Re: [EuroHaCk] Linux 2.2.x ISN vulnerability (fwd)

[scut () NB IN-BERLIN DE (Sebastian)]

Hi.

On Tue, 28 Sep 1999, Jeremy Buhler wrote:

> > A weakness within the TCP stack in Linux 2.2.x kernels
> > has been discovered. The vulnerability makes it possible
> > to "blind-spoof" TCP connections.

> This vulnerability is fixed in kernels 2.2.13pre13 and
> later.  Hopefully 2.2.13 will be released shortly and/or
> the relevant patch from pre13 will be released as an
> erratum versus 2.2.12.  Alan?

A lot of people wrote to us and said that the typo where &secret[3] was
meant but &secret+3 was written within the kernel code was the cause for
this bug.

In our tests, however, we applied nothing but this one-line patch and we
still experienced small-difference ISN's (stealth tested this, but I
believe in his skills :-).

I rather think that changing only a few bits as hash input for the MD4
hash routine is the reason for the weak hash result. Even if the whole
secret rest is zero'd out, a strong hash algorithm should make this up and
create a totally different hash value. As said in (if I remember
correctly) Applied Cryptography every input bit within the hash input
should be able to modify every bit in the hash output value, which is here
obviously not the case.

However, simply using a strong PRNG (as I naivly suggested a few days ago)
is not a solution, as Alan pointed out, there are strict rules for TCP
sequence numbers to avoid data corruption.

ciao,
scut / teso security
[http://teso.scene.at/]

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