Top 6 JavaScript Vulnerabilities to Look Out for in 2025

Anecdotal evidence that JavaScript is the top programming language is easy to come by. Ask most web developers and they will wax lyrical about its versatility, ease of use, and dynamism. This high praise is emphasized by a recent survey showing the majority of developers (62.3%) identified it as the language they want to work with for a tenth straight year – an enduring popularity underpinned by its ability to keep pace with the rapid evolution of web development. However, with widespread adoption and progression comes an increased risk of JavaScript security vulnerabilities.

JavaScript provides threat actors with an irresistible combination of ubiquity and a widening attack surface: most websites (98%) use it client-side to create interactive web pages, while the exciting trends and advancements perpetuating this adoption are susceptible to attacks.

Amid this perfect cyber-attack storm, threat actors are developing new and more advanced techniques to exploit JavaScript’s weak points – a worrying trend is showing no signs of abating in 2025. Let’s investigate the top JavaScript vulnerabilities to look out for in the coming months and how to combat them.

JavaScript Vulnerabilities to Look Out for in 2025

1. Cross-Site Scripting Attacks (XSS)

XSS attacks occur when malicious third-party scripts are injected into trusted websites. Once deployed, threat actors can harness them to exploit vulnerabilities that enable the execution of malicious JavaScript code in another user's browser – leading to data theft, account compromise, or even full system control through further exploitation.

One of the most common JavaScript threats, XSS attacks are constantly being tweaked by perpetrators to maintain their deleterious impact and avoid detection. For instance, as JavaScript frameworks like React, Vue, and Angular evolve in 2025, threat actors will refine XSS attacks that target these blueprints for building web applications.

Prevention tips: Ensure client-side user input is validated and sanitized before being included in HTML output. Additionally, encode any user-generated content before displaying it on web pages to prevent the browser from interpreting it as executable code or clickable web links.

2. E-skimming

This client-side attack occurs when threat actors inject malicious code onto a website to compromise sensitive data – including credit card numbers, expiration dates, and security codes – inputted by users into payment processing pages and other online forms.

Unauthorized access to the site is typically gained by compromising assets originating from outside its security perimeter, such as client-side web assets – the files and resources downloaded and executed by the user's web browser when they visit the site. 

To achieve this, threat actors typically target JavaScript vulnerabilities by injecting the malicious skimming code via a supply chain attack or a direct hack of a website. The code infiltrates the site’s point of sale, allowing sensitive data to be compromised. These attacks can go unnoticed for weeks or months because they occur beyond the boundaries of traditional security controls, creating a blind spot that allows the malicious code to hide in plain sight.

Prevention tips: Audit and monitor JavaScript code, especially third-party script. Implement Subresource Integrity (SRI) to ensure the scripts loaded on the website are not tampered with by validating their integrity. Regular updates and patches to the website platform and any third-party plugins will prevent vulnerabilities in outdated versions from being exploited. 

3. Supply chain attacks

Supply chain attacks target third-party libraries, packages, or dependencies that developers use to provide web functionality – including JavaScript libraries containing pre-written scripts that streamline web development. Once compromised, attackers can inject malicious code into all websites using that third-party resource.

Once the code has been added to a website’s software by targeting JavaScript vulnerabilities, it surreptitiously spreads when users install updates from what they assume is a trusted source – exposing the data of an ever-expanding number of unsuspecting victims. 

Prevention tips: Adopt a multi-layered approach to mitigating supply chain attacks that includes the implementation of web application firewalls (WAFs), a robust Content Security Policy (CSP), and Subresource Integrity (SRI). Augment this by verifying partners' controls and certifications, performing third-party risk assessments, and mapping the flow of traffic and critical data.

4. Formjacking

Formjacking occurs when attackers inject malicious JavaScript code directly into a website’s payment page or login forms, hijacking their functionalities. The script intercepts the input fields, typically targeting payment card information and other sensitive data entered by users, and sends it to the attacker’s server.

This type of attack exploits the trust users have in established, legitimate web applications. Consequently, a formjack can remain undetected for a prolonged period as the form behaves normally.

Prevention tips: Regularly patch and update the website's software, plugins, and extensions, use a secure connection to safeguard data during transmission, and regularly scan the site and server logs for vulnerabilities and suspicious activities.

5. Prototype pollution

All objects in JavaScript contain a prototype that they inherit properties and methods from. This prototype chain allows objects to share common behavior. When an object property is accessed, JavaScript looks for it on the object itself. If it doesn't exist there, it searches further up the chain.

Prototype pollution happens when a threat actor injects arbitrary properties into an object's prototype to modify it and change the behavior of all objects it creates. Having gained unauthorized control of properties in a web application, they can launch attacks from within. For instance, having exploited the client-side JavaScript, the attacker might execute a DOM XSS – a cross-site scripting vulnerability that occurs when a web application's client-side JavaScript modifies the Document Object Model (DOM) to create harm.
 

Prevention tips: Stop untrusted user inputs from directly modifying object properties by validating and sanitizing inputs, especially when merging or copying objects. Avoid using functions that automatically merge object properties. Instead, use libraries and functions that offer safe deep cloning and merging. Block potentially dangerous keys, like proto, constructor, and prototype, from being set or modified. 

6. Insecure Direct Object References (IDOR)

Client-side IDOR attacks occur when an application exposes sensitive objects to users without proper access controls that validate whether they have authorization to view them. The vulnerability stems from the application trusting data or object references manipulated directly in the client-side code. Because the code typically interacts with sensitive data, JavaScript logic can be easily viewed and exploited by threat actors using this technique.

Prevention tips: Implement robust access controls that validate all requests on both the client side and the server. This comprehensive approach will ensure the user is authorized to access or modify the requested resource. Introduce role-based access controls that restrict access to resources based on the user’s role and permissions. Avoid embedding sensitive information in client-side code, including JavaScript, as this helps threat actors discover potential IDOR vulnerabilities.

Conclusion 

JavaScript is a double-edged sword: on one side, it has compelling benefits, such as its versatility and ease of use, that make it the go-to web programming language; on the other, it has innate security vulnerabilities stemming from its widespread use, client-side execution, and dynamic nature.

While the rewards of using JavaScript provide the building blocks for the web, threat actors leverage the risks it poses by wielding the other, more threatening, side of the sword. To blunt the sharp rise in increasingly sophisticated attacks targeting JavaScript vulnerabilities, you must adopt secure coding practices and employ robust defense mechanisms that reinforce your web applications in 2025 – and beyond!