[Python-Dev] Status of the fix for the hash collision vulnerability (original) (raw)

Tim Delaney timothy.c.delaney at gmail.com
Tue Jan 17 00:17:05 CET 2012

On 17 January 2012 10:14, Tim Delaney <timothy.c.delaney at gmail.com> wrote:

On 17 January 2012 09:23, Paul McMillan <paul at mcmillan.ws> wrote:

This is why the "simply throw an error" solution isn't a complete fix. Making portions of an interface unusable for regular users is clearly a bad thing, and is clearly applicable to other types of poisoned data as well. We need to detect collisions and work around them transparently. What if in a pathological collision (e.g. > 1000 collisions), we increased the size of a dict by a small but random amount? Should be transparent, have neglible speed penalty, maximal reuse of existing code, and should be very difficult to attack since the dictionary would change size in a (near) non-deterministic manner when being attacked (i.e. first attack causes non-deterministic remap, next attack should fail). It should also have near-zero effect on existing tests and frameworks since we would only get the non-deterministic behaviour in pathological cases, which we would presumably need new tests for. Thoughts?

And one thought I had immediately after hitting send is that there could be an attack of the form "build a huge dict, then hit it with something that causes it to rehash due to >1000 collisions". But that's not really going to be any worse than just building a huge dict and hitting a resize anyway.

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