Re: CAIS-ALERT: Vulnerability in the sending requests control of BIND

[Robert Tracz <rtracz () tele pw edu pl>]

Hi Ivan,

Ivn Arce wrote:
> > > +  /*
> > > +  * The 16 bit space is very small and brute force attempts are
> > > +  * entirly feasible, we skip a random number of transaction ids
> > > +  * so that an attacker will not get sequential ids.
> > > +  */
> >
> > Using only brute force, the attack is very difficult to be applied. I
> > tried this several times. I did several tests in my experiments. The
> > probability of success is very low to get implement the attack using
> > only brute force.
>
>
> The probability of sucess is exactly:
> m-responses-sent/65535
> If I sent 65535 DNS responses with a different ID on each one one of
> then will hit the right ID.
>
> The attack is basically the same.
> Either you sent N spoofed requests or you send M spoofed responses.
> The network traffic generated is also the same and in both cases
> there is still a race to win against the real DNS.

 As far as I understand the issue Vagner is right at this point. The
birthday paradox comes into play: If you send m requests and m
responses the probability of collision is:

p = 1 - 65535*(65535-1)*(65535-2)*...*(65535-m+1)/65535^m

In practice, if you send m = 256 responses and requests you have already
p = 39.2%, while if you would send 1 request and 511 responses (the
same traffic burden) you would get only p = 0.77%. And sending m = 1024
requests and responses gives you probability of success p = 99.9%.

However I agree with you that it would be better to enhance the
protocol.

Regards,

Robert