Description
After user deletion in MongoDB Server the improper invalidation of authorization sessions allows an authenticated user's session to persist and become conflated with new accounts, if those accounts reuse the names of deleted ones. This issue affects: MongoDB Inc. MongoDB Server v4.0 versions prior to 4.0.9; v3.6 versions prior to 3.6.13; v3.4 versions prior to 3.4.22.

Workarounds
After deleting one or more users, restart any nodes which may have had active user authorization sessions.

Refrain from creating user accounts with the same name as previously deleted accounts.

Credit
Discovered by Mitch Wasson of Cisco's Advanced Malware Protection Group.