Basic Static Techniques

Static Analysis - process of analyzing the code or structure of a program to determine its function

Hashing

• Hashing

  • A common method used to uniquely identify malware

  • Run through a hashing program that produces a unique hash that identifies that malware

  • Uses

    • Hash as a label

    • Share that hash

    • Search for the hash online

Finding Strings

• String

  • Program is a sequence of characters

  • Simple way to get hints about the functionality of a program

  • Usually stored in either ASCII or Unicode format

    • Both store characters in sequences that end with a NULL terminator to indicate that the string is complete

Packed and Obfuscated Malware

• Obfuscated

  • Programs are ones whose execution the malware author has attempted to hide

  • If when you search a program you only find a few strings it is probably obfuscated or packed

• Packed

  • A subset of obfuscated programs in which the malicious program is compressed and cannot be analyzed

• NOTE - They often include at least the LoadLibrary and GetProcAddress which are used to load and gain access to additional functions

• Packing Files

  • On packed programs, a small wrapper program runs to decompress the packed file and then run the unpacked file

Detect Packers with PEiD

• PEiD to detect the type of packer or compiler employed to build an application
• You have to unpack it in order to be able to perform any analysis

• Command to unpack using UPX: upx -d PackedProgram.exe

• You can also detect packed malware using PEview

  • If the Size of Raw Data < Virtual Size then that might be indication that it is packed.

Portable Executable File Format

• Portable Executable (PE)

  • File format used by Windows executables, object code, and DLLs

    • Think of it as a giant array of bytes

  • A data structure that contains the information necessary for the Windows OS loader to manage the wrapped executable code

  • Every file with executable code that is loaded by Windows is in the PE file format

  • Begin with a header that includes information about the code:

    • Type of application

    • Required library functions

    • Space requirements

  • Contains a header followed by a series of sections

    • Header contains metadata about the file itself

  • All Microsoft executables start with MZ

• PE File Headers and Sections

  • .text

    • All other sections store data and supporting information

    • Only section that can execute and only one that includes code

  • .rdata

    • Contains the import and export information

    • Store other read-only data used by the program

    • Sometimes will contain .idata and .edata section which store the import and export information

  • .data

    • Contains the program's global data

    • Accessible from anywhere in the program

    • Local data is not stored in this section, or anywhere else in the PE file

  • .rsrc

    • Includes the resources used by the executable that are not considered part of the executable

    • Strings can be stored either in the .rsrc section or in the main program

Executable	Description
.text	Contains executable code
.rdata	Read-only data globally accessible within the program
.data	Global data accessed through the program
.idata	Stores the import function information
.edata	Stores the export function information
.pdata	Only in 64-bit executables and stores exception-handling information
.rsrc	Stores resources needed by the executable
.reloc	Information for relocation of library files

Linked Libraries and Functions

• Imports

  • Functions used by one program that are actually stored in a different program, like code libraries

  • Executables can import functions by ordinal instead of by name, when they do this the name of the function never appears in the original executable

• Code libraries

  • Can be connected to the main executable by linking

  • Can be linked statically, at runtime, or dynamically

  • Information found in the PE file header depends on how the library code has been linked

• Static Linking

  • Least commonly used method of linking libraries

  • Common in UNIX and Linux programs

  • All code from that library is copied into the executable, making it grow in size

  • Nothing in the PE file header indicates that the file contains linked code

• Runtime Linking

  • Commonly used in malware

  • Connect to libraries only when that function is needed, not at program start

  • Windows functions allow programmers to import linked functions not listed in a program's file header

• Common Functions

  • LoadLibrary - Allow a program to access any function in any library on the system - GetProcAddress - Allow a program to access any function in any library on the system - LdrGetProcAddress - LdrLoadDll

• Dynamic Linking

  • Most common and most interesting

  • Host OS searches for the necessary libraries when the program is loaded

  • Program calls the linked library function, function executes within the library

  • PE file header stores information about every library that will be loaded and every function that will be used by the program

• Common DLLs

DLL	Description
Kernel32.dll	Common DLL contains core functionality like access and manipulation of memory, files and hardware.
Advapi32.dll	Provides access to advanced core Windows components like Service Manager and Registry
User32.dll	Contains user-interface components, like buttons, scroll bars, and components for controlling and responding to user actions.
Gdi32.dll	Functions for displaying and manipulating graphics.
Ntdll.dll	Interface to the Windows kernel. Always imported indirectly by Kernel32.dll. If this is imported it means that the author intended to use functionality not normally available to Windows programs. Tasks like hiding functionality or manipulating processes.
WSock32.dll Ws2_32.dll	Network DLLs, program that uses this likely connects to a network or performs network-related tasks
Wininet.dll	Contains higher-level networking functions that implement protocols such as FTP, HTTP, and NTP

Function Naming Conventions

• Functions that take strings as parameters include an A of a W at the end of their names, like CreateDirectoryW

• Drop the trailing A or W when searching for the function in the Microsoft documentation

• Functions with names ending in "Ex" = new function that is incompatible with old one. Old one still supported

Imported and Exported Functions

• Imported Functions

  • PE file header includes information about specific functions by an executable

• Exported Functions

  • DLL implements functions and exports them for use by an executable that can then import and use them

  • PE file contains information about which functions a file exports

  • DLLs provide functionality for executables = exported functions are common

  • Exported functions are not common in executables

  • Sophisticated malware will often omit function names or use unclear or misleading names

Static Analysis in Practice

• Seeing a lot of imports = file is not packed

• Function Examples

  • SetWindowsHookEx - commonly used in spyware and most popular way that keyloggers receive keyboard inputs

  • RegisterHotKey - registers a hotkey whenever the user presses that hotkey combination the application is notified

  • LowLevelKeyboardProc - an applicaiton defined or library-defined callback function used with the SetWindowsHookEx

• Examine PE Files with PEview

  • All Delphi programs use a compile time of June 19,1992

  • Subsystem description

    • indicates whether this is a console or GUI program

    • Console program = IMAGE_SUBSYSTEM_WINDOWS_CUI

    • GUI programs = IMAGE_SUBSYSTEM_WINDOWS_GUI

  • Virtual Size - How much space is allocated for a section during the loading process

  • Size of raw data - shows how big the section is on disk

  • These should usually be equal Virtual Size = Size of Raw Data

  • Virtual Size > Size of Raw Data = Packed Code

  • Memory Space > Disk Space = Packed Code

  • In Windows it is normal for .data to have virtual size > raw data size - it is not enough to mean its not malicious

• Viewing Resource Section with Resource Hacker

  • Informative Sections

    • Icon - lists images shown when the executable is in a file listing

    • Menu

      • stores all menus that appear in various windows

      • Contains the names of all the menus and the text shown for each

    • Dialog - program's dialog menus

    • String Table - stores strings

    • Version Info - contains a version number and often the company name and a copyright statement

• NOTE - Malware often store an embedded program or driver here and before the program runs, they extract the embedded executable or driver.

• PE Header Summary

Field	Information Revealed
Imports	Functions from other libraries that are used by the malware
Exports	Functions in the malware meant to be called by other programs or libraries
Time Date Stamps	When the program was compiled
Sections	Sections in the file and their sizes on disk and in memory
Subsystem	Command-line or GUI-application
Resources	Strings, icons, menus, and other information included in the file

Tools

• Dependency Walker - lists only dynamically linked functions in an executable
• PEview
• Resource Hacker
• PEBrowse Professional
• PE Explorer