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An existing premium user was matched by username only. After a username was released and reclaimed by another premium account, the new owner logged into the previous user's identity and inherited its permissions. Compare the stored mojangUuid with the Mojang-verified profile and deny the login on mismatch.
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Problem
On login of an existing premium user, NavAuth matches the account by username only. In
UsernameResolutionServicethe branch for an existing premium user (existingUserIgnoreCase != null && isPremium) never compares the storedmojangUuidwith the Mojang verified UUID. After a username is released and reclaimed by a different premium account (Mojang frees a released name after about 37 days), the new owner logs into the previous user's stored account: same UUID, no password, and any permissions bound to that UUID (for example operator).Fix
In that branch, when the name is premium and the stored
mojangUuiddiffers from the Mojang verified profile UUID, deny the login with a dedicated reason and message instead of returning success.Verified
Reproduced end to end on Velocity plus Paper. Before the fix, a premium account with a different Mojang UUID received the stored victim UUID with no password and could run operator commands. After the fix the same login is rejected at PreLogin and never reaches the backend. The UUID mismatch state was seeded directly in the database, since the name release itself is Mojang policy and not part of the plugin.