# leap-frog

## Problem

> Can you jump your way to win in the following program and get the flag? You can find the program in /problems/leap-frog\_4\_32907c7b6e253bd5d9422083e7243619 on the shell server? Source.

* [Program](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Binary%20Exploitation/leap-frog/rop/README.md)
* [Source](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Binary%20Exploitation/leap-frog/rop.c)

## Solution

1. Run the [script.py](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Binary%20Exploitation/leap-frog/script.py) like so: `python script.py USER=username PASSWORD=password`
2. This is not the intended solution. This is a classic ROP challenge. But instead of going through the process as intended, all the win\* variables can be set to 1 by calling gets correctly. Libc's "gets" function is able to write anything typed into stdin to any writable segment of memory. This program writes 0x01 ("true") on top of the win1, win2, and win3 variables in memory to skip the need to call the leap functions. Then display\_flag() is called.
3. The payload is as follows:
   1. padding
   2. gets\_plt (address of `gets@plt`): manipulates the return address of the first `gets()` in the `vuln()` function. So when the first `gets()` finishes, it jumps to the second `gets()`
   3. display\_flag\_addr (address of `display_flag()`): overwrites the return address of the second `gets()`. So when the second `gets()` finishes, it jumps to `display_flag()`.
   4. win1\_addr (address of the `win1` variable): buffer for the second `gets()`. So the second `gets(`) expects another input from the user and writes it to the address of `win1`. The three bytes written will overflow into `win2` and `win3` and set them to true as well.

      Extremely helpful explanation: [StackOverflow answer by d4rwel](https://stackoverflow.com/a/60916625)
4. These write-ups describe the intended ROP solution: [Dvd848](https://github.com/Dvd848/CTFs/blob/master/2019_picoCTF/leap-frog.md) ([Archive](https://web.archive.org/web/20200601203839/https://github.com/Dvd848/CTFs/blob/master/2019_picoCTF/leap-frog.md)) and [Fascinating Confusion](https://fascinating-confusion.io/posts/2019/10/leapfrog-writeup/) ([Archive](https://web.archive.org/web/20200601203736/https://fascinating-confusion.io/posts/2019/10/leapfrog-writeup/))

### Flag

`picoCTF{h0p_r0p_t0p_y0uR_w4y_t0_v1ct0rY_183d3d88}`


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