full-disclosure - Re: TCP Hijacking (aka Man-in-the-Middle)

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Date: Thu, 25 Oct 2007 12:23:00 -0500
From: reepex <reepex@...il.com>
To: Oliver <olivereatsolives@...il.com>, full-disclosure@...ts.grok.org.uk
Subject: Re: TCP Hijacking (aka Man-in-the-Middle)

Hi I am sorry to hear you just woke from your coma. It is now 2007 not 1995.

On 10/25/07, Oliver <olivereatsolives@...il.com> wrote:
>
>
> Hello,
>
> I have been searching all over the place to find an answer to this question,
> but Google has made me feel unlucky these last few days. I hope I could find
> more expertise here. The burning question I have been pondering over is -
> could TCP connections be hijacked both ways? I know there are tools ( e.g.
> Hunt) that sniffs traffic and could arbitrarily reset a connection by
> spoofing the IP and MAC address. But could there be more than just that? Is
> it theoretically possible to not reset the connection with the server or the
> client, but play the man-in-the-middle attack?
>
> An example network scenario of this that I could come up with is that the
> hacker is within the same network as the victim (client), who is connected
> to a server through a persistent TCP connection. Now the hacker could
> pretend to be the server and send a TCP message (not reset/fin) to the
> client and change the seq/ack numbers on the client side, and the hacker
> could pretend to be the client and send a TCP message (not reset/fin) to the
> server and change the seq/ack there. Thus, the seq/ack numbers are
> completely out of sync for the client and server and thus would not
> recognize each others messages. At this point, the hacker could relay ( i.e.
> be man-in-the-middle) the messages from the client to the server and vice
> versa, using the seq/ack numbers that they would accept. While this seems
> pretty pointless so far, the hacker could inject messages at will to either
> side of the connection, and still make the server and client believe that
> they are in sync with each other ( i.e. this would not work if the hacker
> does not relay the messages with the seq/ack numbers the server and client
> would accept). That means the hacker goes undetected and could do whatever
> he chooses, as he has "hijacked" the connection.
>
> Is this possible? Assuming there is no hardware limitation (e.g.
> router/switch blocking MAC/IP addresses from certain port). Would the TCP
> protocol definition and implementation in Windows and *nixes these days
> would interpret this behaviour correctly (correctly for the hacker,
> incorrectly for themselves)? I imagine it would be quite a bit of work
> proving this theory and perhaps some of you could enlighten me or dismiss
> this concept.
>
> Regards,
> Oliver
>
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