CVE-2021-22885: Rails Action Pack Information Disclosure

CVE-2021-22885 Overview

CVE-2021-22885 is an information disclosure and unintended method execution vulnerability affecting Ruby on Rails Action Pack versions 2.0.0 and later. The vulnerability exists when using the redirect_to or polymorphic_url helper methods with untrusted user input, potentially allowing attackers to extract sensitive information or trigger unintended application behavior.

Critical Impact

Attackers can exploit this vulnerability to disclose sensitive application information or execute unintended methods through crafted user input to redirect helpers, potentially compromising application security and exposing confidential data.

Affected Products

• Ruby on Rails (versions >= 2.0.0)
• Ruby on Rails ActionPack Page-Caching
• Debian Linux 10.0

Discovery Timeline

• May 27, 2021 - CVE-2021-22885 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-22885

Vulnerability Analysis

This vulnerability is classified under CWE-209 (Generation of Error Message Containing Sensitive Information). The core issue lies in how Action Pack processes user-controlled input when passed to the redirect_to or polymorphic_url helper methods.

When these helpers receive untrusted input, they may inadvertently expose internal application details or allow attackers to invoke methods that were not intended to be accessible. This occurs because the URL generation logic does not adequately validate or sanitize the input before processing, creating an avenue for information leakage.

The vulnerability is particularly concerning in web applications that dynamically construct redirect URLs or polymorphic routes based on user-supplied parameters without proper validation.

Root Cause

The root cause stems from insufficient input validation in Action Pack's URL helper methods. When redirect_to or polymorphic_url receives user-controlled data, the framework processes this input through its routing system without adequate sanitization. This allows attackers to potentially manipulate the URL generation process to expose sensitive information or trigger unintended method calls within the application context.

Attack Vector

The vulnerability is exploitable over the network and requires no authentication or user interaction. An attacker can craft malicious input and submit it to endpoints that use the vulnerable redirect_to or polymorphic_url helpers with user-controlled parameters.

The attack involves sending specially crafted requests to application endpoints that pass user input directly to these helper methods. When the application processes this input, it may generate error messages containing sensitive information or execute methods that reveal internal application state.

Detection Methods for CVE-2021-22885

Indicators of Compromise

• Unusual redirect behavior or unexpected URL patterns in application logs
• Error messages in responses containing internal application paths or method names
• Anomalous requests to endpoints that handle redirects with suspicious parameter values
• Increased volume of requests probing redirect functionality with varied payloads

Detection Strategies

• Monitor web application logs for abnormal redirect patterns or error responses from redirect_to handlers
• Implement web application firewall (WAF) rules to detect malformed or suspicious redirect parameters
• Deploy application-level logging to capture all inputs passed to redirect_to and polymorphic_url methods
• Use static code analysis tools to identify instances where untrusted input flows to vulnerable helper methods

Monitoring Recommendations

• Enable detailed logging for all redirect operations within Rails applications
• Set up alerts for error rate spikes related to routing or URL generation
• Monitor for information disclosure patterns in HTTP responses
• Implement runtime application self-protection (RASP) to detect exploitation attempts

How to Mitigate CVE-2021-22885

Immediate Actions Required

• Update Ruby on Rails to the latest patched version addressing CVE-2021-22885
• Audit all code paths that use redirect_to or polymorphic_url with user-supplied input
• Implement strict input validation before passing any user data to redirect helpers
• Review and sanitize all dynamic URL generation logic in the application

Patch Information

Security patches are available through official Ruby on Rails releases. Organizations should consult the Debian Security Advisory DSA-4929 for Debian-specific updates. Additional guidance is available through the NetApp Security Advisory NTAP-20210805-0009. The original vulnerability report can be found at HackerOne Report #1106652.

Workarounds

• Avoid passing untrusted user input directly to redirect_to or polymorphic_url methods
• Implement a whitelist of allowed redirect destinations and validate all redirect targets against this list
• Use explicit URL construction instead of polymorphic helpers when dealing with user-controlled data
• Add custom validation middleware to sanitize redirect parameters before they reach the application

# Configuration example - Add input validation in ApplicationController
# config/application.rb
# Ensure only permitted redirect targets are allowed
# Example: Define ALLOWED_REDIRECT_HOSTS = ['example.com', 'app.example.com']
# Validate redirect URLs against whitelist before processing