CVE-2026-25736: Rucio WebUI Stored XSS Vulnerability

CVE-2026-25736 Overview

CVE-2026-25736 is a stored Cross-Site Scripting (XSS) vulnerability affecting Rucio, a software framework designed to organize, manage, and access large volumes of scientific data using customizable policies. The vulnerability exists in the Custom RSE (Rucio Storage Element) Attribute functionality of the WebUI, where attacker-controlled input is persisted by the backend and subsequently rendered in the WebUI 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 with high privileges can inject malicious JavaScript that persists in the database and executes in the browsers of other users viewing affected RSE attribute pages, potentially leading to session hijacking, credential theft, and unauthorized data operations.

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-2026-25736 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-25736

Vulnerability Analysis

This stored XSS vulnerability (CWE-79) occurs in the Custom RSE Attribute handling within the Rucio WebUI. Rucio Storage Elements (RSEs) are abstractions representing storage systems in the Rucio framework, and administrators can define custom attributes for these RSEs. The vulnerability arises because user-supplied input for custom RSE attributes is stored in the backend database without proper sanitization and is later rendered in the WebUI without appropriate output encoding.

When a privileged user (such as an administrator) creates or modifies a custom RSE attribute with malicious JavaScript payload, this payload is persisted in the database. Subsequently, when other users navigate to pages displaying these RSE attributes, the malicious script executes in their browser context, inheriting all the permissions and session data of the viewing user.

Root Cause

The root cause of this vulnerability is the lack of proper output encoding when rendering user-controlled RSE attribute data in the WebUI. The application fails to escape HTML special characters and JavaScript-relevant characters before inserting attribute values into the HTML response. This violates the fundamental principle of contextual output encoding, which dictates that all dynamic content should be encoded appropriately for the context in which it appears (HTML, JavaScript, CSS, URL, etc.).

Attack Vector

The attack requires network access and is executed through the following sequence:

1. An attacker with elevated privileges (administrative access to RSE management) accesses the Rucio WebUI
2. The attacker creates or modifies a custom RSE attribute, injecting a malicious JavaScript payload as the attribute value
3. The backend stores this payload in the database without sanitization
4. When legitimate users browse to pages displaying RSE information, the WebUI renders the malicious attribute value without encoding
5. The injected JavaScript executes in the victim's browser context, allowing the attacker to steal session tokens, perform actions on behalf of the user, or exfiltrate sensitive data

The vulnerability is particularly concerning in scientific data management environments where Rucio is commonly deployed, as it could enable unauthorized access to sensitive research data or manipulation of data management operations.

Detection Methods for CVE-2026-25736

Indicators of Compromise

• Unusual RSE attribute values containing HTML tags or JavaScript code patterns such as <script>, onerror=, onclick=, or javascript:
• Unexpected network requests originating from the Rucio WebUI to external domains
• Session anomalies where users report unauthorized actions performed under their accounts
• Database entries in RSE attribute tables containing encoded or obfuscated script content

Detection Strategies

• Implement Content Security Policy (CSP) headers and monitor for CSP violation reports indicating attempted script injection
• Enable web application firewall (WAF) rules to detect common XSS payload patterns in HTTP requests targeting RSE attribute endpoints
• Review application logs for RSE attribute modifications containing suspicious patterns or unusually long values
• Deploy browser-based security monitoring to detect unexpected DOM modifications or script executions

Monitoring Recommendations

• Configure centralized logging for all RSE attribute create and update operations
• Implement anomaly detection for user session behavior to identify potential session hijacking resulting from XSS exploitation
• Monitor for outbound connections from the WebUI application server to unexpected external hosts
• Regularly audit RSE attribute values in the database for potentially malicious content

How to Mitigate CVE-2026-25736

Immediate Actions Required

• Upgrade Rucio to a patched version immediately: 35.8.3, 38.5.4, or 39.3.1 depending on your current version branch
• Audit existing RSE custom attributes in the database for potential malicious payloads
• Review access logs for suspicious RSE attribute modifications prior to patching
• Implement Content Security Policy (CSP) headers as a defense-in-depth measure
• Consider temporarily restricting RSE attribute modification privileges to essential personnel only until patching is complete

Patch Information

The Rucio development team has released security patches addressing this vulnerability across multiple version branches. Organizations should upgrade to the appropriate fixed version based on their current deployment:

• For users on the 35.x branch: Upgrade to Rucio 35.8.3
• For users on the 38.x branch: Upgrade to Rucio 38.5.4
• For users on the 39.x branch: Upgrade to Rucio 39.3.1

Additional details about the vulnerability and patches are available in the GitHub Security Advisory GHSA-fq4f-4738-rqxm.

Workarounds

• Deploy a web application firewall (WAF) with XSS detection rules to filter malicious payloads targeting RSE attribute endpoints
• Implement strict Content Security Policy headers to prevent inline script execution: Content-Security-Policy: default-src 'self'; script-src 'self'
• Restrict network-level access to the Rucio WebUI to trusted IP ranges or VPN-connected users only
• Temporarily disable or restrict the custom RSE attribute functionality if it is not critical to operations

# Example CSP header configuration for Apache
# Add to your Rucio WebUI virtual host configuration
Header always set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; frame-ancestors 'none';"
Header always set X-Content-Type-Options "nosniff"
Header always set X-XSS-Protection "1; mode=block"