JavaScript Security in 2024: The Complete Guide to Stay Secure

Share article
twitter linkedin medium facebook

Updated on July 8th, 2024

JavaScript security

JavaScript continues to dominate the web development landscape, powering most websites and many web applications. Unfortunately, its near universal adoption combined with some inbuilt security flaws make it an attractive target for cybercriminals, so in this article, we’re going to look at the new and emerging risks associated with JavaScript security and explore what you need to do to mitigate them.

What is JavaScript?

JavaScript is a hugely popular programming language. In 2023, it was used in 95% of all websites, but now a year later that number has jumped to 98.8%. A 2023 Stack Overflow survey found that nearly 64% of professional developers use the language.

It’s so popular because of what it does. In the old days, webpages were static, which means they just displayed things. JavaScript changed that in 1996, allowing developers to add interactive elements that we now take for granted, like forms, dynamic elements like message boxes that update without reloading the whole page, and web page animations like fading images.

JavaScript quickly became one of the three central pillars of web development alongside HTML and CSS, but then developers found they could use it for things like backend tasks in web servers, mobile, and game development, and now it’s helping to power generative AI tasks, virtual and augmented reality devices, and Internet of Things devices.

An extensive list of associated frameworks and libraries has extended its capabilities in these and other directions. It sits at the heart of a growing number of industries, but JavaScript remains as vulnerable as ever, so let’s explore what the risks are.


JavaScript Security Risks

Cross-Site Scripting (XSS)

XSS attacks occur when malicious JavaScript code is injected into a website, which allows attackers to execute arbitrary code in the user’s browser. Such code could be designed to steal sensitive information (like cookies and login credentials), redirect users to malicious websites, or manipulate the website’s functionality.

One vulnerability that allows an XSS attack to happen is poor input data sanitization. For instance, if a web form allows the user to type in JavaScript code without sanitizing it, it could be executed when another user clicks on that form.


JavaScript can be used to manipulate an iFrame, for example, by adjusting its size, position, or z-index to make it appear innocuous. Attackers can use it to trick users into clicking hidden buttons on websites, so they end up inadvertently transferring money or giving away sensitive personal and financial information.

Man-in-the-middle (MITM) attacks

These do what the name suggests. Imagine the attacker sitting in the middle between the user’s browser and a web server, fully able to read, change, or inject code into the browser. If a website serves its JavaScript resources using an unencrypted HTTP connection, an attacker on the network can intercept and modify the JavaScript code before it reaches the client. Alternatively, lots of JavaScript applications depend on third-party frameworks and libraries. Loading them from insecure sources allows an MITM attacker to replace them with malicious versions.


Session hijacking

There are a few methods to beware of with this one:

Session Token Theft: JavaScript running on a website can be used to steal the user’s session token, which is typically stored in a cookie or passed as a parameter in the URL. Attackers can use XSS (Cross-Site Scripting) vulnerabilities to inject malicious JavaScript that sends the session token to the attacker’s server.

Session Fixation: Attackers can use JavaScript to set a known session token on the victim’s browser, for example, by setting a cookie or modifying the URL. If the website does not properly validate the session token, the attacker can then use the fixed session to gain unauthorized access to the victim’s account.

Session Prediction: Some websites may generate predictable session tokens, and JavaScript can be used to analyze the patterns and mechanisms used to generate these tokens. Attackers can then use this information to guess or brute-force valid session tokens.

Session Sidejacking: JavaScript can be used to sniff network traffic and capture the user’s session token, which can then be used by the attacker to impersonate the victim and hijack the session.

JavaScript Injection

Similar to XSS, JavaScript injection occurs when attackers inject malicious JavaScript code into a website’s source code. This can be done through vulnerabilities in third-party libraries, insecure coding practices, or server-side vulnerabilities.

For instance, if a website uses a third-party library with a known vulnerability, an attacker can exploit it to inject malicious JavaScript code into the website’s source code, so they can then execute arbitrary code in the user’s browser.

New and evolving threats

Third-party risks

Chinese CDN company Funnel bought the domain and its associated GitHub account in February 2024. That domain previously hosted a tool that automatically provides JavaScript code to support new features in older browsers, but since February, it’s been injecting malware into mobile devices via websites that embed Polyfill. On July 5, 380,000 websites that embed were hit by a supply chain attack, so if you’re using the service, be sure to remove it immediately.

It’s not known if these activities were state-sponsored, but the fact that Funnel’s activities  haven’t been reined in by the Chinese state at least implies its indifference to the problem.

Another recent third-party event is the targeting of the Open JS Foundation in an attempted takeover. In the Polyfill article, we mentioned the mysterious Jia Tan, probably the alias of a Russian state actor who spent three years infiltrating the XZ Utils project so they could introduce a back door vulnerability. Both cases feature suspected social engineering tactics where the sole maintainers of these community-led open-source projects were put under pressure to hand over control to others, so could this be part of an ongoing trend? It’s too soon to say, but it highlights the need to not rely on the open-source community. As CISA officials Jack Cable and Aeva Black put it, “The burden of security shouldn’t fall on an individual open-source maintainer—as it did in this case to near-disastrous effect.”


Web-skimming attacks that steal customer payment card details and other personal information are old news, but in May we reported that Magento is still vulnerable and facing sophisticated new attacks by dedicated cybercrime gangs. Attackers inject malicious JavaScript code into a company’s payment page, where it waits to pounce on form submissions and forward the details to their drop servers. This again underlines the need to only use trusted JavaScript libraries, track data journeys to ensure they are permitted, and scan for malicious code changes.


What Does JavaScript Security Involve?

Here are some key areas of concern for developers and users.

Client-Side Execution Risks

JavaScript can be executed in the client’s browser, which means user inputs need to be validated and sanitized to prevent threats like XSS attacks.

Dynamic Typing

The language’s dynamic typing can lead to unexpected behaviors if it’s not properly managed. Strict type checking and thorough testing are recommended to prevent type-related vulnerabilities. Improper use of the eval function to dynamically execute code opens up applications to injection attacks. If the code being evaluated comes from an untrusted source such as user input, an attacker could inject malicious code, so developers need to be mindful of this and use safe coding practices.

Cross-Origin Resource Sharing (CORS) Considerations

While CORS enables cross-domain requests, misconfiguration can expose applications to cross-origin attacks. Careful implementation of CORS policies is essential.

Dependency Management

Regular audits and updates of third-party libraries are necessary to address potential security flaws in dependencies.

Secure Object Referencing

Direct references to sensitive data should be avoided or properly secured to prevent unauthorized access.

Client-Side Validation

While useful for user experience, client-side validation should always be supplemented with server-side validation for security purposes.

How to Stay Safe

Use a secure development process

This includes following secure coding practices, such as input validation.

Use a content security policy (CSP)

Can be used to limit the kinds of resources that a web page can load, which can help to reduce XSS attacks.

Implement proper access controls

Require users to authenticate themselves before accessing sensitive functionality or data. Use a secure authentication mechanism such as username/password, tokens, or OAuth. Once authenticated, use authorization checks to establish whether the user has the right permissions to perform specific actions or access certain resources.

Use secure development practices

Become familiar with the OWASP Top Ten security risks for web applications.

Use HTTPS to encrypt data transmitted between the client and server to ensure confidentiality and integrity. Validate and sanitize user inputs to prevent SQL injection, cross-site scripting (XSS), and other attacks. Avoid using raw user input directly in queries or HTML output. Add strong authentication mechanisms, such as multi-factor authentication (MFA) or OAuth. Avoid storing sensitive information like passwords in plain text.

Use threat modeling to identify potential risks and address them early in the development process. Set appropriate security headers in your web server configuration. Headers like Content Security Policy (CSP) and X-Frame-Options enhance security.

Use the least privilege principle to limit permissions for users and processes. Only grant necessary access rights. Avoid revealing sensitive information in error messages. Provide generic error pages and log detailed errors securely. Conduct periodic security audits, and use penetration testing and code reviews to help uncover vulnerabilities.

Use a web application firewall (WAF)

A WAF can help to protect web apps by adding a defensive layer between the websites, traffic, and the web application. It can prevent cookie poisoning and SQL injection, obstruct cross-site scripting, mitigate DoS attacks, and filter out malicious API or Web App requests.

Keep software up to date

The latest software always contains the latest patches for known vulnerabilities, which is why updates are essential. The same goes for JavaScript libraries and frameworks, although having mentioned shady characters who may have begun a campaign of infiltrating open-source projects, we would advise some additional diligence!


How Reflectiz maintains secure

Reflectiz provides a suite of protections against JavaScript-based vulnerabilities, for medium-size businesses up to enterprises.

Continuous Surveillance

Our system provides round-the-clock monitoring of all external scripts and applications integrated into your website, allowing for swift identification of potential security breaches.

Component Mapping and Interdependency Evaluation

We generate a comprehensive catalog of third-party elements within your site, scrutinizing their interconnections to uncover possible weak points.

Threat Evaluation

Utilizing sophisticated algorithms, our platform assesses the security implications of each external component, enabling you to strategically allocate your defense resources.

Crisis Management

Our solution includes robust emergency response features, such as instant notifications, up-to-the-minute threat intelligence, and access to seasoned security professionals for rapid threat mitigation.

Regulatory Adherence Tracking

We ensure your site remains in line with key industry standards like GDPR and HIPAA, assuring both security and legal compliance.


Our platform offers clear insights into the behavior and presence of third-party scripts, crucial for detecting vulnerabilities and unauthorized intrusion attempts.

The client-side nature of JavaScript execution makes it susceptible to manipulation, highlighting the importance of thorough monitoring. Malicious actors can exploit this flexibility, emphasizing the need for comprehensive oversight to detect and counter such threats.

Modern websites are complex ecosystems of interlinked JavaScript files, APIs, and plugins. A deep understanding of these elements and their interactions is critical for identifying potential risks. Undetected vulnerabilities can be exploited by attackers, which makes our platform’s holistic view an invaluable asset.

In Conclusion

Our solution provides unparalleled insight and management capabilities for JavaScript security. It offers a deep analysis of your web ecosystem, detailing all third-party scripts, open-source tools, and external domains. This level of transparency allows for a thorough understanding of your JavaScript usage, enabling proactive security measures.

Through our user-friendly interface, security teams can efficiently oversee all JavaScript components. Real-time alerts notify users of security-impacting changes, while a comprehensive overview of JavaScript elements’ activities and interactions facilitates early threat detection and mitigation, significantly reducing the risk of data breaches and unauthorized access.

While JavaScript’s reliance on third-party scripts, its dynamic nature, and the intricacy of modern web environments present security challenges, our solution’s comprehensive visibility and control empower you to effectively address these issues.

Contact us today to schedule a demonstration and explore how our platform can enhance your website’s security while leveraging the full potential of JavaScript.

Subscribe to our newsletter

Stay updated with the latest news, articles, and insights from Reflectiz.

Your Website looks great!

But what’s happening behind the scenes?

Discover your website blind spots and vulnerabilities before it’s too late!

Try for free