It is crucial for front-end developers to understand and have knowledge of network security when building web applications. Here are some essential front-end network security knowledge, including common types of attacks, preventive measures and some code demonstration.

1. Cross-site scripting attacks (XSS)

Definition and Impact

XSS (Cross-Site Scripting) attacks allow attackers to inject malicious scripts into a user's browser to control user sessions, steal sensitive information, etc. XSS is categorized into Stored XSS, Reflected XSS, and DOM Based XSS.

Preventive Measures

• Input validation and filtering: Strictly validate and filter user input data to avoid injection of malicious scripts.
• Output Encoding: HTML entity encoding or URL encoding of user input to prevent it from being parsed as executable scripts on the page.

Code Demonstration (Output Encoding)

// Suppose there is a piece of user-input text stored in the variable userInput
var userInput = “<script>alert(‘XSS Attack’);</script>”;”;

// Output using HTML entity encoding
var safeOutput = userInput.replace(/&/g, '&')
                          .replace(/</g, '<')
                          .replace(/>/g, '>')
                          .replace(/"/g, ‘"’)
                          .replace(/'/g, ''');

// Insert the encoded text into the DOM
document.getElementById('userContent').textContent = safeOutput; // use textContent instead of innerHTML

2. Cross-Site Request Forgery (CSRF)

Definition and Impact

CSRF (Cross-Site Request Forgery) attack accomplishes the attacker's goals, such as modifying user data, transferring money, etc., by inducing the user to execute a malicious request on a logged-in website.

Preventive Measures

• Add CSRF Token: Add a randomly generated CSRF token to each form or request and validate it on the server side.
• Validate Source (Referer): Although the Referer field can be tampered with, it can be used to some extent as a secondary means of validation.

Code Demonstration (Adding CSRF Token)

<! -- Assuming a CSRF Token is added to the form -->
<form action=“/submit” method=“post”>
    <input type=“hidden” name=“csrf_token” value="{
    { csrf_token }}">
    <! -- Other form fields -->
</form

// The server needs to verify that the csrf_token field in the request matches the token in the session.

3. Clickjacking

Definition and Impact

Clickjacking tricks a user into clicking on a seemingly innocuous area that actually triggers a malicious action or visits a website without the victim's knowledge.

Preventive measures

• Use of X-Frame-Options header: Disable page embedding in iframes by setting the X-Frame-Options header to “deny” or “sameorigin”.
• Use Content Security Policy (CSP): Prevent clickjacking by limiting the resources that can be loaded on the page through CSP policy.

Code Demo (Setting X-Frame-Options)

Setting the response headers in the web server or web application framework:

X-Frame-Options: DENY

Or, set it in the configuration file of the web application (Nginx for example):

add_header X-Frame-Options “SAMEORIGIN”.

4. Other security knowledge

• HTTPS: Use HTTPS protocol to encrypt the communication between the client and the server to prevent man-in-the-middle attacks.
• Content Security Policy (CSP): Reduce the risk of XSS attacks by restricting the sources of scripts, CSS, and other resources that can be executed on a page through CSP policies.
• Regular updates and maintenance: Keep the system secure by updating and fixing application vulnerabilities and security issues in a timely manner.

Front-end developers need to constantly learn and pay attention to the latest developments in the field of network security, combine with experience in practice, and constantly improve their security awareness and skill level. At the same time, it is also necessary to actively participate in teamwork to jointly build a secure Web application environment.