Snort HTTP Inspect Pre-Processor Uricontent Bypass Jun 01 2006 12:19AM
Christian Swartzbaugh (feofil gmail com)
For those of you using snort on this list, this got posted to some of
the snort mailing lists this morning.


"The evasion technique allows an attack to bypass detection of
"uricontent" rules by
adding a carriage return to the end of a URL, directly before the HTTP protocol
This affects thousands of rules in the standard Snort base rule sets.

Due to the seriousness of this vulnerability, we have developed a
working patch for
public review. See below.
This patch addresses the carriage return bug and should catch the known evasion
attempts but further research needs to be done to determine if there
are any other

possible impacts of this bug. The detection for evasion is turned on
by default under
all profiles but can also be used as a server configuration option:
-----HTTP Inspect Server Configuration-----

non_std_cr <yes|no>

This option generates an alert when a non standard carriage return character is
detected in the URI.

More information including a pre-patched tarball, a simple proof of
concept, and a

copy of this patch ..."


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