CVE-2026-5026: API Endpoint SVG XSS Vulnerability

CVE-2026-5026 Overview

CVE-2026-5026 is a stored cross-site scripting (XSS) vulnerability that exists in the /api/v1/files/images/{flow_id}/{file_name} endpoint. This endpoint serves SVG files with the image/svg+xml content type without properly sanitizing their content. Since SVG files can contain embedded JavaScript, an attacker can upload a malicious SVG that executes arbitrary JavaScript when viewed by other users.

This vulnerability allows attackers to steal authentication tokens stored in cookies, including JWT access and refresh tokens, potentially leading to complete account takeover.

Critical Impact

Attackers can execute arbitrary JavaScript in victims' browsers to steal authentication tokens, including JWT access and refresh tokens, enabling session hijacking and unauthorized access to user accounts.

Affected Products

• Applications using the vulnerable /api/v1/files/images/{flow_id}/{file_name} endpoint
• Systems that serve user-uploaded SVG files without content sanitization

Discovery Timeline

• 2026-03-27 - CVE-2026-5026 published to NVD
• 2026-03-30 - Last updated in NVD database

Technical Details for CVE-2026-5026

Vulnerability Analysis

This stored XSS vulnerability (CWE-79) occurs due to insufficient input validation when handling SVG file uploads. The vulnerable endpoint serves SVG files directly to users with the image/svg+xml MIME type without sanitizing the file contents. SVG files are XML-based vector image formats that can legitimately contain embedded JavaScript through <script> elements or event handlers like onload, onclick, and other DOM events.

When a user views a page containing the malicious SVG, the embedded JavaScript executes within the context of the vulnerable application's origin. This gives the attacker access to sensitive data stored in the browser, including cookies, session storage, and local storage. The attack is particularly dangerous because it targets authentication tokens—specifically JWT access and refresh tokens—which can be exfiltrated to attacker-controlled servers.

Root Cause

The root cause of this vulnerability is the lack of content sanitization for user-uploaded SVG files before serving them to other users. The application treats SVG files as static images and serves them with the image/svg+xml content type, which browsers interpret as executable XML that can contain active scripting content. Proper input validation and output encoding were not implemented for this file type.

Attack Vector

The attack requires network access and user interaction. An authenticated attacker uploads a specially crafted SVG file containing malicious JavaScript to the application. When another user accesses a page that renders this SVG, the embedded script executes in their browser session. The malicious JavaScript can then access document cookies and other sensitive data, transmitting authentication tokens to an attacker-controlled endpoint.

The attack mechanism involves embedding JavaScript within SVG elements. Common techniques include using <script> tags directly within the SVG XML structure, utilizing event handlers on SVG elements (such as <svg onload="...">), or employing <foreignObject> elements to embed HTML with script content.

Detection Methods for CVE-2026-5026

Indicators of Compromise

• Unusual SVG file uploads containing <script> tags or JavaScript event handlers
• Network traffic showing authentication tokens being transmitted to external domains
• User reports of unexpected session terminations or unauthorized account access
• Web server logs showing requests to the vulnerable endpoint with suspicious SVG filenames

Detection Strategies

• Implement content inspection rules to detect JavaScript within uploaded SVG files
• Monitor for SVG files containing suspicious elements such as <script>, onload, onerror, and <foreignObject>
• Deploy web application firewall (WAF) rules to block SVG uploads with embedded scripting
• Enable browser developer tool monitoring to detect unexpected cookie access patterns

Monitoring Recommendations

• Log and alert on all SVG file upload activities to the affected endpoint
• Monitor outbound network connections for potential data exfiltration of authentication tokens
• Implement anomaly detection for unusual patterns of token refresh or authentication failures
• Review access logs for the /api/v1/files/images/{flow_id}/{file_name} endpoint regularly

How to Mitigate CVE-2026-5026

Immediate Actions Required

• Disable SVG file uploads temporarily until a proper fix is implemented
• Implement content security policy (CSP) headers to restrict inline script execution
• Review and remove any existing malicious SVG files from the system
• Force token rotation for all users who may have accessed potentially compromised SVG files

Patch Information

Refer to the Tenable Security Research Advisory for detailed patch information and vendor guidance. Apply vendor-provided security updates as soon as they become available.

Workarounds

• Serve SVG files with the Content-Disposition: attachment header to force download instead of inline rendering
• Convert uploaded SVG files to raster formats (PNG, JPEG) before serving to users
• Implement a strict Content Security Policy that blocks inline JavaScript execution
• Use server-side SVG sanitization libraries to strip dangerous elements before storage

# Example: Configure Content-Security-Policy header to mitigate XSS
# Add to web server configuration (Apache example)
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; object-src 'none'"

# Example: Force SVG files to download instead of render inline
<FilesMatch "\.svg$">
    Header set Content-Disposition "attachment"
</FilesMatch>