CVE-2025-27221: Ruby URI Gem Credential Leak Vulnerability

CVE-2025-27221 Overview

CVE-2025-27221 is an Information Leakage vulnerability affecting the URI gem for Ruby versions before 1.0.3. The vulnerability exists in URI handling methods including URI.join, URI#merge, and URI#+, which inadvertently leak authentication credentials. When these methods are used to change the host portion of a URI, the userinfo component (containing credentials) is incorrectly retained, potentially exposing sensitive authentication data to unintended destinations.

Critical Impact

Authentication credentials embedded in URIs may be inadvertently transmitted to third-party hosts when URI manipulation methods are used, potentially leading to credential theft and unauthorized access.

Affected Products

• Ruby-lang URI gem versions before 1.0.3
• Ruby-lang URI gem versions 1.0.0 through 1.0.2
• Applications using vulnerable URI manipulation methods (URI.join, URI#merge, URI#+)

Discovery Timeline

• 2025-03-04 - CVE-2025-27221 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-27221

Vulnerability Analysis

This vulnerability is classified under CWE-212 (Improper Removal of Sensitive Information Before Storage or Transfer). The core issue lies in how the URI gem handles the userinfo component during URI manipulation operations. When a developer uses methods like URI.join, URI#merge, or URI#+ to combine or modify URIs, and the operation results in a change to the host component, the userinfo (which typically contains username and password in the format user:password@host) is not properly stripped from the resulting URI.

This behavior violates the principle that credentials should only be transmitted to their intended destination. In scenarios where a base URI contains embedded credentials and is manipulated to point to a different host, the credentials are inadvertently carried over to the new destination, creating a significant information disclosure risk.

Root Cause

The root cause is improper handling of the userinfo component in the URI gem's join and merge logic. The URI specification expects that when the host changes, authentication credentials should not be preserved, as they were intended for the original host. The vulnerable versions fail to clear the userinfo field when the host portion of the URI is modified through the affected methods.

Attack Vector

An attacker could exploit this vulnerability in scenarios where an application:

1. Constructs URIs with embedded credentials for internal services
2. Uses URI.join, URI#merge, or URI#+ to manipulate these URIs
3. The manipulation results in the host being changed (intentionally or through user-controlled input)

The vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker who can influence the target host in a URI manipulation operation could cause credentials to be leaked to an attacker-controlled server.

The exploitation scenario involves applications that dynamically construct URLs with embedded credentials. When an attacker can influence the host component through input manipulation, the resulting URI retains the original credentials but points to an attacker-controlled destination. This allows credential harvesting without direct access to the original application's secrets.

Detection Methods for CVE-2025-27221

Indicators of Compromise

• Unexpected outbound HTTP requests containing userinfo (credentials) to external or suspicious hosts
• Log entries showing URI requests with user:password@ format being sent to unintended destinations
• Network traffic containing Basic authentication headers to third-party domains
• Application logs showing URI manipulation operations followed by requests to untrusted hosts

Detection Strategies

• Monitor outbound network connections for requests containing embedded credentials in URLs
• Implement application-level logging to track URI manipulation operations and their results
• Use static code analysis tools to identify usage of URI.join, URI#merge, or URI#+ with user-controlled inputs
• Deploy network monitoring to detect credential patterns in outbound request URLs

Monitoring Recommendations

• Configure web application firewalls to alert on outbound requests with embedded credentials
• Implement runtime application security testing to detect credential leakage patterns
• Review application dependency manifests to identify vulnerable URI gem versions
• Enable verbose logging for Ruby applications during security audits to trace URI manipulation flows

How to Mitigate CVE-2025-27221

Immediate Actions Required

• Upgrade the URI gem to version 1.0.3 or later immediately
• Audit application code for usage of URI.join, URI#merge, and URI#+ methods with credential-containing URIs
• Review network logs for potential historical credential leakage
• Consider rotating any credentials that may have been embedded in URIs processed by vulnerable code

Patch Information

The vulnerability has been addressed in URI gem version 1.0.3. Security advisories have been published through multiple channels including the Ruby Security Advisory Database and Debian LTS security announcements. Additional details about the vulnerability discovery are available in HackerOne Report #2957667.

Workarounds

• Avoid embedding credentials directly in URIs; use separate authentication mechanisms instead
• Manually strip userinfo from URIs before performing join or merge operations with untrusted hosts
• Implement input validation to prevent user-controlled data from influencing host components in URI operations
• Use environment variables or secure credential storage instead of embedding credentials in URI strings

# Configuration example - Update the URI gem
gem update uri

# Verify the installed version
gem list uri

# For Bundler-managed projects, update Gemfile
# gem 'uri', '>= 1.0.3'
bundle update uri