Building a Malwarsteinadapting and repurposing malware

hasherezade

About me• I am Hasherezade from twitter ;)

• Programmer & malware analyst

• Write technical articles about malware, crackmes etc

• Author of some tools for malware analysis, i.e. PE-bear, PE-sieve, libPeConv

• Contractor for Malwarebytes, but today I present my personal research

• More: http://hasherezade.net

Agenda1. Who and why repurpose existing malware?

2. A brief case study: Petya.A/Not Petya – probably the most famous Malwarestein in wild

3. Wrapping and modifying executables using libPeConv

4. Demo: meet my little Malwarestein...

Happy Birthday, Petya/NotPetya!

Repurposing malware – who, why, how?

Who, why, how?

•Script kiddies

•Commodity malware authors

•Nation state/politically motivated actors

•Researchers

Who, why, how?• Script kiddies

Why?

to prepare an attack without having a deep technical knowledge

Who, who, how?• Using leaked/published sources (i.e. Hidden Tear, Mirai)

Who, who, how?• Decompiling .NET malware...

Who, why, how?• Stolen DMA Locker

https://blog.malwarebytes.com/cybercrime/malware/2017/05/stolen-version-dma-locker-making-rounds/)

Who, why, how?• Script kiddies

• Modifications are usually small

• Sometimes they are chaotic and corrupting the malware functionality

• Tend to use high-level languages: .NET, Python...

Who, why, how?• Commodity malware authors (middle class)

• to save time and resources dedicated to the development

I am almost like Slavik

Who, why, how?• Using leaked/published sources (i.e. ZeuS based malware, malware

incorporating leaked NSA exploits/modules)

Who, why, how?• Rewriting fragments of known malware using reverse engineering...

Who, why, how?• Using more sophisticated patching (i.e. PetrWrap)

From „ PetrWrap: the new Petya-based ransomware used

in targeted attacks” https://

securelist.com/petrwrap-the-new-petya-based-ransomware-

used-in-targeted-attacks/77762

Who, why, how?• Commodity malware authors (middle class)

• Modifications display good technical knowledge of their author.

• Patches are made in the way that serves exactly the purpose, showing that the author did reverse engineering on the sample and understood it.

Who, why, how?• Sophisticated actors(including possibly nation-state linked)

• Also: to save time and resources dedicated on development

See the talk: „Cyberwar on a

Shoestring: How Kim Jong Un Stole My

Malware” by Kenneth Geers

Who, why, how?• Sophiasticated actors (including possibly nation-state linked)

• To make attribution more difficult

From „The devil is in Rich Headers”

https://securelist.com/the-devils-in-the-

rich-header/84348/

Who, why, how?• Beware of the Red Herrings!

From „The devil is in Rich Headers”

https://securelist.com/the-devils-in-the-

rich-header/84348/

Who, why, how?• Sophiasticated actors (including possibly nation-state linked)

• Used methods:• Same as in commodity malware: using sources, rewriting by reverse engineering,

sophiasticated patching

• Malware may incorporate legitimate applications, repurposed for malicious reasons

• Elements of other, known malware can be used as red herring

Who, why, how?• Researchers (me ;) )

• To join and monitor botnets, to decrypt ransomware, test tools, to learn techniques, to build crackmes...

https://github.com/a0rtega/

pafish

Who, why, how?• Researchers (me ;) )

Rather than writing a full

decryptor from the scratch, I

repurposed DMA locker to work in a

decryption mode...

Who, why, how?• Used methods:

• Reverse engineering samples and reimplementing techniques

• Patching and hooking malware i.e. to intercept its communication with the CnC

• ...and I have my own library, that allows me to wrap malware binaries, export functions of my choice and use them as my library

https://www.youtube.com/watch?v=KMcSAlS9zGE

https://www.youtube.com/watch?v=yV4

jiidyFXw

Petya.A/NotPetya – probably the most famous Malwarestein in wild

Petya.A/NotPetya• Used elements

• Petya (Goldeneye version)

• NSA exploits

• Mimikatz

• Used methods• Binary patching

• Reverse engineering the original code of Petya dropper and coping its functionality

Petya.A/NotPetya• Patches made in the Petya low-level component

Removed the screen with Petya

Skull

Patches were cosmetic: changed

strings, colors... Not the

functionality

Petya.A/NotPetya• Reverse-engineered and rewritten high-level Petya components

Petya.A/NotPetya• Experiment – was it difficult to make it a better wiper?

• Lines responsible for xoring the input with the keystream:

https://github.com/alexwebr/salsa20/blob/master/salsa20.c

Petya.A/NotPetya• Experiment – was it difficult to make it a better wiper?

• What if instead of xoring it with keystream, we just overwrite it with a keystream?

• Minimal patch: just one byte

88 01 -> MOV [BX+DI], AL

Petya.A/NotPetya• Experiment – was it difficult to make it a better wiper?

• To make the changes more visible, we can overwrite it with a defined character i.e. #

B0 23 -> MOV AL, #

88 01 -> MOV [BX+DI], AL

Petya.A/NotPetya• Yes, it works!

Repurposing Petya to become a real wiper can be SUPER EASY, providing that you understood the code

Patching – lessons learned• Patching always requires a good understanding of the binary!

• It is easy to destroy something, hard to keep track on changes and undo them

• It is a simple and powerful technique, but it has its limitations: the size of the patch much be possibly small to fit in the space

• Workarounds?• Injecting a DLL into a process, hooking and redirecting functions...

• What if we want something simpler? And allowing for persistent changes?

• Thinking of it I created my own library, libPeConv

LibPeConv – how it works

libPeConv – how it works• A static library for custom loading of PE files

• Features:• load a PE file from a buffer without dropping it on the

disk.• export any function from the PE file, providing that you

know its offset and API (HexRays decompiler is your friend)

• Easily hook IAT of the loaded PE files• Replace selected functions in the loaded PE file by your

custom functions

libPeConv – how it works• Custom loading of PE files

#include "peconv.h"

https://github.com/hasherezade/libpeconv

Load any PE file from the buffer

(without dropping it on the disk

libPeConv – how it works• Custom loading of PE files

The imports of the loaded executable can be resolved by your own resolver

libPeConv – how it works• Export any function from a malware of your interest – you only need

to know the API and RVA1. Find the function of your

interest and its offset

2. Reconstruct the API

3. Load the EXE with libPeConv and

export the function

4. Use it like your local function

libPeConv – how it works• Custom import resolvers allow you to hook IAT in a very easy way

Whenever the „Sleep” is called by the loaded malware, the function

„my_Sleep” in the loader is executed

instead

libPeConv – how it works• Not only imports: we can redirect any local function from the

malware to our local function

More: https://goo.gl/VCLMn1

libPeConv – how it works• We can also hook a one, chosen call to a particular function...

This time, not the function but the

address of particular call was replaced

The original function „BlueScreen” is

untouched and can be called elsewhere.

libPeConv – how it works• Tutorials:

• https://hshrzd.wordpress.com/tag/libpeconv

• Examples:• https://github.com/hasherezade/libpeconv/tree/master/tests

• Helpers:• https://github.com/hasherezade/libpeconv_wrappers

• https://github.com/hasherezade/libpeconv_project_template

• A compiled (harmless) demo (open source):• https://goo.gl/3YzTjJ

DEMO:Let me introduce my Malwarestein

DEMO

https://youtu.be/zSqwu2y2hSY

DISCLAIMER: This is a Proof-Of-Concept project, created for demonstration purpose only.

It will never be released and used outside the lab.

Thank you!