What are the most important HTML5 security considerations?

1 Cross-Origin Resource Sharing

Cross-Origin Resource Sharing (CORS) is a mechanism that allows web pages to request resources from different domains, such as images, fonts, scripts, or APIs. This can improve the performance and functionality of web applications, but it also poses a potential threat to the confidentiality and integrity of data. If a web page allows CORS requests from any origin, it may expose sensitive information or allow malicious code to execute on the user's browser. To prevent this, web developers should use the Access-Control-Allow-Origin header to specify which origins are allowed to access the resources, and the Access-Control-Allow-Methods and Access-Control-Allow-Headers headers to limit the types and content of the requests.

2 Cross-Site Scripting

Cross-Site Scripting (XSS) is a type of attack that injects malicious code into a web page, usually through user input or a vulnerable script. The code can then access the user's cookies, session data, or other information stored on the browser, or perform actions on behalf of the user, such as stealing credentials, redirecting to phishing sites, or spreading malware. HTML5 introduces new elements and attributes that can be used to execute code, such as <canvas>, <svg>, <audio>, <video>, or data-* attributes. To prevent XSS attacks, web developers should validate and sanitize user input, use Content Security Policy (CSP) to restrict the sources and types of code that can run on the web page, and use HTTP-only and Secure cookies to protect sensitive data.

3 Web Storage

Web Storage is a feature that allows web pages to store data locally on the user's browser, using either the sessionStorage or the localStorage objects. This can improve the user experience and performance of web applications, as well as enable offline access and persistence. However, it also raises some security issues, such as data leakage, unauthorized access, or tampering. If a web page stores sensitive data in web storage, it may be exposed to other web pages from the same origin, or to XSS attacks that can read or modify the data. To prevent this, web developers should encrypt and hash the data before storing it, use HTTPS to protect the data in transit, and use the same-origin policy to restrict the access to the data.

4 Web Workers

Web Workers are a feature that allows web pages to run scripts in the background, without blocking the main thread or the user interface. This can enhance the performance and functionality of web applications, especially for tasks that require intensive computation or network communication. However, it also poses some security risks, such as code injection, resource exhaustion, or denial of service. If a web page creates a web worker from an untrusted source, it may execute malicious code that can access the user's data or resources, or interfere with the web page's functionality. To prevent this, web developers should use the same-origin policy to limit the sources of the web workers, use CSP to restrict the types and content of the scripts, and use the terminate() method to stop the web workers when they are not needed.

5 Web Sockets

Web Sockets are a feature that allows web pages to establish a bidirectional and persistent communication channel with a server, using the WebSocket API and protocol. This can enable real-time and interactive web applications, such as chat, gaming, or streaming. However, it also introduces some security challenges and risks, such as cross-site request forgery, cross-site scripting, or man-in-the-middle attacks. If a web page uses web sockets without proper authentication and authorization, it may allow unauthorized or malicious users to access or manipulate the data or the connection. To prevent this, web developers should use HTTPS to secure the web socket connection, use tokens or cookies to verify the identity and permissions of the users, and use CSP and CORS to control the sources and destinations of the web socket messages.