4 Essential Steps to Hardening Your Application with Emotion.js

In the era of modern web development, leveraging libraries like Emotion.js has become a norm for building dynamic and stylish applications. However, security risks such as CSS injection, XSS attacks, and CSP policy violations can arise if best practices are not followed. This guide outlines four critical steps to ensure the hardening of your application when using Emotion.js.

1- Address Content Security Policy (CSP) Compatibility

Emotion.js often makes inline CSS definitions. This may be incompatible with a specific content security policy (Content Security Policy – CSP). CSP is used to ensure that only content from secure sources is run in a web application. The use of inline CSS without a nonce (one-time token) can cause a CSP violation.Nonce values should be defined in the CSP policy and Emotion.js should be styled accordingly:

/** Compatible use with CSP */
import { CacheProvider } from '@emotion/react';
import createCache from '@emotion/cache';

const nonce = 'randomNonceValue123'; // This value should also be defined in the CSP policy
const cache = createCache({
  key: 'my-app',
  nonce: nonce,
});

// Wrap Emotion.js components with CacheProvider
<CacheProvider value={cache}>
  <StyledDiv>Secure Content</StyledDiv>
</CacheProvider>

/** Compatible use with CSP */
import { CacheProvider } from '@emotion/react';
import createCache from '@emotion/cache';

const nonce = 'randomNonceValue123'; // This value should also be defined in the CSP policy
const cache = createCache({
  key: 'my-app',
  nonce: nonce,
});

// Wrap Emotion.js components with CacheProvider
<CacheProvider value={cache}>
  <StyledDiv>Secure Content</StyledDiv>
</CacheProvider>

2. Validate and Sanitize User Input

Dynamic CSS generation in Emotion.js often relies on user input, making it a potential vector for CSS injection attacks. Directly embedding unvalidated user input into styles can allow malicious actors to execute harmful scripts.

//Unsafe Example:
const userColor = "javascript:alert('XSS')";
const StyledDiv = styled.div`
  color: ${userColor}; // Dangerous: Using user input directly
`;

//Unsafe Example:
const userColor = "javascript:alert('XSS')";
const StyledDiv = styled.div`
  color: ${userColor}; // Dangerous: Using user input directly
`;

By implementing proper validation and sanitization, you can block malicious CSS injection attempts effectively.

3- Protect Against XSS (Cross-Site Scripting)

Improper use of Emotion.js for dynamically generated styles can expose your application to XSS attacks. When user inputs are linked to style properties without filtering, malicious JavaScript can be executed. Implement strong input validation techniques and sanitize all inputs dynamically integrated into your styles.

For example:

• Avoid dynamically binding raw user inputs to style attributes.
• Use libraries like sanitize-html to ensure input integrity.

import sanitizeHtml from 'sanitize-html';

const userInput = "<script>alert('XSS');</script>";
const sanitizedInput = sanitizeHtml(userInput, {
  allowedTags: [],
  allowedAttributes: {},
});

const StyledDiv = styled.div`
  content: "${sanitizedInput}";
`;

import sanitizeHtml from 'sanitize-html';

const userInput = "<script>alert('XSS');</script>";
const sanitizedInput = sanitizeHtml(userInput, {
  allowedTags: [],
  allowedAttributes: {},
});

const StyledDiv = styled.div`
  content: "${sanitizedInput}";
`;

4- Keep Third-Party Dependencies Secure

When using Emotion.js, third-party libraries or components that you include in your application can lead to indirect vulnerabilities. In particular, when using component libraries compatible with Emotion, make sure that these libraries are up-to-date and secure.

• Update library dependencies regularly.
• Follow the official documentation and release notes of Emotion.js.
• Regularly check your project for potential vulnerabilities using a security scanner tool (e.g. npm audit).

What is Emotion.js?

Emotion.js is a powerful library that enables more efficient writing of CSS with JavaScript in modern web development projects. It is designed to integrate with React and other JavaScript frameworks. Emotion makes style definitions dynamic and component-based. It also has a performance-oriented design and supports both static and dynamic styling.

Vulnerabilities Discovered in Emotion.js

Emotion.js itself does not directly cause a vulnerability and has no CVEs (Common Vulnerabilities and Exposures) registered so far. However, incorrect or careless use of the library may indirectly lead to vulnerabilities.