[Python-Dev] Algoritmic Complexity Attack on Python
Phillip J. Eby
Sat, 31 May 2003 09:17:16 -0400
At 05:02 PM 5/30/03 -0500, Scott A Crosby wrote:
>The python community will have to judge whether the performance
>difference of the current hash is worth the risk of the attack.
Note that the "community" doesn't really have to judge. An individual
developer can, if they have an application they deem vulnerable, do
something like this:
# code to return a hash code
safe = SafeString(string_from_untrusted_source)
and then use only these "safe" strings as keys for a given dictionary. Or,
with a little more work, they can subclass 'dict' and make a dictionary
that converts its keys to "safe" strings.
As far as current vulnerability goes, I'd say that the most commonly
available attack point for this would be CGI programs that accept POST
operations. A POST can supply an arbitrarily large number of form field keys.
If you can show that the Python 'cgi' module is vulnerable to such an
attack, in a dramatic disproportion to the size of the data transmitted
(since obviously it's as much of a DoS to flood a script with a large
quantity of data), then it might be worth making changes to the 'cgi'
module, or at least warning the developers of alternatives to CGI (e.g.
Zope, Quixote, SkunkWeb, CherryPy, etc.) that alternate hashes might be a
But based on the discussion so far, I'm not sure I see how this attack
would produce an effect that was dramatically disproportionate to the
amount of data transmitted.