Reversing & malware analysis training part 3 windows pe file format basics

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 Thanks to all the trainers who have devoted their precious time and
countless hours to make it happen.


Reversing & Malware Analysis Training

This presentation is part of our Reverse Engineering & Malware
Analysis Training program. Currently it is delivered only during our local
meet for FREE of cost.

For complete details of this course, visit our Security Training page.


Who am I #1
Amit Malik (sometimes DouBle_Zer0,DZZ)
 Member SecurityXploded & Garage4Hackers
 Security Researcher
 RE, Exploit Analysis/Development, Malware Analysis
 Email: m.amit30@gmail.com


Who am I #2
Swapnil Pathak
 Member SecurityXploded
 Security Researcher
 RE, Malware Analysis, Network Security
 Email: swapnilpathak101@gmail.com


PE File Format
 PE – Portable Executable

 PE is the native Win32 file format.

 32-bit DLL, COM, OCX, Control Panel Applets(.CPL),
.NET, NT kernel mode drivers are all PE File Format.


Why PE File Format
 How windows loader loads the executable in memory.

 How loader build the import and export table for a module in
memory

 From where to start the execution or Address of entry point

 Answer of the question “how binary compiled on a version of
windows works on another version of windows?”

 Where should attacker attack 

 Also today’s malwares are generally encrypted, packed. In order to
rebuild the original binary we need to know how the binary is
structured.

Basic Structure


Basic Structure Cont.
 Most common sections found in executable are
 Executable Code section (.text , CODE)

 Data Sections (.data, .rdata, .bss, DATA)

 Resources section (.rsrc)

 Export Section (.edata)

 Import Section (.idata)

 Debug Information Section (.debug)


Headers – DOS Header
 All PE files start with DOS header
 First 64 bytes of the file.
 Run program in DOS.
 Runs the DOS stub
 Usually the string
“This program must be run under Microsoft Windows”
 e_lfanew is the pointer to PE or NT header
 Structure defined in windows.inc or winnt.h


Header- DOS header cont.

e_magic = 4D, 5A (MZ)
e_lfanew is a DWORD which contains the offset of the PE header


Headers – PE header

 Begins with signature (DWORD) 50h, 45h, 00h, 00h

 Letters “PE” followed by two terminating zeros

 File Header- 20 Bytes – contains info about physical layout and
properties of the file

 Optional Header- 224 Bytes – contains info about the logical layout
of the PE file – size given by member of File header


Headers – PE –> File header

 Machine

 NumberOfSections

 SizeOfOptionalHeader

 Characteristics


Header – PE –> Optional Header


Optional Header Cont.
 AddressOfEntryPoint
 ImageBase
 SectionAlignment
 FileAlignment
 SizeOfImage
 SizeOfHeaders
 Subsystem
 DataDirectory


Header – PE –> Optional –> Data
Directory

 Last 128 bytes of OptionalHeader

 Array of 16 Image_Data_Directory structures

 Each relating to an important data structure like the Import Table

 Members

 Virtual Address : RVA of the data structure

 iSize : size in bytes of the data structure


Data Directories


 IMAGE_DIRECTORY_ENTRY_EXPORT

 IMAGE_DIRECTORY_ENTRY_IMPORT

 IMAGE_DIRECTORY_ENTRY_RESOURCE

 IMAGE_DIRECTORY_ENTRY_TLS

 IMAGE_DIRECTORY_ENTRY_IAT


Headers - Section Header

 Array of IMAGE_SECTION_HEADER

 Equal to the numberofsections – FileHeader member.

 Each structure size = 40 bytes

Section Header cont.
 Name – Virtually can be anything in text

 VirtualSize – Size of section in memory

 VirtualAddress – section entry offset in memory (RVA)

 SizeOfRawData – Size of section on disk

 PointerToRawData – section entry offset on disk

 Characteristics – Type of section (execuatble, data etc.)

 Section Alignment and File Alignment are two important
values from optional header that control the entry point of
next section.

 The structure of PE file on disk is exactly the same as when it is
loaded into memory.

 The windows loader maps the required sections in memory.

 When sections are loaded into memory they are aligned to fit 4KB
memory pages (Section Alignment), each section starting on a new
page.


Type of PE file sections
 Executable code

 Data

 Resources

 Export section

 Import section

 Thread Local Storage (TLS)

 Base Relocations (reloc.)


Export Section
 Relevant to DLLs

 Export functions in two ways

- By name

- By ordinal only

 Ordinal – 16 bit value that uniquely defines a function in
particular DLL


nName
nBase
NumberOfFunctions
NumberOfNames
AddressOfFunctions
AddressOfNames
AddressOfNameOrdinals

Export by Ordinal only
Export Forwarding

Import Section
 Contains information about all functions imported by
executable from DLLs

 Loader maps all the DLLs used by the application into its
address space

 Finds the addresses of all the imported functions and makes
them available to the executable being loaded.


Import Directory
 20 byte structure IMAGE_IMPORT_DESCRIPTOR

 Number of structures = Number of DLLs imported

 Last structure filed with zeros


 OriginalFirstThunk

 Name1

 FirstThunk

Hint
Name1


 Each IMAGE_THUNK_DATA str corresponds to one imported
function from the dll.

 Arrays pointed by OriginalFirstThunk and FirstThunk run
parallelly.

 OriginalFirstThunk – Import Name Table – Never modified

 FirstThunk – Import Address Table – Contain actual function
addresses


Functions exported by ordinal only
- No IMAGE_IMPORT_BY_NAME structure
-IMAGE_THUNK_DATA contains the ordinal of the function
-MSB used to identify the same
- MSB is set, rest 31 bits are treated as an ordinal value.
-Bound Imports

DEMO


Reference
 Complete Reference Guide for Reversing & Malware
Analysis Training


PE file format test
 Write a program in “C” or “ASM” that will modify the Address of
Entry point of an Executable (.exe) file with any random address.

 Write a program in “C” or “ASM” that will add a new section into an
executable (.exe)

 For hints shoot us an email 


Thank You !


Reversing & malware analysis training part 3 windows pe file format basics

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Reversing & malware analysis training part 3 windows pe file format basics