Re: Attacking BFD with NULL auth

[John Scudder <[e-mail]>]

You’re assuming either an on-LAN attacker (and therefore, that BFD is being used on a multiaccess medium) or multihop BFD here, I take it? Because RFC 5881 tells me GTSM is required if there’s no other authentication.

—John

> On Feb 6, 2024, at 8:56 AM, Jeffrey Haas <[e-mail]> wrote:
> 
> 
> My thought over first cup of caffeine for the morning: You can have an active attacker attack a session using NULL auth and knock over a BFD session.  This is counter to the usual "silly" attack of keeping BFD Up.
> 
> Presume the session is in the Up state between A and B and using NULL auth.  The current expected sequence number at A from B is 100.
> 
> An active attacker, seeing that 100 is the sequence number, spoofs packets rapidly in order 101..200.
> 
> Sequence number procedures are, tersely, "accept the larger sequence number as long as it's bigger".  Presume that some portion of that spray of packets gets through and moves the sequence number > 100 + 3 before B would have sent sequence 101.
> 
> B then continues happily sending the meticulously increasing packets, 101, 102, 103.  These packets are discarded because the sequence number is under the "last seen" sequence number.
> 
> The session drops.
> 
> I don't believe there is any mitigation against this attack in NULL auth.
> 
> The impacts of this, if so:
> 1. NULL auth and using the sequence numbers becomes impractical to use for optimizing authentication procedures.  ISAAC and no-auth still work.
> 2. BFD stability really wants that increasing sequence number.  This leads to using either meticulous types from the strong authentication mechanisms, or ISAAC.
> 
> Counter observation 1: Stability doesn't really care about the sequence numbers from a security standpoint, just a dropped packet standpoint.  The attack against stability if the sequence numbers aren't used for authentication of the session to drop the session is to trigger an "unstable" event and whatever trigger might be tied to that mechanism as a client.
> 
> Counter observation 2: If the sequence numbers are ignored as a mechanism for taking the session down, you can't use it to prevent PITM attacks, but it's no worse than no-auth.  The periodic strong authentication becomes more important.
> 
> 
> -- Jeff
> 
> 
>