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Re: TCP/BGP vulnerability - easier than you think

  • From: Priscilla Oppenheimer
  • Date: Tue Apr 27 12:30:07 2004


I didn't have it quite right though because I said the host that sends the ACK could keep sending and then that would trigger a RST. But the ACK triggers an immediate RST from the host that sent the original RST. This time the RST has the right sequence number (derived from the ACK).

Maybe I would have gotten more responses if I hadn't admitted this was for my students. :-) I wear many hats and do work as a network operator too (but for a small network).

Cheers,

Priscilla Oppenheimer


At 09:14 AM 4/27/04, Iljitsch van Beijnum wrote:
On 27-apr-04, at 5:03, Priscilla Oppenheimer wrote:

   C) If the RST bit is set and the sequence number does not exactly
      match the next expected sequence value, yet is within the
      acceptable window (RCV.NXT < SEG.SEQ <= RCV.NXT+RCV.WND) send an
      acknowledgment.

So, per item C, does the recipient of a RST with a sequence number that does not exactly match the next expected sequence value not reset the connection? It sends an ACK but keeps the connection open?
Looks that way to me.

The ACK will go to the correct TCP partner, not the attacker presumably. So then that partner resets. But where does this leave the other partner (the recipient of the RST)? Is the assumption that this side may continue sending, which would cause the other side to RST (since it closed the session) and this RST would have the correct sequence number so the connection would get reset from both partners' points of view?
Yes. I think the idea is that if A has the session open and B has it closed, then the only real RSTs will be those that B sends for packets it receives from A. If those packets have and acknowledgement number in them (which should always be the case for established sessions) then this will be the RST's sequence number so there will be a perfect match between what A expects from an RST and what B sends.

The situation where B legitimately sends a sequence number that isn't an exact match is hard to imagine, as the ACKs A sends depend on the sequence numbers B sent and if the session is dead at B's end presumably B isn't burning up too many sequence numbers. But if this happens for some reason and A sends a dataless ACK packet obviously B will respond with an RST so we're back to the situation where B is sending an RST with the sequence number that A expects.

_______________________________

Priscilla Oppenheimer
www.priscilla.com

When your Daemon is in charge, do not try to think consciously. Drift, wait, and obey. -- Kipling.