直接给了源码:
<?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><?= $message ?></p> <?php } ?> <form action="index.php" method="POST"> <input type="text" name="guess"> <input type="submit"> </form> </body> </html>
分析源码,结合过往做的爆破随机数题目的经验可以看出随机数是个幌子,直接忽略.随后百度$_SERVER['PHP_SELF']
发现文章:[红日安全]代码审计Day15 – $_SERVER[‘PHP_SELF’]导致的防御失效问题
而且源码中还用到了basename(),可以获取到最后的文件名,而get请求source可以显示basename($_SERVER['PHP_SELF'])
文件的内容,结合之前找到的文章构造payload,/index.php/config.php/?source
.但是这里使用了正则对config.php是否为$_SERVER[‘PHP_SELF’]的结尾进行了判断,若config.php是结尾则程序结束.所以我们还需要绕过该正则.basename()函数会去掉非ascii字符,所以我们在config.php后面再加上非ascii字符然后get请求source.最终payload:/index.php/config.php/%ff?source
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