Tags: pwn exploitation 

Rating: 5.0

# Jim Moriarty

## Vulnerability
+ A NULL byte overwrite

If you input an invalid size while allocating the `g_buf`,it would be record. But when you allocate successfully,the program will still append a NULL byte with the invalid size. So you can write a NULL byte on heap or libc.

## Exploit
+ Allocate a large buffer so that it can be in front of libc.
+ You need to calculate the size and the offset of stdin first.
+ Overwrite the last byte of `_IO_buf_base` with null byte in the stdin structure
+ It's very similar to [parrot](https://github.com/scwuaptx/CTF/blob/master/2017-writeup/twctf/Parrot.md)
+ Forge the FILE structure in `g_buf`
+ We want to trigger virtual function call in `_IO_flush_lockp` like [house of orange](http://4ngelboy.blogspot.tw/2016/10/hitcon-ctf-qual-2016-house-of-orange.html).
+ So we need to let
+ `_IO_write_ptr` > `_IO_write_base`
+ `mode` <= 0
+ `vtable` point to the function that we want to call. (We want to call `scanf`)


+ Overwrite the element in the `stdin` structure
+ Use the last `getchar()` to read data
+ Overwrite the `_chain` let point to `g_buf_ptr`
+ But we don't know address of `g_buf_ptr` so that we can't directly control it.
+ So we need to let `g_buf_ptr` be `_chain` pointer.
+ It is very close to `.got.plt`


+ Trigger `_IO_flush_all_lockp`
+ It would be called in `exit`.
+ Because it just want to flush all stream :)
+ Now, the linked list of FILE structure is
+ `_IO_list_all` -> `stderr` -> `stdout` -> `stdin` -> `fake_fp` -> `g_buf_ptr`
+ Finally, it would call `scanf(g_buf_ptr)`
+ Use scanf to do format string attack
+ We can use `scanf("%13$s%9$s")`
+ `%13$s`: Just read until stdin buffer is empty
+ `%9$s` : Read data to stack
+ It would trigger stack overflow and you can do ROP.
+ [exploit](jim_moriarty.py)