[Python-Dev] Status of the fix for the hash collision vulnerability

Victor Stinner victor.stinner at haypocalc.com
Fri Jan 13 02:24:33 CET 2012

Many people proposed their own idea to fix the vulnerability, but only
3 wrote a patch:

- Glenn Linderman proposes to fix the vulnerability by adding a new
"safe" dict type (only accepting string keys). His proof-of-concept
(SafeDict.py) uses a secret of 64 random bits and uses it to compute
the hash of a key.
- Marc Andre Lemburg proposes to fix the vulnerability directly in
dict (for any key type). The patch raises an exception if a lookup
causes more than 1000 collisions.
- I propose to fix the vulnerability only in the Unicode hash (not for
other types). My patch adds a random secret initialized at startup (it
can be disabled or fixed using an environment variable).


I consider that Glenn's proposition is not applicable in practice
because all applications and all libraries have to be patched to use
the new "safe" dict type.

Some people are concerned by possible regression introduced by Marc's
proposition: his patch may raise an exception for legitimate data.

My proposition tries to be "just enough" secure with a low (runtime
performance) overhead. My patch becomes huge (and so backporting is
more complex), whereas Marc's patch is very simple and so trivial to


It is still unclear to me if the fix should be enabled by default for
Python < 3.3. Because the overhead (of my patch) is low, I would
prefer to enable the fix by default, to protect everyone with a simple
Python upgrade.

I prefer to explain how to disable explicitly the randomized hash
(PYTHONHASHSEED=0) (or how to fix application bugs) to people having
troubles with randomized hash, instead of leaving the hole open by


We might change hash() for types other than str, but it looks like web
servers are only concerned by dict with string keys.

We may use Paul's hash function if mine is not enough secure.

My patch doesn't fix the DoS, it just make the attack more complex.
The attacker cannot pregenerate data for an attack: (s)he has first to
compute the hash secret, and then compute hash collisions using the
secret. The hash secret is a least 64 bits long (128 bits on a 64 bit
system). So I hope that computing collisions requires a lot of CPU
time (is slow) to make the attack ineffective with today computers.


I plan to write a nice patch for Python 3.3, then write a simpler
patch for 3.1 and 3.2 (duplicate os.urandom code to keep it unchanged,
maybe don't create a new random.c file, maybe don't touch the test
suite while the patch breaks many tests), and finally write patches
for Python 2.6 and 2.7.

Details about my patch:

- I tested it on Linux (32 and 64 bits) and Windows (Seven 64 bits)
- a new PYTHONSEED environment variable allow to control the
randomized hash: PYTHONSEED=0 disables completly the randomized hash
(restore the previous behaviour), PYTHONSEED=value uses a fixed seed
for processes sharing data and needind same hash values
(multiprocessing users?)
- no overhead on hash(str)
- no startup overhead on Linux
- startup overhead is 10% on Windows (see the issue, I propose another
solution with a startup overhead of 1%)

The patch is not done, some tests are still failing because of the
randomized hash.


FYI, PHP released a version 5.3.9 adding "max_input_vars directive to
prevent attacks based on hash collisions (CVE-2011-4885)".


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