打开题目
看到信息随便输入一个数，显示错误
查看源代码
看到php代码，代码审计
<?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>
basename()
会返回路径重的文件名部分。比如/index.php/config.php使用basename()之后返回config.php。basename()会去掉文件名开头的非ASCII值。
看到提示，有config.php目录，跳转一下看看。
得到
、
我们要想访问config，但是config.php被正则过滤了
本题目利用的是basename()漏洞
用不可显字符绕过正则（后面加 %80 – %ff 的任意字符）
我们构造payload：/index.php/config.php/%ff?source
访问得到flag
