User impersonation due to incorrect handling of the login JWT

Impact

This allows anyone that can connect to the server to forge a LoginPacket with manipulated JWT token allowing impersonation as any Bedrock user. Unless credentials are saved in your configuration, online mode is not affected as users are still required to log in separately. If your credentials are saved, there is no risk of exposing your email or password.

Patches

This was patched as part of https://github.com/GeyserMC/Geyser/commit/b9541505af68ac7b7c093206ac7b1ba88957a5a6 and https://github.com/GeyserMC/Geyser/commit/ab2f5b326fe590e09167e8b45b4b165ac06ecd13. if your Geyser version is 1.4.2-SNAPSHOT or later, the issue has been addressed on your build.

Workarounds

Geyser strongly recommends updating to fix this issue. If this isn’t possible:

• Use online mode and don’t save credentials in your Geyser configuration
• Use an additional authentication method on the Java server

References

This was disclosed to us by a staff member over at Hive; you can read their disclosure here: https://updates.playhive.com/weekend-maintenance-disclosure-2kJMaY

For more information

If you have any questions or comments about this advisory:

• Come talk to us over on our Discord server in the #development channel