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234 lines
7.6 KiB
234 lines
7.6 KiB
4 years ago
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author = "Maik de Kruif"
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title = "Challenge 22 - AdventOfCTF"
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date = 2021-03-04T01:24:34+01:00
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description = "A writeup for challenge 22 of AdventOfCTF."
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cover = "img/adventofctf/6c0810c1568645bcf58da67a1db6e3e7.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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"php",
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"ssrf",
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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: 2200
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## Description
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We have a new service! You can view santa's favorite pictues. Currently there is only one, but it is a very good one! You can get the flag through flag.php.
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Visit https://22.adventofctf.com to start the challenge.
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## Recon
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Upon opening the challenge website, we're greeted with a link with the text "Is this santa?". If we click on it, it redirects to `/index.php?image=cat.jpg`.
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This page shows us a nice picture of a cat.
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## Finding the vulnerability
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When looking at this, you might think of Local File Inclusion (LFI). This makes sense as it is one of the most common vulnerability in opening files.
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We can try to use it to open `flag.php`. To try this, let's replace `cat.jpg` with `flag.php` in the URL.
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When opening the page, we will see a broken image, this is expected as the file it not an image. If we open the source we find the following:
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```html
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<img
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src="data:image/jpeg;base64,PD9waHAKCmluY2x1ZGUoInNlY3JldC5waHAiKTsKCmlmIChzdHJwb3MoY2hlY2tfc2VjcmV0KCksICJhbGxvdyIpICE9PSBmYWxzZSkgewogICBlY2hvIGdldF9mbGFnKCk7IAp9Cgo/Pgo="
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width="100%"
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/>
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```
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Here we see some `base64` encoded data, let's decode it using the following command:
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```bash
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echo -n "PD9waHAKCmluY2x1ZGUoInNlY3JldC5waHAiKTsKCmlmIChzdHJwb3MoY2hlY2tfc2VjcmV0KCksICJhbGxvdyIpICE9PSBmYWxzZSkgewogICBlY2hvIGdldF9mbGFnKCk7IAp9Cgo/Pgo=" | base64 -d
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```
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```php
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<?php
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include("secret.php");
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if (strpos(check_secret(), "allow") !== false) {
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echo get_flag();
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}
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?>
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```
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If we look at this code, we see that if `check_secret()` contains `"allow"`, it will execute the `get_flag()` function (which will probably give us the flag).
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The functions are not defined in this file so they probably come from `secret.php`. Let's try to read that file.
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Alas, we get the cat picture again. That's weird. There might be a filter on the input. Let's verify that by reading the `index.php` file.
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Using the same decoding method, we get the following result:
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{{< code language="php" title="index.php" >}}
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```html
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<!DOCTYPE html>
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<html class="no-js" lang="">
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<head>
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<meta charset="utf-8" />
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<meta http-equiv="x-ua-compatible" content="ie=edge" />
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<title>Advent of CTF 22</title>
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<meta name="description" content="" />
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<meta name="viewport" content="width=device-width, initial-scale=1" />
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<link rel="stylesheet" href="style.css" type="text/css" media="screen" />
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<link
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rel="stylesheet"
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href="https://use.fontawesome.com/releases/v5.6.3/css/all.css"
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integrity="sha384-UHRtZLI+pbxtHCWp1t77Bi1L4ZtiqrqD80Kn4Z8NTSRyMA2Fd33n5dQ8lWUE00s/"
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crossorigin="anonymous"
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/>
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<style>
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.row-margin-05 {
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margin-top: 0.5em;
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}
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.row-margin-10 {
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margin-top: 1em;
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}
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.row-margin-20 {
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margin-top: 2em;
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}
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.row-margin-30 {
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margin-top: 3em;
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}
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</style>
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</head>
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<body>
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<div class="jumbotron bg-transparent mb-0 radius-0">
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<div class="container fluid">
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<div class="row">
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<div class="col-xl-6 mx-auto">
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<h1 class="display-2">
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Advent of CTF <span class="vim-caret">22</span>
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</h1>
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<div class="lead mb-3 text-mono text-warning">
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Your daily dose of CTF for December
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</div>
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<div class="row">
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<div class="col-xl-12 mx-auto">
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<div class="card">
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<div class="card-header text-center">
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<h2>The big reveal</h2>
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</div>
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<div class="card-body">
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<?php
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if (!isset($_GET["image"])) {
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?>
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<a href="/index.php?image=cat.jpg">Is this santa?</a>
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<?php
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} else {
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$path = $_GET["image"];
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if (strpos($path,"secret") !== false) {
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$path="cat.jpg";
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}
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$image = file_get_contents($path);
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echo '<img src="data:image/jpeg;base64,'.base64_encode($image).'" width="100%"/>';
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} ?>
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</div>
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<div class="card-footer text-center">Almost there</div>
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</div>
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</div>
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</div>
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<div class="row row-margin-30">
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<div class="card mb-3 bg-dark text-white">
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<div class="card-body">
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<div class="row">
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<div class="col-md-2">
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<img src="/logo.png" />
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</div>
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<div class="col-md-9 offset-md-1 align-middle">
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<p class="text-center">
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<span class="align-middle">
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The Advent of CTF is brought to you by
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<a href="http://www.novi.nl">NOVI Hogeschool</a>. It
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is built by
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<a
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href="https://twitter.com/credmp/"
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class="icoTwitter"
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title="Twitter"
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><i class="fab fa-twitter"></i> @credmp</a
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>. If you are looking for a Dutch Cyber Security
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Bachelor degree or bootcamp,
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<a href="https://www.novi.nl">check us out</a>.
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</span>
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</p>
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</div>
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</div>
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</div>
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</div>
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</div>
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</div>
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</div>
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</div>
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</div>
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</body>
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</html>
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```
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{{< /code >}}
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Just the PHP part:
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```html
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<?php
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if (!isset($_GET["image"])) {
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?>
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<a href="/index.php?image=cat.jpg">Is this santa?</a>
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<?php
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} else {
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$path = $_GET["image"];
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if (strpos($path,"secret") !== false) {
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$path="cat.jpg";
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}
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$image = file_get_contents($path);
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echo '<img src="data:image/jpeg;base64,'.base64_encode($image).'" width="100%"/>';
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} ?>
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```
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Here we see that we cannot get any file containing "secret". This means we have to find another way to get the flag.
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We see that this code is using the `file_get_contents()` function to open file. Let's have a look at the [PHP documentation for it](https://www.php.net/manual/en/function.file-get-contents.php).
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If we read a bit we find the following example usage:
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```php
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<?php
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$homepage = file_get_contents('http://www.example.com/');
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echo $homepage;
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?>
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```
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This means it can also open URLs and, since we control the input to the function, we have a Server-Side Request Forgery (SSRF) vulnerability. Let's try to use that to open the flag file.
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To test this, we can use the following input; `image=http://localhost/flag.php`.
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_Note: We can not use the `secret.php` file as any input containing "secret" is blocked._
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We, again, got some base64 encoded data back, so let's decode it:
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```bash
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echo -n "Tk9WSXthc2tpbmdfZm9yX2FfZnJpZW5kfQ==" | base64 -d
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```
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## Solution
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We got the flag! It is `NOVI{asking_for_a_friend}`.
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