Overview
CVE-2009-2422 describes a vulnerability in Ruby on Rails (pre-2.3.3) where the Digest HTTP authentication example in http_authentication.rb returns nil when the invoked username does not exist, rather than a value that signals authentication failure. This small mismatch allowed context-dependent attackers to bypass authentication by sending a deliberately invalid username against applications that borrowed the example code. The issue falls under CWE-287: Improper Authentication, since the guard logic could fail to consistently reject invalid credentials. In real deployments, this could let attackers access restricted resources if they relied on the sample code for digest authentication. The vulnerability is rooted in how the example handles the non-existent user path and could be leveraged when the application uses HTTP Digest without additional safeguards or modern authentication controls.
Affected Versions
Rails 2.x before 2.3.3 (specifically 2.3.0-2.3.2)
Code Fix Example
Ruby on Rails API Security Remediation
Vulnerable pattern (sample code returning nil for missing user):
authenticate_or_request_with_http_digest do |username|
user = User.find_by_username(username)
if user
Digest::MD5.hexdigest(user.password_digest)
else
nil
end
end
Fixed pattern (explicitly returning false for missing user, preventing bypass):
authenticate_or_request_with_http_digest do |username|
user = User.find_by_username(username)
if user
Digest::MD5.hexdigest(user.password_digest)
else
false
end
end