JavaScript Security Best Practices

JavaScript is one of the development technologies that made modern web development possible by giving developers to create dynamic, interactive web applications. For exactly that power, there are also grave security considerations. If security measures are not implemented in your JavaScript, those with malicious intent can take advantage of it in attacks such as cross-site scripting (XSS) and many other ways.

What are the best JavaScript security best practices, with an emphasis on preventing XSS and other common security pitfalls.

What is Cross-Site Scripting (XSS) and Why is it Dangerous?

You need first to understand what it is and why it’s such a serious concern.

What is XSS

Cross-site scripting (XSS) is a type of vulnerability that allows attackers to inject malicious scripts into web pages that are viewed by other users. These scripts can run in the context of the victim’s browser, giving the attacker the ability to steal sensitive data (such as cookies or session tokens), hijack user sessions, deface websites, or spread malware.

How XSS Works

XSS exploits a web application’s failure to properly sanitize user-generated content. An attacker injects malicious code, typically in the form of JavaScript, into a page’s content. When other users load the page, the malicious script is executed within their browser, often without their knowledge.

For example, an attacker might inject a script that steals session cookies from a user’s browser and sends them to an external server controlled by the attacker. This can lead to session hijacking, data theft, and other forms of exploitation.

Common Types of XSS

There are three variants of XSS attacks:

Stored XSS

Stored XSS happens when the malicious script gets permanently lodged into the target server. For example, when user input is not sanitized enough, it can be stored in a database or other persistent storage (say, a comment, forum post, or a search term). When another user accesses this data, the malicious script is executed.

Reflected XSS

Reflected XSS occurs when the malicious script is reflected off a web server, usually via a URL or query parameter. The script is then executed when a victim clicks on a malicious link, which passes the script to the server, which then reflects it to the victim’s browser.

DOM-based XSS

DOM-based Xtreme Scripting (XSS) takes place when a malicious script is executed due to a client-side vulnerability in the Document Object Model (DOM) of a browser. This XSS does not follow the traditional path of interacting with the server, and the malicious script is most probably executed by JavaScript for client-side DOM manipulation.

JavaScript Security Best Practices to Prevent XSS

What are the best practices for preventing it and securing your JavaScript code.

 

Sanitize and Escape User Input

Note that one major step in preventing XSS is always sanitizing and escaping user input. Never trust any data from untrusted sources; ensure that user input is sanitized before being rendered on the webpage.

– Sanitizing: This means stripping away any potentially hazardous characters or elements in user input, like `<`, `>`, or `&`, which can be interpreted as HTML or JavaScript.
– Escaping: Basically, it converts characters that have special meaning into their safe HTML entity equivalents; for instance, converting `<` into `<` prevents it from being treated as an HTML tag.

Consider using such libraries as DOMPurify to sanitize user input so that malicious code is never entered.

 

Use Content Security Policy (CSP)

A Content Security Policy (CSP) is a browser feature that helps prevent XSS attacks by restricting the sources from which content can be loaded. CSP allows you to define trusted sources for scripts, images, and other resources, which reduces the risk of malicious content being injected into your application.

a basic CSP header might look like this:

Content-Security-Policy: default-src 'self'; script-src 'self' https://trusted-scripts.example.com;

 

Scripts of the same source (`’self’`) or those from the trusted external source of `https://trusted-scripts.example.com` are allowed by this policy. Inline script execution is disallowed per this specification, and the domains that are not trusted will not be permitted, thus preventing the application from avenues of cross-site scripting attacks.

 

Avoid Inline JavaScript Practices

Inline JavaScript―scripting code directly embedded in HTML code or dynamically injected by event handlers―can be exploited for XSS attacks. Attackers can inject harmful codes through inline events, executing them with full privilege within the context of the page.

Instead of using inline JavaScript, keep your JavaScript code in separate files external to your HTML. Doing so allows you to disallow inline scripts fully and take advantage of CSP.

Use Security-Proof JavaScript APIs

Modern JavaScript builds several APIs aimed at fighting XSS and other threat issues. For example:

– `textContent` vs `innerHTML`: Ideally, whenever working with user-generated content, choose `textContent` or `setAttribute()` over `innerHTML`. This is because `innerHTML` will execute embedded JavaScript, whereas `textContent` adds mere text to an element without executing any script.

element.textContent = userInput;  // Safe
element.innerHTML = userInput;    // Unsafe

– `createElement()` and `appendChild()`: These methods help safely add elements to the DOM without the danger of content-based attacks.

 

Implement Input Validation

Input validation is the primary security step to ensure that every piece of data entered adheres to the expected format. For instance, if you are expecting a phone number, one half of the validation process would be to ensure that the input contains only numbers and dashes. Similarly, one would check to ensure that the input for an email address conforms to the standards of an email address.

Both client-side and server-side validation procedures should be in place to prevent dirtied data from ever being processed or stored.

Other JavaScript Security Best Practices

Besides XSS prevention measures, some other important JavaScript security best practices should be considered:

Securing Cookies and Sessions

– HttpOnly flag: Always set the `HttpOnly“ flag in any cookie that deals with sensitive information such as a session token preventing JavaScript from touching the cookies.
– Secure flag: Make sure cookies are sent only via HTTPS by setting the `Secure` flag.
– SameSite attribute: Also, set the `SameSite` attribute preventing cross-site request forgery (CSRF) attacks.

Never Expose Sensitive Data to JavaScript

Never put sensitive data into any client-side JavaScript, such as API keys, passwords, or user credentials. Any malicious actors may look and find sensitive information from JavaScript that is running on the client’s side using developer tools available for the browser.

If anything needs to be kept in absolute secrecy, the key must be kept on the server side, and communication must be conducted via the HTTPS protocol.

Use Subresource Integrity (SRI)

In the context of JavaScript external libraries and assets downloading, keep in mind that the SRI should be used to check whether the files have been tampered with. SRI allows any browser to verify whether any resource it fetched has been modified or tampered with once; it can never allow maliciously injected scripts into your application.

<'script src="https://example.com/library.js" integrity="sha384-abcdef12345..." crossorigin="anonymous"></script'>

Checking and Auditing Security for JavaScript

No security measure can ever be complete without testing and auditing.

Static Code Analyzers

Analysis before static code execution-where the security tools analyze your JavaScript code-against security vulnerabilities, such as SonarQube and ESLint. They will point out possible deductions in terms of security from unsafe API use or failure to escape user input.

Security Audits

Your application needs regular security audits so that new vulnerabilities find no room to be identified and updated measures in security mechanisms are applied. These processes may include penetration testing and ethical hacking tools, simulating attacks against your system and thereby assessing its weaknesses.

Security Headers

Apart from CSP, other HTTP headers are also helpful in promoting secure communication and preventing attacks such as content type sniffing, the chief among them being `Strict-Transport-Security (HSTS)` and `X-Content-Type-Options`.

Securing JavaScript code remains the main task of any developer trying to protect his/her web application and those using it from attacks such as XSS and other security-related threats.Image by wirestock on Freepik