CVE-2026-25734: Rucio WebUI Stored XSS Vulnerability

CVE-2026-25734 Overview

CVE-2026-25734 is a stored Cross-Site Scripting (XSS) vulnerability affecting Rucio, a software framework that provides functionality to organize, manage, and access large volumes of scientific data using customizable policies. The vulnerability exists in the RSE (Rucio Storage Element) metadata handling of the WebUI, where attacker-controlled input is persisted by the backend and later rendered without proper output encoding. This allows arbitrary JavaScript execution in the context of the WebUI for users who view affected pages, potentially enabling session token theft or unauthorized actions.

Critical Impact

Attackers can inject malicious JavaScript that persists in the database and executes in victim browsers, enabling session hijacking, credential theft, and unauthorized actions within the Rucio scientific data management framework.

Affected Products

• Rucio versions prior to 35.8.3
• Rucio versions prior to 38.5.4
• Rucio versions prior to 39.3.1

Discovery Timeline

• 2026-02-25 - CVE-2026-25734 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-25734

Vulnerability Analysis

This stored XSS vulnerability (CWE-79) occurs when RSE metadata containing malicious payloads is saved to the Rucio backend and subsequently rendered in the WebUI without proper sanitization or output encoding. Unlike reflected XSS attacks that require victims to click malicious links, stored XSS payloads persist in the application's data store, affecting any user who views the compromised RSE metadata page.

The attack requires high-privilege access to initially inject the malicious payload into RSE metadata, but once stored, the payload executes automatically for any authenticated user viewing the affected pages. This can lead to session token theft via document.cookie access, unauthorized actions performed in the victim's authenticated session, keylogging or credential harvesting, and defacement or phishing attacks within the trusted application context.

Root Cause

The root cause is improper output encoding when rendering user-supplied RSE metadata in the WebUI. The Rucio backend accepts and stores metadata values that may contain HTML and JavaScript, but the frontend fails to properly encode or sanitize these values before inserting them into the DOM. This violates the fundamental XSS prevention principle of treating all user-supplied data as untrusted and encoding it appropriately for the output context.

Attack Vector

The attack vector is network-based and requires an attacker with sufficient privileges to modify RSE metadata. The attacker crafts a malicious payload containing JavaScript code and submits it as RSE metadata through the Rucio API or WebUI. The malicious metadata is stored in the backend database. When any user navigates to the WebUI page displaying the affected RSE metadata, the JavaScript executes in their browser context with full access to their session.

The exploitation scenario typically involves crafting a payload such as event handlers or script tags embedded in metadata fields that bypass basic input validation but execute when rendered in HTML context.

Detection Methods for CVE-2026-25734

Indicators of Compromise

• RSE metadata entries containing suspicious HTML tags such as <script>, <img>, <svg>, or event handlers like onerror, onload, onclick
• Unexpected outbound requests from user browsers to external domains after viewing Rucio WebUI pages
• Session tokens or cookies being transmitted to unauthorized external endpoints
• User reports of unexpected behavior or redirects when accessing RSE metadata pages

Detection Strategies

• Implement Content Security Policy (CSP) headers to detect and block unauthorized script execution, monitoring CSP violation reports for XSS attempts
• Deploy web application firewall (WAF) rules to detect common XSS patterns in API requests to RSE metadata endpoints
• Enable detailed logging of RSE metadata modifications and audit for suspicious content patterns
• Configure browser-based monitoring to detect DOM manipulation indicative of XSS exploitation

Monitoring Recommendations

• Monitor HTTP response headers to ensure proper Content-Type and X-Content-Type-Options are set
• Set up alerts for CSP violation reports that may indicate XSS exploitation attempts
• Audit user session activity for anomalous actions following RSE metadata page views
• Review database entries for RSE metadata containing HTML or JavaScript patterns

How to Mitigate CVE-2026-25734

Immediate Actions Required

• Upgrade Rucio to version 35.8.3, 38.5.4, or 39.3.1 depending on your current release branch
• Audit existing RSE metadata entries for potentially malicious content and sanitize as needed
• Implement Content Security Policy headers to provide defense-in-depth against script injection
• Review user accounts with RSE metadata modification privileges and apply principle of least privilege

Patch Information

The Rucio development team has released patched versions that implement proper output encoding for RSE metadata rendered in the WebUI. The following versions contain the fix:

• Rucio Release 35.8.3 for the 35.x branch
• Rucio Release 38.5.4 for the 38.x branch
• Rucio Release 39.3.1 for the 39.x branch

For detailed information about the vulnerability and fix, refer to the GitHub Security Advisory GHSA-h9fp-p2p9-873q.

Workarounds

• Implement a reverse proxy or WAF rule to sanitize RSE metadata responses before they reach end users
• Restrict RSE metadata modification permissions to only essential administrative accounts until patching is complete
• Deploy Content Security Policy headers with script-src 'self' to prevent inline script execution
• Consider temporarily disabling RSE metadata display in the WebUI if patching cannot be performed immediately

# Example: Add Content Security Policy header in nginx configuration
# Add to server or location block for Rucio WebUI
add_header Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; frame-ancestors 'self';" always;
add_header X-Content-Type-Options "nosniff" always;
add_header X-XSS-Protection "1; mode=block" always;