wxsBSD

This started with a tweet from Steve Miller (https://twitter.com/stvemillertime/status/1508441489923313664) in which he asked what is better for performance: 1 rule with 10k strings or 10k rules with 1 string each? Based upon my understanding of YARA I guessed it wouldn't matter for search time and the difference in bytecode evaluation would be in the noise. Effectively, I guessed you would not be able to tell the difference between the two.

Costin was the first to provide actual results and he claimed a 35 second vs 31 second difference between the two (https://twitter.com/craiu/status/1508445059129163783). That didn't make much sense to me so I asked for his rules so I could test them. He provided me with two rules files (10k.yara and 10kv2.yara) and a text file with a bunch of strings in it.

This is my attempt to replicate his findings and also document why he was getting the warning he was getting. Because I wanted the run to take a bit of time I ended up not using his text file with all the strings (it

wxsBSD

wxs@wxs-mbp yara % cat rules/sets.yara
rule a0 { condition: false }
rule a1 { condition: true }
rule b { condition: 1 of (a*) }
rule c { condition: 2 of (a*) }
rule d { condition: 50% of (a*) }
rule e { condition: 1 of (a1) }
rule f { condition: all of (a1, e) }
wxs@wxs-mbp yara %

wxsBSD

One way to find PE files that start at offset 0 and have a single byte xor key:

rule single_byte_xor_pe_and_mz {
  meta:
    author = "Wesley Shields <[email protected]>"
    description = "Look for single byte xor of a PE starting at offset 0"
  strings:
    $b = "PE\x00\x00" xor(0x01-0xff)
 condition:

wxsBSD

Test rules:

wxs@wxs-mbp yara % cat rules/test.yara
rule b {
  strings:
    $a = "LSCOLORS"
  condition:
    $a
}

wxsBSD

import platform
import yara

print(f"Platform version: {platform.version()}")
print(f"Python version: {platform.python_version()}")
print(f"YARA version: {yara.YARA_VERSION}")


r = """

wxsBSD

#include <ctype.h>
#include <stdio.h>

int main(void) {
  for (int i = 0; i <= 255; i++)
    printf("0x%02x %u\n", i, !isalnum(i));
  return 0;
}

wxsBSD

import platform
import yara

print(f"Platform version: {platform.version()}")
print(f"Python version: {platform.python_version()}")
print(f"YARA version: {yara.YARA_VERSION}")

rules = yara.compile(source='rule a { strings: $a = "foo" fullword condition: $a }')
for c in range(256):

wxsBSD

Counting number of times strings match in YARA with awk...

wxs@wxs-mbp yara % cat rules/test.yara
rule a { strings: $a = "FreeBSD" nocase  $b = "usage: " condition: any of them }
wxs@wxs-mbp yara % ./yara -s rules/test.yara /bin/ls
a /bin/ls
0xb8e1:$a: FreeBSD
0xb9a1:$a: FreeBSD
0xb9f1:$a: FreeBSD

wxsBSD

fib.bf - A Fibonacci Generator Written In Brainfuck Running On YARA

Wait, What?

Back in January I wrote bf2y which is a brainfuck to YARA compiler. bf2y takes in an arbitrary brainfuck program and outputs the instructions to execute the brainfuck code on the YARA virtual machine (well, a slightly modified VM). If you want the full details of how it works go read the code, but I want to talk about writing a Fibonacii number generator for it.

First, A BF Primer

wxsBSD

Here's an example of how part of yrrc works. Starting with these rules:

wxs@wxs-mbp yrrc % cat rules/test.yara
rule a {
  meta:
    sample = "24c422e681f1c1bd08286c7aaf5d23a5f088dcdb0b219806b3a9e579244f00c5"
  condition:
    true
}