ModSecurity ASCIIZ Evasion Issue

It has been brought to our attention that a fault in the ModSecurity
parsing code has been discovered and published. (No, we have not been
contacted by the author, either before or after the publication. We
learned about the problem from a third party.) The fault makes it
possible to sneak an attack payload past a class (not all, as you will
see below) of ModSecurity rules.

The rules that use variables that refer to request parameters (e.g.
ARGS) can be evaded in certain circumstances, as follows: 1)
parameters must be transported in an application/x-www-form-urlencoded
request body,  2) an un-encoded NULL byte (ASCIIZ) must be embedded in
the payload and 3) the parser used by the web application must do
things differently. (Apparently, PHP < 5.2.0 cannot be attacked in
this way but >= 5.2.0 can.) The ASCIIZ byte will cause the parser to
terminate prematurely, missing some data.

The logging features are not affected.

More information is available here:
http://www.php-security.org/MOPB/BONUS-12-2007.html

A ModSecurity update will be released to deal with this issue.
However, an update is not the only way to deal with the problem. it is
possible to use ModSecurity itself to detect and prevent evasion.
Simply add the following rule to your rule set:

SecRule REQUEST_BODY " <at> validateByteRange 1-255" \
"log,deny,phase:2,t:none,msg:'ModSecurity ASCIIZ Evasion Attempt'"

All your other rules can remain intact. The above rule should not
result in false positives under normal circumstances.

Please note that, not having been notified in advance, there was not
enough time to research all aspects of this evasion technique as
thoroughly as we would like. We are sending this email to notify our
users and help them mitigate the problem quickly. The information
presented here may end up being a work in progress, in which case we
will follow up as soon as we learn more.

And, for the record, I am not at all happy with the fact the issue was
not disclosed to us in advance. We take security very seriously; a
responsible disclosure would have allowed us to have an updated
version of ModSecurity available for download at the same time as the
disclosure.

--

-- 
Ivan Ristic

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