How do XSS vulnerabilities work?
What is the principle of XSS attack, specific code examples are needed
With the popularity and development of the Internet, the security of Web applications has gradually become the focus of attention. Among them, Cross-Site Scripting (XSS) is a common security vulnerability that web developers must pay attention to.
XSS attack is to inject malicious script code into the web page and execute it in the user's browser. This way the attacker can control the user's browser, obtain the user's sensitive information, or perform other malicious operations. . XSS attacks can be divided into three types: storage, reflection and DOM.
A stored XSS attack is when the attacker stores malicious script code in the database of the target website. When the user browses the attacked page, the server sends the malicious script to the user's browser for execution. This attack can steal users' sensitive information, such as login credentials, personal data, etc.
Reflected XSS attack is when the attacker constructs a malicious URL and sends the URL containing malicious script code to the target user. After the user clicks on the URL, the server will return the malicious script code as a parameter to the user's browser, and the browser will execute the script. This type of attack is commonly seen on phishing websites and social engineering attacks.
DOM-type XSS attacks are carried out by modifying the DOM structure of the page. The attacker constructs a URL that contains malicious script code. When the user clicks on this URL, the browser will execute the script and change the DOM structure of the page, thus achieving the attack. This attack method is common in some highly interactive web applications, such as online editors, message boards, etc.
The following uses specific code examples to demonstrate the principles of XSS attacks.
Suppose there is a webpage with a guestbook function, where users can post messages and display them. The following is the code for a simple message display function:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>留言本</title>
</head>
<body>
<h1 id="留言本">留言本</h1>
<div id="messages">
<!-- 留言内容展示在这里 -->
</div>
<form action="save_message.php" method="POST">
<input type="text" name="message" placeholder="请输入留言">
<input type="submit" value="提交留言">
</form>
</body>
</html>In the above code, after the user enters the message content in the text box and clicks the "Submit Message" button, the message will be sent to save_message. php to save. The following is the code of save_message.php:
<?php $message = $_POST['message']; // 实现留言的保存操作，略... echo "<div>" . $message . "</div>"; ?>
In this simple example, the message is stored on the server side, and the message content is dynamically displayed on the < div id="messages">. However, without proper verification and filtering measures, attackers can inject malicious script code into the message content to conduct XSS attacks.
For example, an attacker may enter the following content as the message content:
<script>
alert('你的帐号已被攻击');
// 或者发送用户的cookie信息到攻击者的服务器
</script>When other users browse the guestbook page, this malicious script code will be dynamically generated into <div> to execute in their browser. This will pop up a dialog box prompting the user that their account has been attacked. <p>To prevent XSS attacks, web developers need to perform input validation and output filtering. Input validation refers to checking the data entered by the user to ensure that it conforms to the expected format and content. Output filtering refers to processing the data to be output to the page and escaping special characters in it to protect the security of the user's browser. </p>
<p>To sum up, the principle of XSS attack is to perform malicious operations in the user's browser by injecting malicious script code. To protect the security of web applications, developers should pay attention to input validation and output filtering to prevent XSS attacks from occurring. </p>
</div>
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