AltAuth has not been updated to recent Minecraft versions. Since Mojang accounts have become completly unusable I have no need anymore for the modification, although the banned and multiplayer disabled accounts functionality should still work.

AltAuth

AltAuth is a solution to allow players with Mojang account, (untested) banned accounts and (untested) multiplayer disabled accounts to join online mode Minecraft servers depending on client and server support. This works by redirecting the session server requests to a proxy specified by the server.

Trusting the AltAuth proxy session server used by the server is required, as a malicious proxy server can use your Minecraft account for up to 48 (Mojang account) or 24 (Microsoft account) hours on ALL servers. You can select in-game which AltAuth proxy servers you trust. A server using AltAuth has to trust its selected AltAuth proxy server that it prevents unauthenticated users from joining. More information on setting up an AltAuth proxy and a technical overview can be found on the GitHub page https://github.com/lixfel/AltAuth.