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
}

wxsBSD

bf2y: Brainfuck to YARA

bf2y is a brainfuck compiler which targets the YARA virtual machine. You can get it from my bf2y branch at https://github.com/wxsBSD/yara/tree/bf2y.

Requirements

You will need python3 and ply (https://pypi.org/project/ply/)

The Toolchain

wxsBSD

wxs@wxs-mbp yara % cat rules/test.yara
rule a {
  strings:
    // This program cannot VGhpcyBwcm9ncmFtIGNhbm5vdA==
    // AThis program cannot QVRoaXMgcHJvZ3JhbSBjYW5ub3Q=
    // AAThis program cannot QUFUaGlzIHByb2dyYW0gY2Fubm90
    $a = "This program cannot" base64 ascii

    // Custom alphabets are supported, but I have it commented out for now. ;)

wxsBSD

wxs@wxs-mbp yara % cat rules/test.yara
rule a {
  strings:
    // This program cannot VGhpcyBwcm9ncmFtIGNhbm5vdA==
    // AThis program cannot QVRoaXMgcHJvZ3JhbSBjYW5ub3Q=
    // AAThis program cannot QUFUaGlzIHByb2dyYW0gY2Fubm90
    $a = "This program cannot" base64

    // Custom alphabets are supported, but I have it commented out for now. ;)

wxsBSD

Keybase proof

I hereby claim:

• I am wxsbsd on github.
• I am wxs (https://keybase.io/wxs) on keybase.
• I have a public key whose fingerprint is 96D1 2E6B F61C 2F3D 83EF 8F0B BE54 310C 17F0 AA37

To claim this, I am signing this object:

wxsBSD

YARA Loop Optimization Details

Let's look at the bytecode without my optimizations. Before we do that let's set some terminology, because I find it easier to use names compared YARA VM memory locations. These are the names I've mostly borrowed from the comments in the grammar:

• memory 0: lower bound
• memory 1: boolean_expression accumulator
• memory 2: iteration counter
• memory 3: upper bound

We'll be using this rule for the first example:

wxsBSD

I've been working on optimizing the YARA compiler to generate better bytecode for loops. The goal is to skip as much of loops as possible by not iterating further once the loop condition is met. Here's the rule I'm using. Completely contrived and excessive, but it's to show the performance improvement:

wxs@wxs-mbp yara % cat rules/test.yara
rule a {
  condition:
    for any i in (0..100000000): (i == 1)
}
wxs@wxs-mbp yara %

wxsBSD

/*
 * fmtid + 24 == number of property identifiers and offsets
 * fmtid + 28 == start of property identifier and offsets (4 bytes each)
 */
rule test {
  strings:
    //$fmtid = { 02 d5 cd d5 9c 2e 1b 10 93 97 08 00 2b 2c f9 ae }
    $fmtid = { e0 85 9f f2 f9 4f 68 10 ab 91 08 00 2b 27 b3 d9 }
    $redacted_author = "REDACTED AUTHOR"
  condition:

wxsBSD

Using YARA python interface to parse files

I've shared this technique with some people privately, but might as well share it publicly now since I was asked about it. I've been using this for a while now with good success. It works well for parsing .NET droppers and other things.

If you don't know what the -D flag to YARA does I suggest you import a module and run a file through using that flag. It will print, to stdout, everything the module parsed that doesn't involve you calling a function. This is a great way to get a quick idea for the structure of a file.

For example:

wxs@mbp yara % cat always_false.yara