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Wesley Shields wxsBSD

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View gist:8c92d10dfad9d4b38b04

I was recently asked how to check the entropy of a given section in YARA, and because the person who asked is clearly looking to learn how to fish instead of just being given fish I went into some detail on the explanation. With his permission I am sharing my response here.

It's a combination of a number of things:

math.in_range(test, lower, upper):

Given a test value, check to see if it is in range of the lower and upper bounds. This is an inclusive test.

math.entropy(offset, length):

View gist:476c652a772465075d46

Here's what I was thinking of doing...

{
  // Description of the YARA rules to use. Each key is a group name used in additional_monitoring
  // or in scheduled_queries if you want.
  "yara": {
    "sig_group_1": [ "foo.sig", "bar.sig" ],
    "sig_group_2": [ "baz.sig" ]
  },
View gist:4d5d7677578f80cdf82a

Someone recently asked me if it is possible to test if a string is in a section or not in YARA. This is my attempt at an answer, and please note that some of the capabilities are still pending a merge to master.

// Make sure the string is in the .rsrc section.
rule test_in {
  strings:
    $a = { DE AD BE EF 00 00 DE AD BE EF }
  condition:
    $a in ((pe.sections[pe.section_index(".rsrc")].raw_data_offset)..(pe.sections[pe.section_index(".rsrc")].raw_data_offset + pe.sections[pe.section_index(".rsrc")].raw_data_size))
}
View gist:6ba82bcb8f658ed71b84

With the release of YARA 3.2.0 I wanted to show people how to utilize some of the new features. In particular I'll show an example for rich_signatures, import hashing and Authenticode signatures.

You'll need both of these for obvious reasons. :)

import "pe"
import "hash"
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