Da: Urabe S. [mailto:[email protected]]
Inviato: mercoled 28 dicembre 2011 14:33
A: ruby-talk ML; [email protected]
Oggetto: [ANN] CVE-2011-4815: Denial of service attack was found for
Denial of service attack was found for Ruby’s Hash algorithm
This is something related to computational complexity. Specially
crafted series of strings that intentionally collide their hash values
each other was found. With such sequences an attacker can issue a
denial of service attack by, for instance, giving them as POST
parameters of HTTP requests for your Rails application.
The situation is similar to the one found for Perl in 2003. In 1.8
series of Ruby, we use a deterministic hash function to hash a string.
Here the “deterministic” means no other bits of information than the
string itself is involved to generate a hash value. So you can
precalculate a string’s hash value beforehand. By collecting a series
strings that have the identical hash value, an attacker can let ruby
process collide bins of hash tables (including
instances). Hash tables’ amortized O(1) attribute depends on
uniformity of distribution of hash values. By giving such crafted
input, an attacker can let hash tables work much slower than expected
(namely O(n2) to construct a n-elements table this case).
- Ruby 1.8.7-p352 and all prior versions.
All Ruby 1.9 series are not affected by this kind of attack. They do
share hash implementations with Ruby 1.8 series.
Our solution is to scramble the string hash function by some
PRNG-generated random bits. By doing so a string’s hashed value is no
longer deterministic. That is, a
String#hash result is consistent
only for current process lifetime and will generate a different number
the next boot. To break this situation an attacker must create a set
strings which are robust to this kind of scrambling. This is believed
be quite difficult.
Please upgrade to the latest version of ruby via my previous post.
Bear in mind that the solution does not mean our hash
algorithm is cryptographically secure. To put it simple, we fixed
the hash table but we didn’t fix
String#hash weakness. An
attacker could still exploit it once he / she got a pair of a string
and its hash value returned from
String#hash. You must not
String#hash outputs. If you need to do such things,
consider using secure hash algorithms instead. Some of them (such
as SHA256) are provided in Ruby’s standard library.
For those who knows alternative hash algorithms inside our code
base: we do not support them (they are disabled by default). By
choosing them we consider you can read C, and you can understand
what was wrong with the default one. Make sure that your choice is
safe at your own risk.
Credit to Alexander Klink [email protected] and Julian
Waelde [email protected] for reporting this
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