Flask-AppBuilder is an application development framework, built on top of the Flask web framework. In affected versions there exists a user enumeration vulnerability. This vulnerability allows for a non authenticated user to enumerate existing accounts by timing the response time from the server when you are logging in. Users are advised to upgrade to version 3.4.4rc1 as soon as possible. There are no known workarounds for this issue.
CWE-203 - Observable Discrepancy
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.