stoerchl / yara_zip_module

yara_zip_module

This yara module can be used to search for strings inside a zip (.docx word file format) file. The files inside a zip are compressed and therefore not very well searchable for strings. This yara module unzips a requested file in memory and searches for a given string.

Installation

The installation of the module should be pretty simple, but yara has to be built from source..

1. Clone the yara repository (https://github.com/VirusTotal/yara)
2. Copy yara_zip_modules files into the libyara folder of the yara clone:

	yara_zip_module/miniz.c --> yara/libyara/miniz.c
	
	yara_zip_module/include/yara/miniz.h --> yara/libyara/include/yara/miniz.h

	yara_zip_module/modules/zip.c --> yara/libyara/modules/zip.c

3. Edit the file yara/libyara/Makefile.am and add the module as well as the miniz library:

	MODULES += modules/zip.c

	yarainclude_HEADERS = include/yara/miniz.h
	
	libyara_la_SOURCES = miniz.c

4. Add the module to the module_list file in the modules folder:

	MODULE(zip)

5. Now you can build yara by executing the make command inside the root folder.

More information can be found here: https://yara.readthedocs.io/en/v3.7.0/writingmodules.html

Usage

The yara zip module has at the moment only one function has_string(<file_inside_zip>, <search_string>) Following an example of a yara rule using the has_string(..) function. If the given string was found, then the return value of the has_string function equals the offset inside the requested file.

import "zip"

rule embedded_html
{
    meta:
    author = "@stoerchl"
    info = "searches for a given string inside a zip file"
  
    condition:
        zip.has_string("word/document.xml", "wp15:webVideoPr") > 0
}

Thanks to

This module would not be working without this data compression library:

https://github.com/richgel999/miniz/

And without the yara project there obviously would have been no new yara module ;-)

https://github.com/VirusTotal/yara