Re: [quicwg/base-drafts] Add initial threat model to security considerations (#2925)

[Martin Thomson <[e-mail]>]

martinthomson commented on this pull request.



> +{{handshake-properties}}.  Similarly, any active attacker that observes QUIC
+packets and attempts to insert new data or modify existing data in those packets
+should not be able to generate packets deemed valid by the receiving endpoint.
+
+A spoofing attack, in which an active attacker rewrites unprotected parts of a
+QUIC packet that it forwards or injects, such as the source or destination
+address, is only effective if the attacker can forward packets to the original
+endpoint.  Packet protection ensures that the packet payloads can only be
+processed by the endpoints that completed the handshake, and invalid QUIC
+packets are ignored by those endpoints.
+
+An attacker can also modify the boundaries between QUIC packets and UDP
+datagrams, causing multiple packets to be coalesced into a single datagram, or
+splitting coalesced packets into multiple datagrams.  Such modification has no
+functional effect on a QUIC connection, although it might change the performance
+characteristics exhibited by the receiving endpoint.

Of note here is that we allow Initial packets to be smaller than 1200 bytes, as long as they are coalesced with other stuff to make up a 1200 byte UDP datagram.  That too is vulnerable to this sort of composition problem.  I think that it's fine to make these assumptions and risk modification as long as they are recognized as such.  For Initial, we are doing so on the understanding that elements on the path are able to disrupt the handshake at that point anyway.  For PMTUD, we might make a different assessment as the conditions are different.

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