JavaScript Security and Risks

As of 2021, over 95% of websites are estimated to use the JavaScript-interpreted programming language. It owes its popularity to its Omni-platform capabilities, stable frameworks, and inherent ease of use. Through JavaScript, organizations can develop the back-ends and front-ends of dynamic web pages that offer enhanced user experiences such as animations, activity tracking, form submissions, and more.

While JavaScript continues to be a popular programming language for web application development, JavaScript vulnerabilities are widely exploited by attackers and malicious users to manipulate data or gain control of web systems. As a result, it takes a focused approach to tackle security concerns that involve activities undertaken to detect, investigate, prevent and solve such vulnerabilities.

Understanding JavaScript Vulnerabilities

This article delves into the most common vulnerabilities associated with JavaScript security issues applications, various best practices to ensure your code, and a list of Javascript vulnerability tools that will help you prevent them.

If you don’t have time to read, let’s start now by running a Javascript Security Scanner to find which vulnerabilities you could be exposed to. Free-running this new tool is the fastest way to get reports, know your specific vulnerabilities, and clearly explain how to prevent each. Enjoy!

Javascript Security Issues and Risks

The growing popularity of dynamic web applications built on JavaScript introduces several security issues that can make applications vulnerable. These include:

JS Source Code Vulnerabilities

JavaScript developers typically rely on integrating numerous public or open-source packages and libraries containing hidden vulnerabilities and exposing security issues within the source code. Open-source vulnerabilities might not be easily recognizable at first, and using a vulnerable package expands the attack surface of your web app.

Unintended Script Execution

JavaScript embeds functions on HTML web pages when interacting with a web page’s Document Object Model (DOM). Attackers can embed scripts into these functions, then execute them by all client computers that connect to the web page. 

Exclusive Reliance on Client-Side Validation

Most organizations only ensure validation on users’ browsers. Though this offers a certain level of security, hackers who use advanced techniques to send unverified data to servers can corrupt records and configurations.

Exposure of Session Data

Attackers leverage the power of client-side browser scripts to access all communication between the browser and the web application. This communication may include sensitive session data, such as user session IDs used for unauthorized access.

Unintentional User Activity

Attackers may use several techniques that manipulate a browser into performing malicious activities on various websites users are already logged on to. Additionally, hackers take advantage of JavaScript security gaps that execute threat vectors on other sites in the background as soon as a browser is logged in. 

JavaScript Vulnerability in a graphic

JavaScript Security Risks You Must Know About

Attackers commonly use well-known JavaScript security vulnerabilities to navigate through security loopholes of an application’s attack surface to compromise user and system data. As a result, they often show up in a security audit report generated by a security scanner.

These vulnerabilities include:

XSS JavaScript

Cross-Site Scripting vulnerabilities involve a code javascript injection attack from the client side. Where hackers embed malicious scripts on legitimate web pages, the application passes unvalidated data to web browsers. Web applications that factor in unfiltered user input in the generated outputs are most susceptible to Cross-Site Scripting attacks. XSS common attacks or potential vulnerabilities can be prevented through:

  • Filtering & sanitizing user input
  • Using effective response headers
  • Encode data before output generation
  • Utilize Content Security Policies
  • Using regularly an XSS Scanner before every release

Cross-Site Request Forgery (CSRF or XSRF)

CSRF is a widespread security vulnerability in which threat actors manipulate legitimate users into submitting malicious requests to web applications they are ambushed to visit. When the web application fails to differentiate between valid user requests and forged requests, attackers can execute any malicious actions under the guise of legitimate end-users. 

CSRF attacks can be prevented by:

  • Implementing secure random tokens
  • Logging off unused web applications
  • Disallowing automatic password entries by browsers
  • Securing session credentials
  • Using regularly a CSRF Scanner before every release

Server-Side JavaScript Injection Attacks

This mechanism injects and executes malicious or arbitrary code on a web application’s server without sanitizing and filtering user inputs. Attackers typically look for functions that parse user-generated data without proper validations to ingest insecure scripts to be executed by the server. 

Server-side JavaScript injection attacks are typically prevented by properly validating and filtering user inputs.

Client-Side Logic Attacks

JavaScript engines are known for their speed and performance in client-side processing. However, when organizations place sensitive operations, such as hard-coding usernames and passwords, the user can fully control the environment where logical decision-making is performed. This allows attackers to gain quick access and control of the web app through a browser. 

Client-Side Logic attacks can be prevented by avoiding operations with sensitive security controls on the client side.

And when securing API keys in a client-side JavaScript application,  the most commonly-used option is to set an API Access and Secret Key pair.
JavaScript Vulnerabilities Prevention Guide

Prevention Guide

JavaScript Vulnerability Prevention Guide

Learn how to detect and prevent JavaScript Vulnerabilities and secure your web assets.


How to secure JavaScript Code?

Here we show you the best practices to help maintain JavaScript applications safe and some tools that could help you with this task.

Best Practices for JavaScript Vulnerabilities

Some best practices to keep JavaScript applications safe include:

Avoid Evaluating User Input – The eval() and new Function () command executes arguments passed in user inputs as JS expressions. As hackers manipulate user input to run malicious scripts, it is recommended to avoid evaluating user inputs or parsing JSON data through the above constructors.

Enable TLS/SSL Encryption – Encrypting data between servers and clients helps prevent CSRF and XSS attacks.

Secure API Access – It is essential to assign tokens for each user accessing the web app through the API, enabling secure access.

Setting Secure Cookies – By setting cookies as secure, each cookie can only be used for a single web page, ensuring encrypted access.

Defining Content Security Policies – Content security policies ensure that attackers don’t inject malicious scripts into web applications to manipulate state changes.

Also, hire security professionals or embed continuous security by employing a security scanner and generating a vulnerability report before pushing new production changes. You can read about DevSepOps and SecDevOps in our blog posts (and yes, they are different things).

JavaScript Security Scanning Tools

Some popular vulnerability scanning tools for JavaScript applications include:

Crashtest Security Suite

Crashtest Security involves a collection of JavaScript scanning mechanisms that enable end-to-end vulnerability assessments for APIs, Web Applications, and Microservices with a meager false positives rate. The suite integrates with all modern development stacks to reduce the risk of being hacked through an API or Web App. CS security scanner also provides downloadable security audit reports you can share with your team or clients.

When did you last check your JavaScript app security level?


A fast, portable, and adaptable web application scanner designed to identify potential vulnerabilities on small-scale websites. Grabber can be used to identify such issues as cross-site scripting, SQL injection, File Inclusion, AJX checks, and JavaScript Code Vulnerabilities, among others.


The Zed Attack Proxy (ZAP) is an open-source web application security scanner part of the OWASP project. The tool uses a combination of AJAX Spidering and Fuzzing to expose potential vulnerabilities by sending the application into an undesired state. ZAP encapsulates most AppSec features in a single, open-source, easy-to-use platform that enables manual and automated security scans.


An open-source web application audit and scanning tool that relies on straightforward CLI commands to enable ‘black box’ scanning. The tool uses fuzzy methods to inject payloads into the web application and check for vulnerabilities. Wapiti can extract URLs from basic JavaScript interpreters, making it ideal for dynamic applications. 

JavaScript Security Video Explanation

All Risks and Security Possibilities of JavaScript explained in a video


Why is JavaScript Security Relevant to Your Website?

Attackers and malicious users widely exploit JavaScript vulnerabilities to manipulate data or gain control of web systems. This includes; cross-site scripting, clickjacking, cookie theft, session fixation, etc. As a result, it takes a focused approach to tackle security concerns that involve activities undertaken to detect, investigate, prevent and solve such vulnerabilities.

Which are the most common JavaScript Vulnerabilities?

Cross-Site Scripting (XSS), Cross-Site Request Forgery (CSRF or XSRF), Server-Side JavaScript Injection, and Client-Side Logic Attacks. Read more about JavaScript vulnerabilities in our penetration testing blog.


JavaScript security identification and remediation is often considered the first step to securing modern applications. While JavaScript has vulnerabilities and known security risks, organizations can best maintain application security using the programming language with pragmatic best practices and the right tools. 

The Crashtest Security Suite is trusted by many software vendors and organizations globally to deploy safer web applications through vulnerability scanning and assessment. Start your 14-day trial with the suite to explore how Crashtest Security can help improve developer productivity and reduce security testing budgets.

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