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Monday, July 2, 2012

Tools for Reverse Engineering and Malware Analysis

Reverse engineering is the process of analyzing a subject system to identify the system's components and their relationships, and to create representations of the system in another form or at a higher level of abstraction. The process of reverse engineering, which is part of malware analysis, is accomplished using specific tools that are categorized as hex editors, disassemblers/debuggers, decompiles and monitoring tools.
Disassemblers/debuggers occupy important position in the list of reverse engineering tools. A disassembler converts binary code into assembly code. Disassemblers also extract strings, used libraries, and imported and exported functions. Debuggers expand the functionality of disassemblers by supporting the viewing of the stack, the CPU registers, and the hex dumping of the program as it executes. Debuggers allow breakpoints to be set and the assembly code to be edited at runtime.
One must be familiar with the Portable Executable (PE)[1]file format before diving into reverse engineering for Windows executables. In this article we will get into important aspects of Hiew, OllyDbg and IDA Pro from reverse engineer's perspective.

Hiew[2] short for Hacker's view is a great disassembler (not that this is not debugger) designed for hackers, asthe name suggests. 
It supports three modes - Text, Hexadecimal and Decode (Dis-assembly) mode. Enter/F4 key is used to switch between these modes. In each mode the Function Line, corresponding to function keys from F1 to F12, which appears at the bottom of the Hiew screen, changes and its functionality with CTRL, SHIFT and ALT combinations.

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Fig. 1 Hiew – Three modes: Text, Hex, Decode

PE Header
PE Header could be viewed by pressing F8 from Hex or Decode view. In this mode we could see important properties of PE file using following shortcuts:
Sections Table

Import Table

Export Table

Data Directories

Jump to Entry Point

Jump to end of last section

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Fig. 2 Hiew – PE Header

Search in file
Hiew supports to search in a file for ASCII or HEX sequence of bytes by pressing F7 key. It also supports byte wild character.

Wild character

To repeat search

To change search direction

ASCII and Unicode strings are viewed from Text/Hex mode by pressing Alt-F6 key. This helps to search for juicy strings like suspicious urls, FTP, SMTP or IRC commands, files names, registry keys etc in the file. You could jump to selected string from string window by pressing ENTER key. +/- keys are used to change the minimum length of displayed strings, this will help to filter out smaller strings. You could apply filter for displayed strings using F9 key.

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Fig. 3 Hiew – Strings from file

Moving around

You could directly jump to specific location by pressing F5 key and providing offset (offset values are hexadecimal?). To specify relative offset + or - sign could be used as prefix to offset. When specified offset is a Virtual Address, it should start with ".". Alt-F1 key is used to toggle between Virtual Address and file offset.
If you want jump to specific function or offset which appears as part of control transfer instruction like call, jmp or conditional jump, you could press the key that appears at the end of instruction. Please see Fig.1 marked for label 4. In this case if you press key “4”, it will take you to offset 0x010073DA.
0 or Backspace key is used to jump back the previous instruction.

Simple Decryption

Hiew supports decryption of block using simple encryptions like xor, add, rol etc. Press F3 from Hex or Decode view to enter in edit mode and then press F7 to add simple decryption routine. You could set operand size as byte, word or dword by pressing F2.
Hiew works great when used in combination with File Manager like FAR[3] by configuring its command line. This is very helpful disassembler to quickly get different aspects of file under analysis like file header, section information, data directories, imported / exported functions and strings. 

OllyDbg[4][5]is an application-level debugger. OllyDbg interface shows the disassembly, hex dump, stack, and CPU registers. Additionally, OllyDbg supports run tracing, conditional breakpoints, PE header viewing, hex editing, and plug-in support.
At first Startup, OllyDbg asks to setup User Data Directory (UDD) and Plugins directory.UDD is used to keep debugged application specific information like breakpoints and other information and obviously you need to save plugins in Plugins directory. It provides wide Debugging Options like break on new module or when thread is created, how to process exceptions etc. OllyDbg supports setting of Hardware Breakpoints, Software Breakpoints, Memory Breakpoints and even Conditional Breakpoints.
OllyDbg supports plugins to enhance its functionality. 

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Fig. 4 OllyDbgAdvanced Window

Olly Advanced Plugin

There were some bugs reported with Olly v1.10 related to string parsing routine, parsing of faulty executables. This plugin fixes most of these bugs. Some malware samples are loaded with Anti-Debugging techniques [7], Olly Advanced plugin helps to counter most of them.

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Fig. 5 OllyDbg Advanced Plugin

Olly DumpPlugin
Olly Dump is used to dump debugged process memory. You could trace the packed file till it reaches original entry point and then dump unpacked version of file from process memory. It provides options to rebuild Import Address Table (IAT).

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Fig. 6 OllyDbgDump Plugin

Olly ScriptPlugin
OllyScriptis a plugin to that lets you to automate OllyDbg by writing scripts in an assembly-like language. Many tasks involve a lot of repetitive work just to get to some point in the debugged application. By using this plugin you could write a script once and it could be used with other similar samples. OpenRCE[8]hosts dozens of scripts that helpful to find original entry point (OEP) of many packers.

IDA Pro is a powerful disassembler that presents the disassembly in well-organized format, shows Graph view of selected function. However, it is less frequently used as a debugger in reverse engineers community where OllyDbg steals the top rank. IDA Pro's features include hex editing, string extraction, and import and export viewing. IDA Pro also features a window for viewing all of the functions called by a program, and provides accurate analyses of the program, summarizing them in a color-coded bar at the top of the screen, which classifies the various sections of the program's code. Below figure shows IDA Pro's interface, including the disassembly and the color-coded analysis bar at the top of the screen. The titles of the other windows are visible on the tabs above the disassembly.

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Fig. 7 IDA Pro Main Windows

IDA Pro supports wide variety of processors like ARM, DEC, Intel, Motorola etc.
IDA Pro provides selection of debuggers
• Bochs
• Win Debugger
• WinDbg

IDA Pro with Boch semulator make an interesting combination that is used to debug Operating system starting from booting process and it is helpful in debugging even ROM BIOS and Master Boot Record code.
Analysis done on particular sample, comments added, functions marked could be saved as an .idb file.

IDA Shortcuts
Below is the list of some important IDA Shortcuts, for complete list please visit reference [9].

Goto address or variable

Go back to previous location

Add inline comment

Add comment

Rename label, variable, functions etc.

Show cross reference

Substitute enum

*Dont forget to* Save changes

Extending IDA 

IDA supports writing IDC Scripts which is very similar to C like language on top of powerful IDA disassembler. The functionality of disassembler could be utilized even through python scripts and by writing plugins. 

Fast Library Identification and Recognition Technology
One of the challenges with disassembly of programs developed with modern high level languages is to identify library functions. One may end up in spending considerable time to go through these functions. On the other hand identification of library functions can considerably ease the analysis of a program. IDA comes with FLIRT to recognize the standard library functions. 
One must understand the power of each tool to choose appropriate tool for specific requirement during reverse engineering.

1. Portable Executable File Format – A Reverse Engineer View
2. Hiew
3. FAR Manager
4. OllyDbg
5. OllyDbg Quick Start Guide
6. OllyDbg Plugins
7. Anti-Debugging
8. Olly Scripts
9. IDA Shortcuts
©2012, copyright BLACK BURN


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