WM03 - 250pts - NCS Competition 2021 Writeup

Briefing
Visit the site at https://cfta-wm03.allyourbases.co and find a way to bypass the password check.
Solution
1.The source code for the HTML index has a PHP snippet that shows how the site validates the password:
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 <!--
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 TODO: remove, taken from OSS project, login contains:
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 -->
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 return function ($event) {
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     require_once("flag.php");
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     $hash = "0e747135815419029880333118591372";
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     $salt = "e361bfc569ba48dc";
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     if (isset($event['password']) && is_string($event['password'])) {
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         if (md5($salt . $event['password']) == $hash) {
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             return $flag;
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         }
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     }
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     return "Incorrect";
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 };
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2.I tried bruteforcing the salt and hash using hashcat, but that did not succeed.
3.The bug is that the hash of the salt and password is compared to the saved hash using == (double equals) instead of === (Triple equals). PHP interprets strings with only numbers and an e in them as float number format strings (numerical strings). If you use == in php, when you compare a number with a string or the comparison involves numerical strings, then each string is converted to a number and the comparison is performed numerically.
4.If we can find a password that when salted hashes to 0e followed by only digits then when it is compared to the stored hash, it will evaluate to true. Both of the strings are converted to 0 when compared with ==. If you want to compare them as strings, you should use === (strict comparison) instead. More info: https://stackoverflow.com/a/22140266​
5.There might be a better way, but I simply wrote a bruteforce algorithm to try all the permutations of 8 ascii lowercase letters, prepend the salt, hash the string, and then check if the hash starts wth 0e and contains only digits after that. It took about 3 minutes 30 seconds to find a valid password doing about 600,000 to 700,000 attempts per second. The discovered password was acpgvxjy and entering it into the site displays the flag.
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theLOOSEtheMATH&theTRUTHY

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Briefing

Solution

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