How To Detect Potentially Malicious PHP Files

How To Detect Potentially Malicious PHP Files ?

Here is the tool called PHP-malware-finder by nbs-system.

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:

Best PHP Obfuscator
Carbylamine
Cipher Design
Cyklodev
Joes Web Tools Obfuscator
P.A.S
PHP Jiami
Php Obfuscator Encode
SpinObf
Weevely3
atomiku
cobra obfuscator
phpencode
tennc
web-malware-collection
webtoolsvn
novahot
nano

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".

Installation

Install Yara.
This is also possible via some Linux package managers:
Debian: sudo apt-get install yara
Red Hat: yum install yara (requires the EPEL repository)

Download php-malware-finder
git clone https://github.com/nbs-system/php-malware-finder.git

How to use it?

$ ./phpmalwarefinder -h
Usage phpmalwarefinder [-cfhtvl] <file|folder> ...
-c Optional path to a rule file
-f Fast mode
-h Show this help message
-t Specify the number of threads to use (8 by default)
-v Verbose mode

Or if you prefer to use yara-

$ yara -r ./php.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.

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
It has a complete testsuite, to avoid regressions
Its whitelist system doesn't rely on filenames
It doesn't rely on (slow) entropy computation
It uses a ghetto-style static analysis, instead of relying on file hashes
Thanks to the aforementioned pseudo-static analysis, it works (especially) well on obfuscated files

Licensing

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

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

Download