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Comment by willscott | Hacker News Reader

From the linked paper, the 'trick' here is for the attacker to improperly respond to a tcp connection initiated from the target.

The standard handshake looks like:

Syn ->

<- Syn / Ack

Syn ->

The 'split handshake' looks like:

Syn ->

<- Syn

Syn / Ack ->

<- Ack

As a result, the hypothesis is that the intermediary router could incorrectly flip the roles of client and server, and incorrectly forward untrusted packets from the outside 'client'.

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I guess I don't fully understand the NSS labs attack here. In both of the scenarios laid out by Macrothink it appears that control of the server (that would be protected by the firewall) is needed for the attack to be carried out. Can an outside attacker really gain access to an otherwise blocked server this way, or is this more of a case of evading logging or IDS/IPS rules? Unless I misunderstand (which seems likely) it seems something like disabling the security cameras after you break into a bank so that they can't record you taking the money out.

Could anyone explain to a simpleton how the NSS version of the attack works using only client access?

The "server" doesn't necessarily need to be the target in an attack. The attacker would be in control of the server in this scenario, and the "client" is a machine within the targeted network. The "client" and "server" labels are only representative of who is initiating the tcp connection.

Hi! Thanks for your response. I didn't mean to get us hung up on the terminology. Let's call one machine the "Internet side" and one machine the "Protected side". I have read the Macrothink paper another time, and I still believe that you need to modify the behavior of the "Protected side" host (ie, you'd need to have prior access) before this attack would work. Is that incorrect?

Unfortunately, I haven't had time to read through the paper, and only got through the first few pages. From what I see though, the point of the attack is to punch through by using the improper handling of the split handshake by a client, which apparently fools some firewalls. For this to happen, you would only need to get a client to open a tcp connection to your malicious server, which is trivial. I'll have to take a closer look at the paper later on to to give more detail though. Hopefully someone else will chime in in the meantime.

And, from the Macrothink paper, it looks like it's not so much "open an inbound connection through the firewall" as "trick the firewall into not scanning the payload". Not relevant for, say, home routers that don't do scanning in the first place.

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