Uninstalling Mattermost Launcher for Windows (64-bit), then reinstalling keeps you logged in without authentication

How it was found

Hello Team,

Hope you are doing great and enjoying a lot. This issue affected me directly and I was very amazed by it, so I felt it was important to report it in case it was not known. It is resulting unintended behavior:

In addition to this report is very similar to both of already been Resolved hackerone reports https://hackerone.com/reports/238260 https://hackerone.com/reports/1278261

Steps to reproduce:

1. Install The Mattermost Desktop App for Windows (64-bit).
2. Enter the Display name with Server URL
3. Login to Mattermost Desktop App
4. Uninstall Mattermost Desktop App
5. Reinstall Mattermost Desktop App

Conclusion: You will automatically be logged back in to your account, even though you uninstalled Mattermost Desktop App from your computer and did not enter a username/password to login to the fresh Mattermost Desktop App installation.

Thanks and have a good day ;) Regards @annonmous

Impact

The Mattermost Desktop App uninstall process is fully automatic, there is no prompt or indication that there is data left behind. I believe it is reasonable to expect that when uninstalling Mattermost Desktop App users session data should have been removed. If I am a user on a shared user account (for example, if I borrowed a computer and I installed Mattermost Desktop App, but uninstalled it later), they can take full control of my account after the fact.

When testing this, I could access all of my messages and data, and even access the Mattermost Desktop App admin panel for my team that I am an administrator of.