Tags: python regex bypassfilter
Rating:
TheHiddenOne [App Script, 249 points, 13 solves]
========
### Description
>You can send a python file to the administrator.
>
>But he **cat** everything to exec only simple print!
>
>Hint: `(^print\([\"'][a-z A-Z]*[\"']\)[;]?$)|(^#.*)`
>
>[http://remote1.thcon.party:10200](http://remote1.thcon.party:10200)
>
>[http://remote2.thcon.party:10200](http://remote2.thcon.party:10200)
>
>**Creators** : gus (Discord: gus#4864)
### Overview
This challenge involves uploading a Python file that has to pass some validation before being run. After solving this challenge, however, I realized that my solution was unintended :)
### Ideas
The challenge description mentions that an administrator will examine the uploaded file (using `cat`) and that it must also match a provided regex before being run. I wasn't sure what was meant about the administrator checking the source with `cat`, but that didn't matter because I knew of a way to run arbitrary Python code while matching the provided regex. In fact, the only part of the regex I cared about was the `#.*` part, which was _intended_ to only allow comments. Python has a little-known feature where you can, on the first or second line of a script file, write a special comment that gets interpreted as a directive to change the encoding used to read the file. This is most commonly used to change the encoding to UTF-8 (to allow non-ASCII characters) like so:
```python
#!/usr/bin/env python
# -*- coding: utf-8 -*-
#...rest of file...
```
However, the encoding you pick can be any of Python's register codecs. One of these supported codecs is `raw_unicode_escape`, which allows you to intersperse Unicode escape sequences like `\u265F` for a black chess pawn Unicode character. You can also encode arbitrary ASCII characters this way, such as a newline (`\u000a`). This means that you can "hide" Python code on a comment line such that it will actually execute. My [final exploit script](test.py) is below:
```python
# coding: raw_unicode_escape
#\u000aimport os
#\u000aos.system("ls -laF")
#\u000aos.system("cat *flag*")
```
This results in the flag being displayed: `THCon21{D0nt_tRusT_c4t_4nd_use_c4t_-v}`.
### Unintended solution?
After solving the challenge, the text in the flag made me wonder if my solution was unintended. I looked up the `-v` flag for `cat` and found that it controls whether unprintable characters like ASCII control codes are printed. I then used my ability to execute code to download the challenge's source code to see how it was actually validating the uploaded Python code. The web service's [index.php](index.php) effectively just passes the uploaded file to [/srv/chall.py](chall.py). This script then sets up a terminal emulator using a Python library called "pyte", with a fixed window size. After effectively running `cat uploaded.py` in this terminal emulator, the script then matches the contents of each line against the regex. If any line doesn't match the regex, it fails validation and refuses to run the script. The use of a terminal emulator is unnecessary for my exploit, so that leads me to believe that the intended solution is to use ASCII control codes like `\x7f` (backspace) to have a line that starts with Python code, then a bunch of backspace characters, then a '#' character so that the script thinks the line is a comment and will allow it. Or some other combination of other control codes to do something similar.
Regardless, my solution worked, and we got the points for the challenge, so I'm happy :D
