CVE-2026-25733: Rucio WebUI Stored XSS Vulnerability

CVE-2026-25733 Overview

A stored Cross-Site Scripting (XSS) vulnerability has been identified in 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 Custom Rules function of the WebUI where attacker-controlled input is persisted by the backend and later 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 can inject malicious JavaScript that executes in victim browsers, potentially stealing session tokens, performing unauthorized actions on behalf of authenticated users, or compromising sensitive scientific data managed by the Rucio platform.

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 CVE-2026-25733 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-25733

Vulnerability Analysis

This stored XSS vulnerability (CWE-79) occurs within the Custom Rules function of the Rucio WebUI. The underlying issue stems from the application's failure to properly sanitize and encode user-supplied input before storing it in the backend database and subsequently rendering it in the web interface. When a user with appropriate permissions creates or modifies custom rules, malicious script content can be embedded within the rule data. This payload is then stored persistently and executed each time another user views the affected page through their browser.

The stored nature of this XSS vulnerability makes it particularly dangerous as it does not require victim interaction beyond normal application usage. Unlike reflected XSS attacks that require social engineering to trick users into clicking malicious links, stored XSS payloads are automatically executed when legitimate users access the compromised content through normal workflows.

Root Cause

The root cause of this vulnerability is improper output encoding when rendering user-controlled data in the WebUI. The Custom Rules function accepts user input that is stored in the backend without adequate validation. When this data is later retrieved and displayed in the WebUI, the application fails to apply proper HTML entity encoding or contextual escaping, allowing embedded JavaScript to execute in the browser context.

This type of vulnerability typically occurs when developers trust data retrieved from their own database without recognizing that the data originated from untrusted user input. The fix requires implementing proper output encoding at the point of rendering, following the principle that all data displayed in HTML context should be treated as untrusted.

Attack Vector

The attack vector for CVE-2026-25733 is network-based and requires low privileges with user interaction. An authenticated attacker with permissions to create or modify custom rules can inject malicious JavaScript payloads into rule definitions. These payloads persist in the Rucio backend database and execute whenever other users view pages that render the affected custom rules.

A successful attack could enable the attacker to steal session tokens from other users (including administrators), perform actions on behalf of authenticated users, modify or exfiltrate sensitive scientific data, redirect users to malicious websites, or deface the WebUI interface. The impact is significant in environments managing large volumes of scientific data where data integrity and user trust are paramount.

Detection Methods for CVE-2026-25733

Indicators of Compromise

• Unexpected JavaScript tags or event handlers within custom rule definitions stored in the Rucio database
• Unusual outbound network connections from user browsers when accessing the WebUI Custom Rules pages
• Reports from users about unexpected browser behavior, pop-ups, or redirects when viewing custom rules
• Session tokens appearing in external server logs that could indicate token exfiltration

Detection Strategies

• Implement Content Security Policy (CSP) headers and monitor CSP violation reports for injection attempts
• Deploy Web Application Firewall (WAF) rules to detect common XSS payloads in HTTP requests to the Custom Rules endpoints
• Review stored custom rule data for suspicious patterns including <script>, javascript:, event handlers like onerror, onload, and encoded variants
• Monitor application logs for unusual patterns in custom rule creation or modification activities

Monitoring Recommendations

• Enable detailed logging for all Custom Rules CRUD operations including the full content of rule definitions
• Implement anomaly detection for users creating rules with unusual content patterns or excessive special characters
• Set up alerting for CSP violation reports specifically related to inline script execution
• Regularly audit stored custom rules for potential malicious content using automated scanning tools

How to Mitigate CVE-2026-25733

Immediate Actions Required

• Upgrade Rucio installations to patched versions 35.8.3, 38.5.4, or 39.3.1 immediately
• Audit existing custom rules in the database for potentially malicious content before and after patching
• Implement Content Security Policy headers to provide defense-in-depth against XSS attacks
• Consider temporarily restricting access to the Custom Rules functionality until patches are applied

Patch Information

Rucio has released security patches that address this stored XSS vulnerability. Organizations should upgrade to the following fixed versions based on their current major version:

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

For detailed information about this vulnerability and the security fix, refer to the GitHub Security Advisory GHSA-rwj9-7j48-9f7q.

Workarounds

• Restrict access to the Custom Rules functionality to only trusted administrators until patches can be applied
• Implement a Web Application Firewall (WAF) with rules to block common XSS payloads targeting the WebUI
• Deploy strict Content Security Policy headers that disallow inline script execution as a defense-in-depth measure
• Regularly scan stored custom rules data for suspicious content patterns using automated tools

# Example: Configure Content Security Policy headers in your web server
# For Apache, add to httpd.conf or .htaccess:
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; frame-ancestors 'none';"

# For Nginx, add to server block:
add_header Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; frame-ancestors 'none';" always;