From ad2163fcf44e0a7227eaec57409932f566c89dd7 Mon Sep 17 00:00:00 2001
From: Matthias Prader <e1625755@student.tuwien.ac.at>
Date: Fri, 13 Dec 2019 15:43:50 +0000
Subject: [PATCH] Add asis19
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writeups/matthias/asis19.md | 54 +++++++++++++++++++++++++++++++++++++
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+## The CTF in retrospective
+---
+The Asis CTF was great fun. The two web challenges I looked at involved a little too much guesswork for my taste but the teamwork on Mattermost was great! I spend about 20 hours on these challenges.
+
+## Web: Protected Area (solved)
+---
+The link to the challenge leads to a website with a title `This site has a protected area`, the test `welcome` and a line of text (`I have to go GYM I should fix my site bugs`), that appear shortly after opening the site. A look into the pages HTML code showed, that jQuery is loaded and a custom is executed. The script consists of three methods:
+- `file_check(file)` makes an Ajax request to `/check_perm/readable/` with `file` added as the value of a parameter named file (`file_check(testfile)` would make the request `check_perm/readable?file=testfile`). If the response to that request is `True` the method `file_read(file)` is called. If not `fail` is printed to the console.
+- `file_read(file)` makes an Ajax request to `/read_file` and adds the same parameter. The result of that request is passed to the function `update_page(text)`
+- `update_page(text)` appends the `text` to an element in the HTML Document.
+The `$(document).ready()` function prints `ready` to the console and calls `file_check('public.txt')`
+
+We (the people on Mattermost) assumed that the backend could be in python because of `True` as a possible result of `check_perm/readable`. Also, we assumed, that we have to read the Flag somehow via `/read_file?file=xyz`.
+
+After a lot of trying and guessing we found out that:
+- `/read_file?file=xyz` returns `security` if it does not end with `.txit`. We interpreted it so, that the value of the parameter `file` must end tith `.txt`
+- `/read_file?file=xyz` replaces `../` in the file value only once. So `/read_file?file=....//xyz` is possible
+- if the filename does not end tith `.txt` or contains multiple `.txt` the server responds with `security`. If the file ends with a single `.txt` but does not exist the server responds with `500`.
+
+Someone then tried to add other parameters. It turned out that `/read_file/?file=private.txt&debug=true` resulted in `security` but `/read_file/?debug=true&file=private.txt` in `500`. So we tried to add a second parameter after `file` and end the value of that parameter with `.txt`:
+`/read_file/?file=xyz&test=ha.txt` worked! So our interpretation was wrong. Its only checked if the whole query string ends with `.txt` and nor the value of the `file` parameter.
+
+In the meantime we already could look at Protected Area 2, which pointed to a Github repo of a docker container that was apparently used for these challenges. So we knew that there was a folder `/app/` that contains a `main.py`
+`/read_file/?file=....//....//....///app/main.py&test=ha.txt` allowed us to read the content of the python file.
+By looking at the python files that are imported in the main file we found the protected endpoint `/protected_area_0098` which sends the flag back (in the file `/app/application/api.py`). But a `check_login` method is executed before. In a file imported by the `api.py`, the file `/api/application/functions.py` was the `check_login` method. The method looks for a header named `ah` and checks if its value is equal than a MD5 hash of some values located in a configfile. We found the config.py at `/app/config.py` so we had the values, could craft the correct header. `curl -H "ah: cbd54a3499ba0f4b221218af1958e281" http://66.172.33.148:8008/protected_area_0098` resulted in `ASIS{f70a0203d638a0c90a490ad46a94e394}`
+
+##Web: Protected Area 2 (not solved)
+---
+Protected Area 2 had exactly the same structure as Protected Area 2. The only defference in the pages sources was a hint to a file named `private.txt`. This file contained the link to the already mentioned Github repo.
+We could move between directories with `read_file` as in Protected Area 1. But this time the trick with adding another parameter to bypass the check if the file ends with `.txt` did not work.
+We found out, that this time the `check_perm` function was implemented differently. `http://66.172.33.148:5008/check_perm/wtf/?file=public.txt` showed the error message `'_io.TextIOWrapper' object has no attribute 'wtf'`. So apparently we can decide which method to execute. By changing the `wtf` to `read` we were able to read files.
+We looked at the `nginx.conf` file
+```
+server {
+ listen 80;
+ location / {
+ try_files $uri @app;
+ }
+
+ location @app {
+ include uwsgi_params;
+ uwsgi_pass unix:///tmp/uwsgi.sock;
+ }
+
+ location /static {
+ alias /opt/py/app/static;
+ }
+}
+```
+which told us, that the base bolder for the app must be in `/opt/py/app`. In that folder we found a `config.py` which contained the path to the flag but unfortunately we couldn't read it with our method (no permission i think).
+
+As we saw that all requests are passed to `uwsgi` we searched for a config file for it but found only the two standard ones. And thats where we couldn't make any progress anymore.
+
+After the CTF has ended, I looked at a Writeup and saw, that the `uwsgi.ini` we looked for was located in `opt/py/app`. I am sure that i've tried to look in that folder for this file. Probably i misspelled `uwsgi` that happened a lot during that ctf. I often wrote `uswgi`. Too bad.
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