The example code for the digest authentication functionality (http_authentication.rb) in Ruby on Rails before 2.3.3 defines an authenticate_or_request_with_http_digest block that returns nil instead of false when the user does not exist, which allows context-dependent attackers to bypass authentication for applications that are derived from this example by sending an invalid username without a password.
CWE-287 - Improper Authentication
Improper (or broken) authentication attacks are widespread, and have accounted for many of the worst data breaches in recent years. Improper authentication attacks are a class of vulnerabilities where an attacker impersonates a legitimate user by exploiting weaknesses in either session management or credential management to gain access to the user’s account. This can result in disclosure of sensitive information, and can lead to system compromise, theft, identity theft, and fraud.