Zulip is an open source group chat application. Starting with version 4.0 and prior to version 4.11 Zulip is vulnerable to a race condition during account deactivation where a simultaneous access by the user being deactivated may in rare cases allow continued access by the deactivated user. A patch is available in version 4.11 on the 4.x branch and version 5.0-rc1 on the 5.x branch. Upgrading to a fixed version will as a side effect deactivate any cached sessions that may have been leaked through this bug. There are currently no known workarounds.