JavaScript (JS) is a high-level programming language used for web development, particularly to create dynamic and interactive content. When creating a JavaScript app, you need to hire reliable, knowledgeable JavaScript programmers and invest in JavaScript cybersecurity. Otherwise, threat actors can exploit JavaScript vulnerabilities, leading to attacks that compromise website integrity and put user information in danger.
That’s why you need to implement security measures to protect JavaScript apps and their users. This guide covers what JavaScript security is and why it’s important. You’ll also learn about the types of JavaScript attacks and 15 ways to avoid JavaScript security vulnerabilities.
JavaScript powers many modern web apps, which makes it a constant target. Because most JavaScript runs in the browser, attackers can easily access the app’s code and exploit vulnerabilities. Threat actors can also bypass front-end protections by sending requests to the server-side apps and modifying the code in the browser.
Many companies have fallen victim to cyberattacks. In 2022, Segway’s online store was impacted by an e-skimming attack when a third-party script used on its checkout page was compromised. Attackers injected a malicious JavaScript skimmer that captured payment data entered in the browser.
Incidents like this highlight that if you don’t proactively secure your JavaScript, someone else will proactively exploit it. Staying ahead means finding and fixing vulnerabilities before an attacker can exploit them and put you and your users at risk.
Given JavaScript’s weaknesses, there’s been an explosion of JavaScript attacks. Here are the main types of JavaScript attacks:
E-skimming is the practice of threat actors extracting user data from e-commerce sites to steal customer payment information during checkout. It targets areas where people enter payment details.
Magecart is a type of e-skimming attack. It involves extracting payment information from online shopping cart systems, especially those using Magento.
Formjacking is injecting malicious code into websites. The goal is to hijack their form functionalities to lift user data.
JavaScript sniffers (also called payment card skimmers or web skimmers) are malicious scripts designed to steal payment or transactional data from e-commerce sites. They’re commonly associated with Magecart-style attacks, where attackers inject skimming malware into checkout pages to capture credit card details.
JavaScript injection refers to client-side code injection, typically a form of DOM-based XSS, in which attackers manipulate page input or DOM APIs to cause the browser to execute malicious JavaScript.
JavaScript security vulnerabilities are flaws and weaknesses in your code and its dependencies that attackers can exploit to steal data or take control of systems.
JavaScript is especially vulnerable to attack because JavaScript runs in a restricted browser sandbox without OS-level permissions. However, anything exposed through the DOM, cookies, storage APIs, permissions APIs, or insecure server endpoints can be accessed unless properly secured.
While browsers follow W3C standards that enforce security policies, such as the same-origin policy and sandboxing, it’s still up to website owners and developers to properly configure and maintain them. A single misconfiguration or outdated dependency can create an opening for attacks.
Below are some of the top JavaScript vulnerabilities developers need to watch for and address early in the development process.
Source code vulnerabilities arise from weaknesses in the web app’s own logic. These are design-level flaws that give attackers unlimited access to data or functionality. Examples include unvalidated assumptions, such as trusting certain inputs or states will “never happen,” or hard-coding application programming interface (API) keys directly in the source code.
Coding errors can creep in when developers are tired and stressed, leading to vulnerabilities. This is especially dangerous for client-side JavaScript, as threat actors can easily review the code and identify exploitable gaps.
This happens when untrusted code runs inside a web page’s Document Object Model (DOM). Attackers can put malicious scripts into these DOM functions, causing them to execute in every browser that loads the affected page.
Most JavaScript web apps are built with third-party or open-source libraries. If those libraries contain malicious updates or unpatched bugs, they can introduce vulnerabilities into your project without you realizing it.
Cross-site scripting (XSS) lets attackers inject scripts that steal sessions, deface pages, or turn into other attacks. Variants include stored, reflected, and DOM-based XSS. You can prevent it by applying sanitization (escaping, filtering, and validating) when handling user input and server output.
Threat actors run XSS attacks by injecting data that contains special characters in JavaScript, HTML, or CSS. When a browser encounters these characters during rendering, it may interpret them as code instead of plain text. An attacker can then “break out” of the text field and run malicious code on the client side.
To prevent this, always encode or escape any user-supplied data before sending it in a response, whether it’s later pulled from a database or displayed immediately. Escaping ensures that the browser treats special characters as text rather than executable code.
Sometimes, the best safeguard is to filter risky characters from user input before processing it. However, skilled attackers can easily find ways around simple filters, so filtering input should just be an extra defense layer, not your only one.
Systems should always validate browser-supplied data to ensure it only has expected characters. For example, phone number fields should only accept numbers, parentheses, and dashes, and reject all other characters.
The safest approach is to allow lists that define what’s valid, rather than blocking every possible bad character. Without proper validation, attackers can sneak malicious payloads and trigger attacks.
Client-side validation improves the user experience. It can tell users when they leave a form field blank, check password strength, and ensure they enter the correct email address.
However, relying solely on client-side validation can leave your web app vulnerable to attackers. Threat actors can use tools like Postman, cURL, and custom scripts to bypass JavaScript checks in your forms and send malicious code directly to the backend. This can corrupt stored information, trigger injection flaws, and even give attackers access to sensitive parts of your website.
The solution? Validate the server, even if you have already validated the browser. Protecting both the front- and back-end ensures that only safe, expected data is processed and that users have a great experience.
Client-side browser scripts are among the most valuable targets for attackers. It includes cookies that may contain sensitive data, including user session IDs. XSS attacks can expose user session ID tokens to threat actors, allowing them to hijack the session.
To prevent this, browsers now support the HttpOnly cookie attribute. By telling the browser not to allow the cookie to be accessed from the DOM, the attribute prevents client-side script-based attacks from accessing sensitive data in the cookie.
Threat actors can also steal session and local storage browser data in the same way. Accordingly, you should avoid storing sensitive information, including tokens, in browser storage.
Cross-site request forgery (CSRF) attacks happen when attackers trick an authenticated user’s browser into performing unwanted actions, such as transferring funds or changing the user’s password.
CSRF attacks exploit the fact that browsers automatically attach cookies (including session cookies) to requests. If a user is logged into a site—like their bank—an attacker can trick their browser into sending an unwanted request to that site, such as changing an email address or initiating a transfer. Unlike phishing, the malicious site doesn’t need to look like the real service; it just needs the victim’s browser to submit the request using their existing authenticated session.
This attack injects and executes arbitrary or malicious code on a web app’s server, bypassing filtering or sanitization of user input. Threat actors often target functions that parse user-generated data without proper validation. You can prevent these attacks by properly filtering and validating user inputs.
As a prototype-based language, JavaScript inherits properties and methods from its prototype, another object. Thanks to this functionality, multiple objects can share properties and methods from a single prototype. This process is the result of the prototype chain, which JavaScript traverses to help you access a particular property or method of an object.
Unfortunately, an attacker can inject methods or properties into an object’s prototype, resulting in prototype pollution. This can affect all objects linked to that prototype, resulting in remote code execution and data corruption.
Sensitive data exposure occurs when important information, such as authentication keys, login credentials, or personally identifiable information (PII), is compromised, allowing attackers to access it.
Sensitive data exposure can happen when you:
Serialization in web apps is important since data must be stored and transferred in formats that other systems can easily interpret. It involves converting data to JSON, binary, and other system-readable formats before sending or storing it.
Deserialization is the opposite of this process. It fetches serialized data from the system and restores it to its original form. Insecure deserialization primarily affects server-side platforms such as Java, PHP, and .NET. In JavaScript environments, the main risk comes from parsing untrusted JSON or object structures that influence application logic, but this generally does not enable remote code execution.
Unintentional user activity
This happens when attackers manipulate a browser into performing malicious actions on sites where users are already logged in. Attackers may also exploit JavaScript vulnerabilities to trigger unintended actions on other sites where the user is already authenticated, often by abusing insecure event handlers or cross-site interactions.
Although there are many JavaScript security holes, you can protect apps and servers from them by adopting the following JavaScript security best practices and state-of-the-art scanning tools.
A content security policy (CSP) lets you restrict access to or control how your website loads external resources, such as images, inline scripts, or fonts.
CSPs help prevent clickjacking, XSS, and other code-injection attacks by allowing your site to load only authorized resources. To implement a CSP, add it to your server’s HTTP headers. Configure CSP by stating which types of domains and resources your site can load content from.
In Waterfall-style development, security checks often happen later in the SDLC. This doesn’t inherently create more vulnerabilities, but it does delay detection, making issues harder and more expensive to fix.
If you adopt the DevSecOps project management method, which bakes security practices into every phase of the SDLC, you may be able to decrease coding errors.
Secure coding practices are general principles and guidelines that help you write more secure code. It can minimize the impact of, or even prevent, common security bugs in JS. Some of the most important coding practices you should train developers on include:
Inline event handlers let you attach JavaScript directly to HTML elements using attributes such as onchange, onmouseover, and onclick. Unfortunately, because threat actors can use them for XSS attacks, you should avoid them.
Don’t trust any user input.
Always validate it so it follows the exact format you expect. You should also sanitize it to remove unsafe elements and encode it before displaying it in the browser. This process prevents attackers from slipping malicious payloads into forms, URLs, or text fields.
You should also regularly scan your codebase, since even small validation gaps can turn into major vulnerabilities.
Modern JavaScript apps rely heavily on third-party packages. Unfortunately, some of them may contain malicious code, so unchecked dependencies can increase your attack surface. What’s more, vulnerabilities in one npm module can spread throughout your entire app, especially when transitive dependencies pull in code you’ve never seen.
Reduce this risk by using automated tools to scan your dependencies for outdated versions — which are more likely to have security holes — and known security issues. You should also keep a list of what your project uses, update packages regularly, and remove libraries you no longer use.
The eval() function violates the programming rule to “never trust user input.” That’s because it lets users, including attackers, run arbitrary code in your program.
Global variables seem convenient, but they create hidden security risks. Since they can be modified by every script running on the page, malicious or compromised code can read, overwrite, or hijack them. This can break application logic, expose sensitive data, or allow attackers to manipulate how your app behaves.
As such, you should avoid global variables. For example, avoid this:
var globalVar = "placeholder";
Replace it with this:
(function() {
var privateVar = "placeholder";
})();
Encryption involves transforming data into an unreadable format through a secret algorithm or key. It can prevent unauthorized third parties from accessing or modifying your data. It can be applied to data in transit, at rest, and in use.
Servers and browsers are always communicating, exchanging sensitive data such as authentication tokens and login credentials. To protect this information from threat actors, you need to ensure it’s encrypted, remains in its original format, and is sent to the right server or location.
You can do this by using HyperText Transfer Protocol Secure (HTTPS), a protocol for improving the security of data transmission between servers and browsers. It ensures data sent to the server is unchanged and protected from attackers.
To prevent actions in your code that can cause security holes, use the “use strict” directive in JavaScript. In this mode, JavaScript flags errors when certain red flags occur, such as using undeclared variables, accessing global objects, and making coding mistakes.
To implement this mode, just add the following to the beginning of a script or function:
'use strict'Cookies often store sensitive data, such as session IDs, so you need to protect them. Marking a cookie as Secure ensures it’s sent only over HTTPS, preventing attackers from intercepting it through network sniffing or man-in-the-middle attacks. You should also set the Http-Only attribute on cookies to block malicious JavaScript from stealing session IDs and other sensitive data. For even stronger protection, set SameSite to Strict or Lax so the browser limits when cookies can be sent in cross-site requests.
To defend against CSRF attacks, add an anti-CSRF token in your web app. This token generates a unique identifier for every user session and is included with every request that performs a sensitive action.
When the browser makes a request, the server checks whether the CSRF token from the browser matches the one stored on the server. If they don’t match, the server will reject the request, preventing attackers from triggering unauthorized actions on the user’s behalf.
If you directly manipulate the DOM with untrusted data, you may expose the app and its users to serious security risks. Attackers can slip malicious scripts into user input, which is then written directly to the page if you use unsafe methods like innerHTML. This is one of the most common pathways for DOM-based XSS attacks.
To protect your app and its users, avoid mixing raw user input with DOM operations. Use safe APIs, such as textContent, or libraries that automatically escape dangerous characters before rendering.
If you’re using frameworks like React, Angular, or Vue, their templating systems automatically escape values when used correctly, which significantly reduces the risk of DOM-based XSS.
Because browser-delivered code is readable by design, even well-secured JavaScript can be reverse-engineered, copied, or tampered with. JavaScript obfuscation adds an extra layer of defense by transforming your code into a version that’s functionally identical but extremely difficult for humans or automated tools to interpret. This process can include renaming variables, encrypting strings, flattening control flow, and hiding critical logic paths.
Protecting your apps and users from JavaScript cybersecurity threats can be challenging if you don’t have the right tools or a small team.
That’s where JSDefender comes in; this professional-grade JavaScript obfuscator has been helping teams scramble and obfuscate the most vulnerable parts of their code, so bad actors can’t tamper with it.
JSDefender provides many benefits, including:
To see how JSDefender can harden your security stance, get a free trial today.