Technical Advisory: PDFTron JavaScript URLs Allowed in WebViewer UI (CVE-2021-39307)

Vendor: PDFTron
Vendor URL:
Versions affected: WebViewer UI 8.0 or below
Systems Affected: Web applications hosting the affected software
Author: Liyun Li <[at]nccgroup[dot]com>
CVE Identifier: CVE-2021-39307


PDFTron’s WebViewer UI 8.0 or below renders dangerous URLs as hyperlinks in supported documents, including JavaScript URLs, allowing the execution of arbitrary JavaScript code.


An attacker could steal a victim’s session tokens, log their keystrokes, steal private data, or perform privileged actions in the context of a victim’s session.


JavaScript URLs are dangerous because they can be used to execute arbitrary JavaScript code when visited. Built-in PDF readers in modern browsers, such as Mozilla’s pdf.js, do not render code-execution-capable URLs as hyperlinks to avoid this issue.

To reproduce this issue, first create the following HTML document and save the rendered content as PDF on a modern browser.

<h2><a href="javascript:document.write`
    <form method='GET' action=''>
      <input type='submit' value='NCC Group'>
`">Click me</a></h2>

After that, use the “d” parameter to include the uploaded PDF file (e.g. http://webviewer-instance/#d=https://domain.tld/test.pdf).

Support for rendering clickable JavaScript and Data URL should be removed.

Recommendation to Users

Upgrade WebViewer UI to 8.1, available at

Vendor Communication

2021-08-16: Issue reported to PDFTron
2021-08-17: PDFTron confirmed the vulnerability
2021-08-23: PDFTron issued patch to nightly build
2021-09-09: PDFTron WebViewer 8.1 released 
2021-09-14: Advisory released by NCC Group

About NCC Group

NCC Group is a global expert in cybersecurity and risk mitigation, working with businesses to protect their brand, value and reputation against the ever-evolving threat landscape. With our knowledge, experience and global footprint, we are best placed to help businesses identify, assess, mitigate respond to the risks they face. We are passionate about making the Internet safer and revolutionizing the way in which organizations think about cybersecurity.

Published date:  September 14, 2021

Written by:  Liyun Li

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