It is possible to check if a user exists in Bitbucket (either internal or external directories) on a login page

Issue Summary

It is possible to check if a user exists in Bitbucket (either internal or external directories) on a login page by typing a username & incorrect password combination.

This is reproducible on Data Center: (yes)

Steps to Reproduce

1. Go to the Bitbucket logging page.
2. Try to log in with a user that exists in any of the Bitbucket user's directories.
3. Try to log in with a user that doesn't exist in any of the Bitbucket user's directories.

Expected Results

Error messages in both cases are the same:

Actual Results

Error messages are different in both cases allowing usernames discovery:

User exists in a directory:

User doesn't exist in a directory:

Workaround

Currently, there is no known workaround for this behavior. A workaround will be added here when available