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Re: Re: Re: hash collision DOS

by crazyinsomniac (Prior)
on Jun 01, 2003 at 13:33 UTC ( #262202=note: print w/ replies, xml ) Need Help??


in reply to Re: Re: hash collision DOS
in thread hash collision DOS

It's like When Hashes Go Wrong, you just are not gonna run into it that often, if at all.

That's what I call a non-issue. Sure, keep an eye out, but don't strain your neck.

 
______crazyinsomniac_____________________________
Of all the things I've lost, I miss my mind the most.
perl -e "$q=$_;map({chr unpack qq;H*;,$_}split(q;;,q*H*));print;$q/$q;"


Comment on Re: Re: Re: hash collision DOS
Re: Re: Re: Re: hash collision DOS
by kschwab (Priest) on Jun 01, 2003 at 13:42 UTC
    Agreed. I do find it interesting that the authors of this white paper chose also chose to use 10,000 inputs to trigger the behavior. Hmm...
Re: Re: Re: Re: hash collision DOS
by iburrell (Chaplain) on Jun 02, 2003 at 20:57 UTC
    The problem is that the attacker is generating the strings that go into the hash table. He chooses strings that collide and produce the worst-case performance. The worst-case performance isn't likely in normal use, but is easy for a malicious attacker to construct the strings.

    There are some limits on how many strings can be inserted by an attacker. CGI.pm limits POST sizes. With a 1 MB limit and 10 bytes per string, that is 100,000 strings all trying to go into one hash bucket. Instead of taking a fraction of a second to parse, it takes the web server thousands of seconds.

    It would be possible to add checks to prevent this attack. One easy way is limit the number of parameters in CGI.pm. 1000 is probably a reasonable limit. The proper solution is change the Perl hashing function so it isn't deterministic. If the attacker can't predict the behavior, they can't the worst-case strings.

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