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Are PDFs with JavaScript Trustworthy?
2/29/2024 - Brian O'Neill

man looking at computer screen thoughtfully

JavaScript is an extremely versatile programming language supported by every major web browser. In addition to web application development (its primary use-case), it's also widely used to add custom interactive elements and dynamic behavior to PDF documents, largely in an effort to improve user-supplied data capture and efficiently link multimedia web content for document viewers.

Despite the numerous benefits PDFs enhanced with custom JavaScript can offer, however, we need to be extremely careful about opening PDFs containing JavaScript when they come from unknown, untrusted or potentially compromised external clients. If we came across a PDF containing JavaScript code from an unverified source via client-side file upload, or as an email attachment, we’re at risk of initiating a cyber-attack when we open the document in our web browser.

In this article, we'll examine some of the high-level benefits and risks associated with PDFs containing JavaScript code.

Benefits of JavaScript in PDFs

Many popular PDF viewers give us the option to enable JavaScript features for our document or write custom JavaScript code through a built-in editor. This code can carry out useful actions in our PDF, especially when capturing data inputs through dynamic input fields.

We could, for example, write code to calculate values from different numbers entered in separate form fields within our PDF. This could save a lot of time otherwise spent collecting data from PDFs and performing manual calculations, significantly improving the efficiency of web-based PDFs used for important data collection purposes.

We could also use JavaScript to validate important input fields in our PDF to help prevent malformed data entry. If one of our PDF fields asked for user-supplied email addresses, for example, we could write a script to ensure the user receives an error message if they wrote their address with incorrect syntax.

JavaScript can facilitate multimedia integrations in our PDF, allowing us to include audio or video elements with controlled playback. JavaScript-enabled buttons or triggers can allow users to submit data directly to a database or navigate quickly to other important web pages and documents.

Risks of JavaScript in PDFs

Threat actors can write malicious JavaScript in a specially crafted PDF file to exploit vulnerabilities in our web browsers. Moreover, they can obfuscate their code relatively easily with a basic understanding of PDF file structure, ensuring the average document viewer most likely won’t be aware of malicious scripts running in the background.

There are a wide variety of outcomes threat actors can target with malicious JavaScript.

By writing code that exploits a vulnerable PDF reader in a victim’s web browser, a threat actor could initiate a connection with a remote URL to steal important login credentials, session tokens, and other sensitive personal data from the user. They could also force the user’s vulnerable web browser to make an extremely high volume of requests to a target server, resulting in a Denial-of-Service attack.

By exploiting a vulnerable web browser’s download capability, a threat actor could also possibly force a remote malware download onto the user’s device, ultimately encrypting their victim’s data with Ransomware or taking direct control over other sensitive processes.

A threat actor could even exploit JavaScript buttons and triggers in a PDF to redirect users to other malicious documents or web pages.

Blocking PDFs with JavaScript using the Cloudmersive Advanced Virus Scan API

PDFs containing JavaScript (or HTML) code can be detected and blocked using custom request parameters available via the Cloudmersive Advanced Virus Scan API. This API provides 360-degree content protection for file uploads, scanning content for virus & malware signatures while additionally providing the option to block threatening content types including scripts, executables, macros, password-protected files, and more.

For more information about the Cloudmersive Advanced Virus Scan API, please do not hesitate to contact a member of our team.

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