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112 lines
4.5 KiB
112 lines
4.5 KiB
+++
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author = "Maik de Kruif"
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title = "Challenge 14"
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subtitle = "Challenge 14 - AdventOfCTF"
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date = 2020-12-14T19:45:51+01:00
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description = "A writeup for challenge 14 of AdventOfCTF."
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cover = "img/writeups/adventofctf/2020/dd04640480d764ab09eea047cde749cd.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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]
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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: 1400
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## Description
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We are testing a new 2-factor security system for Santa's deepest secrets. It should be pretty secure!
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Visit <https://14.adventofctf.com> to start the challenge.
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## Finding the vulnerability
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Upon opening the challenge website we're greeted with some PHP code, as well as two input fields. The PHP code is the following:
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```php
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<?php
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ini_set('display_errors', 0);
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include("flag.php");
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if (isset($_POST["password"], $_POST["verifier"])) {
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$password = $_POST["password"];
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$verifier = $_POST["verifier"];
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$hash = sha1($password + $secret_salt);
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$reference = substr($hash, 0, 7);
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if ($verifier === $reference) {
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echo $flag;
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die();
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}
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}
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header("Location: /index.php?error=That was not right.");
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exit();
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?>
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```
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As we can see, we only get the flag if `$verifier` is equal to `$reference`, where `$reference` is the `SHA-1` hash of the password in our request together with `$secret_salt`.
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_Note: when I started with this challenge, [@credmp](https://twitter.com/credmp) had already posted a hint which I, unfortunately, saw. It said "The salt is a number."._
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## Type Juggling
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Because we already know `$secret_salt` is a number, we can leverage PHP type juggling to generate a known value for the hashing algorithm. But first, what is type juggling?
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Because PHP does not support explicit type definition in a variable declaration, a variable's type is determined by the context in which the variable is used. For instance, if a `string` value is assigned to a variable `$var`, `$var` becomes a string. But if an `int` value is then assigned to `$var`, it becomes an int. This means that if we have a `string` and add an `int` to it, the outcome will be an `int`. The value of this `int` depends on what is inside the `string`. If the `string` starts with a number, it will be interpreted as an `int` with the value of the number inside the `string`, and if it starts with a letter, it will be interpreted as `0`.
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For example, if we have a variable with the value `"hello"`, it will be converted to `0`. And if we have a variable with the value `"24"` or `"12ab"` it will be interpreted as `24` and `12` respectively.
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## Generating a verifier value
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To see what input will be converted to what output, we can create a little script that will generate those values for us. An example would be the following:
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```php
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<?php
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$password = "1a";
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$secret_salt = "11";
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$hash = sha1($password + $secret_salt);
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$reference = substr($hash, 0, 7);
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echo ($password + $secret_salt)."\n";
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echo $hash."\n";
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echo $reference."\n";
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?>
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```
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We can then run this script on our local machine or on a website like <https://sandbox.onlinephpfunctions.com/>.
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This script will take the two inputs and give us the outcome of the addition with the `$secret_salt` (I used 11 as the salt number but it could be any number), the generated hash and thus the `$verifier` input we need for the original script.
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If we make `$password` a really big number like `922337203685477580792233720368547758079223372036854775807` we see that the output of the addition will be written in the scientific notation if it is converted to a string. And because the `sha1()` function wants a `string` for the input, it will also get the shrunk-down version. This means that if we make `$password` big enough, the value of `$secret_salt` will not matter as it is not significant enough for the output.
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If we thus enter `922337203685477580792233720368547758079223372036854775807` as `$password` in the above script, we get the following output:
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```text
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9.2233720368548E+56
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48a888ebec04f516e8b765bc3879354411ac2387
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48a888e
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```
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If we thus use `922337203685477580792233720368547758079223372036854775807` as the password and `48a888e` as the verifier, it should echo the flag.
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## Solution
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After entering the above form data and submitting it, we get the flag: `NOVI{typ3_juggl1ng_f0r_l1fe}`!
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This flag can then be submitted for the [challenge](https://ctfd.adventofctf.com/challenges#14-15).
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