Category Archives: Malware Reversing

Reverse Engineering Malware – What you need to know?


Every now and then, a nasty piece of Malware raises its ugly head and wreck havoc on the Enterprise Infrastructure. It is often necessary to analyze the Malware and understand its working so that

  • The impact of the Malware on IT Systems can be ascertained AND
  • The nature of preventative controls that can be put in place so that this threat does not spread further.

In such scenarios, Reverse Engineering of the Malware becomes a requirement. Reverse engineering of a malware or an unknown piece of binary file is the process of analyzing and understanding its characteristics, behavior. There are several approaches that several different people use. But in this Blog post, the goal is to give a quick little guide for Malware Reversing so that anyone with an inclination to pick it can do so very easily. This in my mind is an essential tool in the hands of a Security Analyst. The basic skills needed to do this are listed below:

  • Some programming skills, or at the very least be able to understand and read source code
  • Logical Mindset capable of analyzing and interpreting the Vectors used by the Malware Code
  • Lots and Lots of Patience and Interest

Let us now get into the basics. We need to understand that Malware programs have several stages before they infect and compromise a machine. Typically a malware performs the following:

  • Get itself persistent by adding its executable path in the registry, autorun, etc – Exploit
  • Comfortably elevate itself to another process, so that it cannot be easily found – Masquerade
  • Deter the process of it’s analysis by rapidly changing its code signature – Polymorphism
  • Make connections to remote servers sometimes to update itself or also to report back to its Master – Callback
  • Perform the necessary tasks on the affected system – Data Exfiltration or Zombie

All these Malicious Programs, have one goal or the other, but eventually they end up handing over control of your machine to strangers and potentially bad guys as well. Some Intentions of Malware are listed below:

  • Steal sensitive information / Key-logging / identity theft / usernames and password / banking information / Company patents / source code / etc (including personal data that may have been part of the system)
  • Access private networks
  • Perform DDoS Attacks
  • Spamming
  • Browser hijacking, ad-wares to perform fraud
  • Ransom-ware: deny access to the users’ own data and demand money to give access back – in other words extortion
  • Data exfiltration

Reverse Engineering Methodology:
This effort involves determining not only what the malware can do specifically, but also establishing how to identify the presence of such programs on affected systems. There might be so many (right) ways to do this, but for something quick we shall follow the steps laid out here.

Why Quick?? Because in an Enterprise sometimes we might not have all the time to really perform in-depth analysis, as time is a major factor when responding to incidents of this kind.

The Setup:
In order to perform an Effective Malware Analysis, we need to have a Tool Kit and an Environment for Analysis. Some of the key things to take care while setting up the environment are:

  • The Environment should be isolated with no connections to the Enterprise Data Sensitive network.
  • The environment should have its own  Proxy service so that it does not have scope to spread. The Proxy can be a sink hole where it just logs the connections made.
  • Set-up 2 sand boxes, 1 physical and 1 VM, as some malware programs only work on  on a physical box as they are VM-aware.
  • Make sure these sandboxes are standard images, with bare minimum corporate patching done. This should theoretically be equivalent to the weakest link in the organization.
  • Install all the required tools listed below to do certain type of analysis.
  • Tools required: strings, ida pro, pmdump, volatility framework, upx, packerid, pescanner, pe explorer, md5hashollydbg, deep freeze, winalysis, lp

The Analysis: The analysis of Malware is usually a two-phased approach – Behavior Analysis and Code Analysis. These two analysis methods yield so much information that detection and response becomes easy.

  • Behavioral analysis: Observing malware interactions with its environment like network connections, files dropped, evasive measures taken etc. This can be identified by installing the Malware, “getting infected” as you may call it.
    • Once infected, you can capture the network packets, to look at potential domains and IP addresses the software tries to connect. This will help in perimeter filtering and Endpoint ‘Firewall’ing
    • If the Malware drops some files using C2, then that also can be observed as a part of the getting infected process. This will help in gather SHA and MD5 values for the dropped files and banning them in Endpoint solutions from execution
  • Code analysis: Examining the code that comprises the program to infer what exactly the malware is capable of doing when executed. This does not help in response schemes, but is very important from a Forensics Purpose. Code Analysis can help in determining the extent of loss, the extent of vulnerability in the system that is being exploited etc.
  • Code Analysis can be done as follows:
    • Firstly Identify if either the unknown file is protected, obfuscated, encrypted (armoring) and/or packed (the original code is compressed, encrypted or both). To do this, we can use packerid or pe explorer.This technique is applied in an attempt to evade signature based malware detection, and to deter the efforts of static analysis. Identifying the packer specifically, can exactly tell you what you are missing in terms of detection using Perimeter tools.
    • Then with basic analysis like enumerating exports, imports, function use, syscalls, winapi, mutex, dll dependencies, strings and some grepping, using the winalysis or other similar tools that you might be comfortable with, you can come up with several theories about the file. These theories will give an understanding of the various attack vectors employed by the file. This can help lock down a system to these kinds of Malware attempts.
    • Drilling down further into the specific attack functions and looking at the code itself can help understand the vulnerability being exploited. This is very useful for Developers in fixing the holes in the software. This will help in a sort of retroactive patching methodology.
Post Analysis Steps: 
  1. Once the analysis is done on the Behavior and the Code aspects of the Malware, you have lots of data about at hand. Documenting the Analysis is very key because, future variants may use the same Attack Vector, Same Exploit Code etc to gain access to a machine/application.
  2. Use the documentation prepared as above to compare against subsequent analysis. This will save a great deal of time in detecting and responding to future threats posed.
  3. Snapshot of the VM also can be retained for future reference.
  4. Destroy the Analysis VM and start over again!!!
Practical Example
There will be a follow up post to this with a Hands-On Tutorial of how its done!!!! Keep Following this blog and Happy Reversing!!!!
Additional Resources:

 

Website to get malware samples for analysis:
http://oc.gtisc.gatech.edu:8080/

Websites to assist you in malware analysis:
REMnux (Linux distribution for malware analysis) – http://zeltser.com/remnux/
ISEC Labs Anubis Tool – http://anubis.iseclab.org/
GFI Sandbox – http://www.gfi.com/malware-analysis-tool
Hex to Binary/ASCII – http://home.paulschou.net/tools/xlate/
Hex to ASCII – http://www.dolcevie.com/js/converter.html
Jsunpack – http://jsunpack.jeek.org/