Re: [tsvwg] Regarding DTLS and UDP options

[Tom Herbert <[e-mail]>]

>> A related concern I have is around security and whether ambiguity can
>> be exploited. For instance, someone could easily forge a UDP fragment
>> using the UDP option. If a device supports UDP options they will
>> interpret this as fragment, if they don't they will see this as plain
>> UDP packet. This are very different interpretations of the same bits
>> however both devices could claim protocol conformance-- that's a
>> potential problem. This becomes a real problem if the ambiguity can be
>> somehow exploited to bypass security.
> UDP packets and TCP packets can already be forged.
>
> If that's your concern, you need to use true transport-layer or network
> layer security.
>
1) Yes, that is my concern. UDP is already dicey in terms of security,
the risks introduced with these extensions needs to be thoroughly
vetted before I'd ever deploy in my network
2) It can't be a requirement to turn up security everywhere just to
use UDP options
3) My example is a contrived attack against the new protocol that
might be deployed, the mitigations to defend such an attack should be
part of the protocol not deferred to some other layer

> And endpoints can claim conformance all they want; forged packets are
> already noncompliant. No protocol is robust to forged packets except
> those that support authentication or encryption.
>
That's misses the crux of my argument. In my example, an attacker
forged a packet with can be "correctly" interpreted as either fragment
or a non-fragment. Which interpretation depends on the node that
processes it (sort of a Copenhagen Interpretation for network
protocols I suppose :-) ). But a packet cannot simultaneously be a
fragment and a not fragment, that is nonsensical (packets don't have
duality in this instance). This must be considered an error condition.
This ambiguity does not occur in IP or IPv6 and has been introduced by
the protocol specification not incorrect implementation. It's unlikely
in practice that this would cause problems, however, as I mentioned
above, if this can be exploited as a security vulnerability that will
be a concern of mine.

I thought a one point there was the idea that the UDP checksum would
be incorrect so that in the case I describe above a fragment would be
dropped and not misinterpreted by a node that doesn't support
fragments. Is there a reason that wouldn't work?

Tom