webpress urbano: Introduction To Reversing Golang Binaries

Golang binaries are a bit hard to analyze but there are some tricks to locate the things and view what is doing the code.

Is possible to list all the go files compiled in the binary even in an striped binaries, in this case we have only one file gohello.go this is a good clue to guess what is doing the program.

On stripped binaries the runtime functions are not resolved so is more difficult to locate the user algorithms:

If we start from the entry point, we will found this mess:

The golang string initialization are encoded and is not displayed on the strings window.

How to locate main? if its not stripped just bp on [package name].main for example bp main.main, (you can locate the package-name searching strings with ".main")

And here is our main.main:

The code is:

So in a stripped binary we cant find the string "hello world" neither the initialization 0x1337 nor the comparator 0x1337, all this is obfuscated.

The initialization sequence is:

The procedure for locating main.main in stripped binaries is:

1. Click on the entry point and locate the runtime.mainPC pointer:

2. click on runtime.main function (LAB_0042B030):

3. locate the main.main call after the zero ifs:

4. click on it and here is the main:

The runtime is not obvious for example the fmt.Scanf() call perform several internal calls until reach the syscall, and in a stripped binary there are no function names.

In order to identify the functions one option is compile another binary with symbols and make function fingerprinting.

In Ghidra we have the script golang_renamer.py which is very useful:

After applying this plugin the main looks like more clear:

This script is an example of function fingerprinting, in this case all the opcodes are included on the crc hashing:

# This script fingerprints the functions
#@author: sha0coder
#@category fingerprinting

print "Fingerprinting..."

import zlib


# loop through program functions
function = getFirstFunction()
while function is not None:
    name = str(function.getName())
    entry = function.getEntryPoint()
    body = function.getBody()
    addresses = body.getAddresses(True)

    if not addresses.hasNext():
        # empty function
        continue 

    ins = getInstructionAt(body.getMinAddress())
    opcodes = ''
    while ins and ins.getMinAddress() <= body.getMaxAddress():
        for b in ins.bytes:
            opcodes += chr(b & 0xff)
        ins = getInstructionAfter(ins)
    
    
    crchash = zlib.crc32(opcodes) & 0xffffffff

    print name, hex(crchash)


    function = getFunctionAfter(function)

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sábado, 22 de agosto de 2020

Introduction To Reversing Golang Binaries

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Ali Garces

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