Impact

The server uses ECDH to calculate a shared secret for the symmetric encryption key used to encrypt network packets after logging in. ECDH requires that the keys used must both belong to the same elliptic curve. In Minecraft: Bedrock Edition, the curve used is secp384r1.

Using any other curve (for example secp256r1) to sign the LoginPacket JWTs would lead to successfully verifying the login chain, but would later crash due to an uncaught exception during ECDH key derivation due to the client-provided key belonging to a different curve than the server's key. It's also theoretically possible that a non-EC key could be used (e.g. RSA or DH), which would also pass login verification as long as SHA384 hashing algorithm was used for the JWT signatures, and also lead to a crash.

Patches

The problem was fixed in 4.23.1 and 5.3.1 in the following commit: 4e646d19a4a1e0d082bd4d1f5a58ae0182a268d9 While 4.x would not have crashed when this was encountered, the faulty validation code has also been corrected there.

Workarounds

A plugin could handle LoginPacket and check that all of the identityPublicKeys provided in the JWT bodies actually belong to secp384r1. This can be checked by verifying that openssl_pkey_get_details($key)["ec"]["curve_name"] is set and equal to secp384r1. Beware that this element may not exist if the key is not an EC key at all.