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90 lines
3.2 KiB
90 lines
3.2 KiB
+++
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author = "Maik de Kruif"
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title = "Challenge 8 - AdventOfCTF"
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date = 2020-12-08T09:34:24+01:00
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description = "A writeup for challenge 8 of AdventOfCTF."
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cover = "img/adventofctf/da781419d6bf02d0a580e48414b9cbde.png"
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tags = [
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"AdventOfCTF",
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"challenge",
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"ctf",
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"hacking",
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"writeup",
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"web",
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]
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categories = [
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"ctf",
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"writeups",
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"hacking",
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]
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+++
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- Points: 800
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## Description
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If only you could figure out where to go.
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Visit <https://08.adventofctf.com> to start the challenge.
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## Finding the vulnerability
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When opening the website we're greeted with the following message:
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> Did you know that the fastest robot can solve a rubiks cube in 0.887 seconds?
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This is talking about robots, which my be a hint to look at the [`robots.txt`](https://08.adventofctf.com/robots.txt).
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### What is a robots.txt file?
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A `robots.txt` file lives at the root of a website. So, for the site www.example.com, a robots.txt file would live at www.example.com/robots.txt. robots.txt is a plain text file that follows the [Robots Exclusion Standard](http://en.wikipedia.org/wiki/Robots_exclusion_standard#About_the_standard). A robots.txt file consists of one or more rules. Each rule blocks (or allows) access for a given crawler to a specified file path in that website.
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### Opening the file
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The file shows the following:
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```text
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# robots.txt generated by *************.com
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User-agent: *
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Disallow: /
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Disallow: /cgi-bin/
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Disallow: /encryption/is/a/right
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Disallow: /fnagn/unf/znal/cynprf/gb/tb
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```
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This probably means there is some sensitive information on one of the `Disallow` locations. Let's look at them one by one.
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**`/cgi-bin/`**
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When opening [`/cgi-bin/`](https://08.adventofctf.com/cgi-bin/), we get a `404` error. So let's skip this one for now.
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**`/encryption/is/a/right`**
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Upon opening [`/encryption/is/a/right`](https://08.adventofctf.com/encryption/is/a/right/), we get some encoded string back. It looks like `base64` so let's try to decode it using `base64 -d` in the terminal:
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```bash
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echo "RW5jb2RpbmcgYW5kIGVuY3J5cHRpb24gYXJlIDIgZGlmZmVyZW50IHRoaW5ncy4=" | base64 -d
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> Encoding and encryption are 2 different things.
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```
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This doesn't mean a lot so let's have a look at the next one.
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**`/fnagn/unf/znal/cynprf/gb/tb`**
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After opening [`/fnagn/unf/znal/cynprf/gb/tb`](https://08.adventofctf.com/fnagn/unf/znal/cynprf/gb/tb/), we're greeted with the following text:
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> "Oh, the places you'll go", my favorite poem... but this is the wrong place. Maybe you read that wrong?
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Hmm, it says "Maybe you read that wrong?". The URL also looks kinda weird. It might be `rot13` encoded. So let's try to decode it using `rot13`:
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```bash
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echo "/fnagn/unf/znal/cynprf/gb/tb" | rot13
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> /santa/has/many/places/to/go
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```
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_Note: `rot13` is not a program on linux, I just programmed it as an alias for `tr 'A-Za-z' 'N-ZA-Mn-za-m'`_
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We got new url (I hope 😀). Let's try to [access it](https://08.adventofctf.com/santa/has/many/places/to/go/). We got the flag! It is `NOVI{you_have_br@1ns_in_your_head}`.
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This flag can then be submitted for the [challenge](https://ctfd.adventofctf.com/challenges#8-9).
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