Что такое XSS и как защитить свое веб-приложение от атак

XSS (Cross-Site Scripting)

XSS (Cross-Site Scripting) is a type of vulnerability in web applications that allows an attacker to inject malicious code into web pages that are accessed by other users. In a successful attack, the attacker can gain access to user-entered information, including login credentials, passwords, payment card data, and other confidential information.

XSS attacks are divided into three types: persistent, reflected, and DOM-based. Let's take a closer look at each type.

Persistent XSS Attacks

Persistent XSS attacks, also known as stored XSS, occur when an attacker enters malicious code that is then stored on the server and delivered to other users with each request. For example, consider a web application that allows users to leave comments on a page. If the application does not properly sanitize user-entered data before storing it on the server, an attacker can enter malicious code into a comment. When other users view the page, their browsers will execute this code, which can lead to session theft or the injection of malicious scripts.

Reflected XSS Attacks

Reflected XSS attacks occur when malicious code is injected into URL parameters or user-inputted data on a page and then reflected back to the user. For example, if a web application allows users to search for information using keywords, and the search is done by appending the keywords to URL parameters, an attacker can create a link with malicious code and send it to another user. When the user opens this link, the malicious code will be executed in the victim's browser.

DOM-based XSS Attacks

DOM-based XSS attacks occur when client-side JavaScript code is used to modify the DOM structure of a web page. If a web application does not perform necessary input validation, an attacker can create a URL with malicious parameters that manipulate the DOM structure of the page. For example, if a URL parameter is used to modify the classes of DOM elements, an attacker can enter a malicious parameter value that will trigger the execution of malicious scripts.

Preventing XSS attacks in web applications requires a series of measures. It is important to properly filter all user-entered data before displaying it on a web page. Filtering can be implemented using functions such as htmlspecialchars() or htmlentities(), which replace special characters with their escaped equivalents.

Below are examples of PHP and JavaScript code that demonstrate the use of these functions to filter user input:

PHP:

JavaScript:

In these examples, the htmlspecialchars() function in PHP and the encodeURIComponent() function in JavaScript are used to escape special characters such as "<", ">", "&", "'", "«", and "»". This allows the user-entered data to be displayed without the risk of executing malicious code.

In conclusion, XSS attacks pose a serious threat to web applications and require careful attention during development and testing. Proper filtering of user input and the use of special functions to escape special characters can help prevent XSS attacks and ensure the security of web applications.