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AV Evasion

This documentation has been made using the box AV Evasion Shellcode from Tryhackme. You can work on it here

Portable Executable

What is it?

  • .text stores the actual code of the program

  • .data holds the initialized and defined variables

  • .bss holds the uninitialized data (declared variables with no assigned values)

  • .rdata contains the read-only data

  • .edata: contains exportable objects and related table information

  • .idata imported objects and related table information

  • .reloc image relocation information

  • .rsrc links external resources used by the program such as images, icons, embedded binaries, and manifest file, which has all information about program versions, authors, company, and copyright!

Example steps in which the Windows loader reads an executable binary and runs it as a process

  1. Header sections: DOS, Windows, and optional headers are parsed to provide information about the EXE file. For example,

    • The magic number starts with "MZ," which tells the loader that this is an EXE file.

    • File Signatures

    • Whether the file is compiled for x86 or x64 CPU architecture.

    • Creation timestamp.

  2. Parsing the section table details, such as

    • Number of Sections the file contains.

  3. Mapping the file contents into memory based on

    • The EntryPoint address and the offset of the ImageBase.

    • RVA: Relative Virtual Address, Addresses related to Imagebase.

  4. Imports, DLLs, and other objects are loaded into the memory.

  5. The EntryPoint address is located and the main execution function runs.

How to store shellcode in PE (Examples)

  1. Defining the shellcode as a local variable within the main function will store it in the .TEXT PE section.

  2. Defining the shellcode as a global variable will store it in the .Data section.

  3. Another technique involves storing the shellcode as a raw binary in an icon image and linking it within the code, so in this case, it shows up in the .rsrc Data section.

  4. We can add a custom data section to store the shellcode.

PE-Bear

PE-Bear is a software which helps to check the PE structure: Headers, Sections, etc. PE-Bear provides a graphic user interface to show all relevant EXE details.

Introduction to Shellcode

Shellcode is a set of crafted machine code instructions that tell the vulnerable program to run additional functions and, in most cases, provide access to a system shell or create a reverse command shell.

Once the shellcode is injected into a process and executed by the vulnerable software or program, it modifies the code run flow to update registers and functions of the program to execute the attacker's code.

It is generally written in Assembly language and translated into hexadecimal opcodes (operational codes). Writing unique and custom shellcode helps in evading AV software significantly.

Methodology:

  1. First we write our code in assembly. Here is an example provided by tryhackme

global _start

section .text
_start:
    jmp MESSAGE      ; 1) let's jump to MESSAGE

GOBACK:
    mov rax, 0x1
    mov rdi, 0x1
    pop rsi          ; 3) we are popping into `rsi`; now we have the
                     ; address of "THM, Rocks!\r\n"
    mov rdx, 0xd
    syscall

    mov rax, 0x3c
    mov rdi, 0x0
    syscall

MESSAGE:
    call GOBACK       ; 2) we are going back, since we used `call`, that means
                      ; the return address, which is, in this case, the address
                      ; of "THM, Rocks!\r\n", is pushed into the stack.
    db "THM, Rocks!", 0dh, 0ah

  1. We compile and link the ASM code to create an x64 linux executable file and execute the program.

nasm -f elf64 thm.asm
ld thm.o -o thm
./thm

  1. We extract the shellcode with the objdump command by dumping the .text section of the compiled binary.

objdump -d thm

  1. We use objcopy to dump the .text section into a new file called thm.text in a binary format objcopy -j .text -O binary thm thm.text

  2. The thm.text contains our shellcode in binary format, so to be able to use it, we will need to convert it to hex first. The xxd command has the -i option that will output the binary file in a C string directly xxd -i thm.text

  3. To confirm that the extracted shellcode works as we expected, we can execute our shellcode and inject it into a C program.

#include <stdio.h>

int main(int argc, char **argv) {
    unsigned char message[] = {
        0xeb, 0x1e, 0xb8, 0x01, 0x00, 0x00, 0x00, 0xbf, 0x01, 0x00, 0x00, 0x00,
        0x5e, 0xba, 0x0d, 0x00, 0x00, 0x00, 0x0f, 0x05, 0xb8, 0x3c, 0x00, 0x00,
        0x00, 0xbf, 0x00, 0x00, 0x00, 0x00, 0x0f, 0x05, 0xe8, 0xdd, 0xff, 0xff,
        0xff, 0x54, 0x48, 0x4d, 0x2c, 0x20, 0x52, 0x6f, 0x63, 0x6b, 0x73, 0x21,
        0x0d, 0x0a
    };
    
    (*(void(*)())message)();
    return 0;
}

  1. Then, we compile gcc -g -Wall -z execstack thm.c -o thmx and we execute it ./thmx

Generate shellcode using Public Tools

  • msfvenom -a x86 --platform windows -p windows/exec cmd=calc.exe -f c this shellcode generated with msfvenom will launch calc in the target.

Shellcode injection

Hackers inject shellcode into a running or new thread and process using various techniques. Shellcode injection techniques modify the program's execution flow to update registers and functions of the program to execute the attacker's own code.

AMSI Bypass

Resources

  • This documentation has been made from my notes of this room on TryHackme:

  • Other convenient resources on the subject includes

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