F5 BIG-IP devices suffer from a vulnerability in which not all bytes of the Finished packet in a TLS connection are validated. The sort-of technical description can be found here. F5 don't consider it a vulnerability, so they won't patch it, and it is currently causing me a headache right now for PCI compliance reasons. At the moment nobody seems to agree on whether there's a bug or not, and there's not really a clear explanation of the actual impact and attack requirements.
My understanding is that the Client and Server Finished packets in TLS are designed to post facto mutually authenticate the handshake messages, including the master secret key exchange. My thought process is that, if an attacker were to man-in-the-middle the TLS connection, they could leverage this vulnerability to downgrade the connection by spoofing the Client Finished packet, or possibly even compromise the master secret. I'm unsure as to how the process would go from there, with the validation of the Server Finished packet on the cient.
Assuming that some arbitrary number of bytes in the Finished packet are not properly validated, what's the real impact of this vulnerability? With this bug in play, does the security of the connection mostly rely on the client's validation of the Server Finished packet?