Zitadel is an open source identity management system. ZITADEL administrators can enable a setting called "Ignoring unknown usernames" which helps mitigate attacks that try to guess/enumerate usernames. If enabled, ZITADEL will show the password prompt even if the user doesn't exist and report "Username or Password invalid". Due to a implementation change to prevent deadlocks calling the database, the flag would not be correctly respected in all cases and an attacker would gain information if an account exist within ZITADEL, since the error message shows "object not found" instead of the generic error message. This vulnerability is fixed in 2.58.1, 2.57.1, 2.56.2, 2.55.5, 2.54.8, and 2.53.9.
The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not.