CVE-2026-25138: Rucio Auth Bypass Vulnerability

CVE-2026-25138 Overview

CVE-2026-25138 is a username enumeration vulnerability affecting Rucio, a software framework that provides functionality to organize, manage, and access large volumes of scientific data using customizable policies. The WebUI login endpoint returns distinct error messages depending on whether a supplied username exists, allowing unauthenticated attackers to enumerate valid usernames within the system.

Critical Impact

Unauthenticated attackers can enumerate valid usernames through the WebUI login endpoint, enabling targeted credential attacks and social engineering campaigns against known users.

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-2026-25138 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-25138

Vulnerability Analysis

This vulnerability is classified as CWE-204 (Observable Response Discrepancy), which occurs when a system generates different observable responses based on internal state that should remain confidential. In the case of Rucio, the WebUI login endpoint provides distinguishable error messages when processing authentication requests with valid versus invalid usernames.

When an attacker submits a login attempt with a username that exists in the system, the application returns one type of error message (such as "invalid password"). However, when a non-existent username is provided, a different error message is returned (such as "user not found"). This behavioral difference allows attackers to systematically probe the login endpoint to build a list of valid user accounts without requiring any authentication.

The vulnerability is exploitable over the network without any privileges or user interaction, making it accessible to any remote attacker who can reach the Rucio WebUI login endpoint.

Root Cause

The root cause of this vulnerability lies in improper handling of authentication error responses. The login endpoint implementation fails to follow security best practices for authentication error messages as outlined in the OWASP Authentication Cheat Sheet. Instead of returning generic, uniform error messages for all authentication failures, the application exposes whether the submitted username exists within the system through differentiated responses.

Attack Vector

The attack vector is network-based, requiring only HTTP access to the Rucio WebUI login endpoint. An attacker can automate the enumeration process by:

1. Compiling a list of potential usernames (common names, email patterns, or leaked credential databases)
2. Submitting authentication requests to the WebUI login endpoint with each username and an arbitrary password
3. Analyzing the response messages to determine which usernames are valid
4. Using the enumerated valid usernames for subsequent targeted attacks such as password spraying, credential stuffing, or social engineering

This information disclosure enables attackers to reduce the attack surface for brute force attempts by targeting only confirmed valid accounts, significantly increasing the efficiency of credential-based attacks.

Detection Methods for CVE-2026-25138

Indicators of Compromise

• High volume of failed login attempts from a single IP address targeting multiple usernames
• Sequential or alphabetical patterns in attempted usernames suggesting automated enumeration
• Rapid succession of login requests with minimal time between attempts
• Login attempts using common username wordlists or patterns typical of enumeration tools

Detection Strategies

• Monitor authentication logs for unusual patterns of failed login attempts across different usernames
• Implement rate limiting on the login endpoint and alert when thresholds are exceeded
• Deploy web application firewalls (WAF) to detect and block automated enumeration tools
• Correlate authentication failure events with network flow data to identify scanning behavior

Monitoring Recommendations

• Enable detailed logging on the Rucio WebUI login endpoint capturing source IP, username attempted, and response timing
• Set up alerts for authentication failure rates exceeding normal baseline thresholds
• Monitor for connections from known malicious IP addresses or Tor exit nodes targeting the login endpoint
• Implement honeypot usernames to detect enumeration attempts

How to Mitigate CVE-2026-25138

Immediate Actions Required

• Upgrade Rucio to patched versions 35.8.3, 38.5.4, or 39.3.1 immediately
• Implement rate limiting on the WebUI login endpoint to slow enumeration attempts
• Review authentication logs for evidence of prior enumeration activity
• Consider placing the Rucio WebUI behind a VPN or network segmentation to reduce exposure

Patch Information

The Rucio development team has released patched versions that address this vulnerability by implementing uniform error messages for authentication failures. Organizations should upgrade to one of the following fixed versions based on their current deployment branch:

• 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 additional details, refer to the GitHub Security Advisory GHSA-38wq-6q2w-hcf9.

Workarounds

• Deploy a reverse proxy or WAF in front of the Rucio WebUI to normalize error responses
• Implement account lockout policies to mitigate automated enumeration and credential attacks
• Restrict access to the WebUI login endpoint to trusted IP ranges or networks
• Enable CAPTCHA or multi-factor authentication to add friction for automated attacks

# Example rate limiting configuration using nginx
# Add to location block for Rucio WebUI login endpoint
limit_req_zone $binary_remote_addr zone=login:10m rate=5r/m;

location /webui/login {
    limit_req zone=login burst=10 nodelay;
    limit_req_status 429;
    # ... existing proxy configuration
}