validation or sanitization of their XML content.

SVG files are XML-based vector image formats that can legitimately contain JavaScript through <script> elements or event handlers like onload, onclick, and other inline event attributes. When the application fails to sanitize these elements before storing and serving the SVG files, any embedded malicious code will execute in the browser context of users who view or access the image.

The stored nature of this XSS vulnerability makes it particularly dangerous, as the malicious payload persists on the server and can affect multiple users over an extended period without requiring the attacker to maintain an active presence.

Root Cause

The root cause of this vulnerability is the absence of proper input sanitization for SVG file uploads. The application accepts SVG files as valid image formats but does not strip or neutralize potentially dangerous XML elements and attributes that can execute JavaScript code. This includes <script> tags, <foreignObject> elements, and inline event handlers embedded within the SVG markup.

Attack Vector

An authenticated attacker with file upload privileges can craft a malicious SVG file containing embedded JavaScript code. When this file is uploaded to the Kubysoft application, it is stored on the server without sanitization. Subsequently, when any user accesses a page that renders or displays the malicious SVG, the embedded script executes within that user's browser session.

The attack is network-based and requires the attacker to have authenticated access to the upload functionality. User interaction is required in that a victim must access the page containing the malicious SVG for the payload to execute. The malicious SVG might contain payloads that steal session cookies, redirect users to phishing sites, modify page content, or perform actions on behalf of the victim user.

Detection Methods for CVE-2025-59903

Indicators of Compromise

• Presence of SVG files containing <script> tags, <foreignObject> elements, or inline event handlers (onload, onerror, onclick, etc.)
• Unusual network requests originating from user browsers after viewing SVG images, particularly requests to external domains
• Unexpected changes to user sessions or authentication states following SVG file access

Detection Strategies

• Implement file content inspection rules to detect JavaScript or event handlers within uploaded SVG files
• Monitor web application logs for SVG file uploads followed by suspicious client-side activity
• Deploy browser-based XSS detection tools that can identify script execution from image resources

Monitoring Recommendations

• Enable detailed logging of all file upload activities, including file type, content hash, and uploading user
• Configure Content Security Policy (CSP) headers to restrict script execution sources and monitor CSP violation reports
• Implement real-time alerting for any SVG files containing potentially malicious content identified during upload scanning

How to Mitigate CVE-2025-59903

Immediate Actions Required

• Audit all existing SVG files on the server for potentially malicious content and remove or quarantine suspicious files
• Implement SVG sanitization at the upload point to strip all script elements, event handlers, and <foreignObject> tags
• Consider converting uploaded SVG files to raster formats (PNG, JPEG) to eliminate script execution risk entirely

Patch Information

No vendor patch information is currently available for this vulnerability. Organizations should consult the INCIBE CERT Vulnerability Notice for the latest updates on remediation guidance from the vendor.

Workarounds

• Disable SVG file uploads entirely until a proper sanitization solution is implemented
• Serve SVG files with Content-Type: image/svg+xml and Content-Disposition: attachment headers to prevent inline rendering
• Implement a strict Content Security Policy (CSP) that includes script-src 'self' to prevent execution of inline scripts within SVG files
• Use a dedicated SVG sanitization library (such as DOMPurify) to process all uploaded SVG content before storage

# Example Apache configuration to mitigate SVG-based XSS
# Add to .htaccess or server configuration

# Force SVG files to download instead of rendering inline
<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'"