Writing YARA rules based on executable code within malware can be a tedious task. An analyst cannot simply copy and paste raw executable code into a YARA rule, because this code contains variable values, such as memory addresses and offsets. The analyst has to disassemble the code and wildcard all the pieces in the code that can change between samples. mkYARA aims to automate this part of writing rules by generating executable code signatures that wildcard all these little pieces of executable code that are not static.
Installation is as easy as installing the pip package.
pip install mkyara
import codecs
from capstone import CS_ARCH_X86, CS_MODE_32
from mkyara import YaraGenerator
gen = YaraGenerator("normal", CS_ARCH_X86, CS_MODE_32)
gen.add_chunk(b"\x90\x90\x90", offset=1000)
gen.add_chunk(codecs.decode("6830800000E896FEFFFFC3", "hex"), offset=0x100)
gen.add_chunk(b"\x90\x90\x90\xFF\xD7", is_data=True)
rule = gen.generate_rule()
rule_str = rule.get_rule_string()
print(rule_str)mkYARA comes with a standalone tool that is cross platform, as in, it can create signatures for Windows binaries running under Linux.
usage: mkyara [-h] [-i {x86}] [-a {32,64,x86,x64}] -f FILE_PATH [-n RULENAME]
-o OFFSET -s SIZE [-m {loose,normal,strict}] [-r RESULT] [-v]
Generate a Yara rule based on disassembled code
optional arguments:
-h, --help show this help message and exit
-i {x86}, --instruction_set {x86}
Instruction set
-a {32,64,x86,x64}, --instruction_mode {32,64,x86,x64}
Instruction mode
-f FILE_PATH, --file_path FILE_PATH
Sample file path
-n RULENAME, --rulename RULENAME
Generated rule name
-o OFFSET, --offset OFFSET
File offset for signature
-s SIZE, --size SIZE Size of desired signature
-m {loose,normal,strict}, --mode {loose,normal,strict}
Wildcard mode for yara rule generation
loose = wildcard all operands
normal = wildcard only displacement operands
strict = wildcard only jmp/call addresses
-r RESULT, --result RESULT
Output file
-v, --verbose Increase verbosity
mkYARA comes with a IDA plugin to easily create YARA signatures by selecting a set of instructions and choosing one of the mkYARA -> Generate YARA rule options. Installation is as easy as installing the pip package and copying the mkyara_plugin.py to your IDA plugin directory.
