Validation of an MLS (RFC 9420) external commit's proposal list in the `validateExternalCachedProposals()` function of `org.bouncycastle.mls.protocol.Group` failed to verify that a removed leaf was associated with the joiner. While RFC 9420 allows a joiner to remove an old version of themselves via a Remove proposal, the implementation did not ensure the LeafNode met the necessary credential criteria for the removed participant. Consequently, any party with the group's public GroupInfo could submit a Remove proposal specifying any member's `LeafIndex`, allowing them to evict that member and occupy their slot in the ratchet tree. This issue affected callers of the `Group.externalJoin` and `Group.handle` APIs, as the required credential check was only present in the gRPC interop harness.