PHP Malware Finder
5 March, 2021 by
PHP Malware Finder
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https://github.com/nbs-system/php-malware-finder

 

What does it detect?

PHP-malware-finder does its very best to detect obfuscated/dodgy code as well as files using PHP functions often used in malwares/webshells.

The following list of encoders/obfuscators/webshells are also detected:

Of course it’s trivial to bypass PMF, but its goal is to catch kiddies and idiots, not people with a working brain.

If you report a stupid tailored bypass for PMF, you likely belong to one (or both) category, and should re-read the previous sentence.

How does it work?

Detection is performed by crawling the filesystem and testing files against a set of YARA rules. Yes, it’s that simple!

Instead of using an hash-based approach, PMF tries as much as possible to use semantic patterns, to detect things like “a$_GET variable is decoded two times, unziped, and then passed to some dangerous function like system“.

How to use it?

$ ./phpmalwarefinder -h
Usage phpmalwarefinder [-cfhtv] [-l (php|asp)] <file|folder> ...
    -c  Optional path to a configuration file
    -f  Fast mode
    -h  Show this help message
    -t  Specify the number of threads to use (8 by default)
    -v  Verbose mode
    -l  Set language ('asp', 'php')

Or if you prefer to use yara:

$ yara -r ./php.yar /var/www
$ yara -r ./asp.yar /var/www

Please keep in mind that you should use at least YARA 3.4 because we’re using hashes for the whitelist system, and greedy regexps. Please note that if you plan to build yara from sources, libssl-dev must be installed on your system in order to have support for hashes.

Ho, and by the way, you can run the comprehensive testsuite with make test.

Whitelisting

Check the whitelist.yar file. If you’re lazy, you can generate whitelists for entire folders with the generate_whitelist.py script.

Why should I use it instead of something else?

Because:

  • It doesn’t use a single rule per sample, since it only cares about finding malicious patterns, not specific webshells
  • Its whitelist system doesn’t rely on filenames
  • It doesn’t rely on (slow) entropy calculation
  • It uses a ghetto-style static analysis, instead of relying on file hashes
  • Thanks to the aforementioned pseudo-static analysis, it works on obfuscated files too

Licensing

PHP-malware-finder is licensed under the GNU General Public License v3.

The amazing YARA project is licensed under the Apache v2.0 license.

Patches, whitelists or samples are of course more than welcome.

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