CVE-2022-39333 - HTML Injection in Nextcloud Desktop Client

Nextcloud is a popular open-source platform for file sharing and collaboration. Its Desktop Sync Client makes it easy for users to sync files between their computers and their Nextcloud server. But, like all software, it isn’t immune to security vulnerabilities.

In this post, we’ll dive into a real security issue, CVE-2022-39333. Here’s what you need to know, how this bug works, and how to stay safe.

What is CVE-2022-39333?

CVE-2022-39333 is a security vulnerability found in the Nextcloud Desktop client (before version 3.6.1) that lets an attacker inject arbitrary HTML into the application window. In plain English, a bad actor could make malicious HTML code show up in your Nextcloud Desktop app. This happens due to improper sanitization of user-controlled content in parts of the client.

Impact: Arbitrary HTML injection – can lead to phishing, information theft, or worse

- Exploitability: Requires the attacker to trick the target into interacting with malicious content

Understanding the Risk

HTML injection is dangerous because it can act as a starting point for more serious attacks, like cross-site scripting (XSS) or phishing. Imagine if someone sent you a shared link or file name with HTML content inside – and your desktop app renders it as real HTML, not just text.

For example

- A file or folder with a name like <img src=x onerror=alert('Hacked!')> can display a pop-up or even steal information from the app.

How it Can Be Exploited (with Example)

Attackers may leverage this vulnerability by crafting a malicious share, folder, or file name containing HTML markup. When the vulnerable Nextcloud Desktop Client renders this content, the HTML gets executed as part of the app’s UI.

> ⚡ Note: The actual injection points may vary depending on how the client displays synced content or notifications.

Example Attack Scenario

Let’s say Alice is using an old Nextcloud Desktop Client (version < 3.6.1). Bob, the attacker, has access to upload files on the shared Nextcloud server (perhaps through a public share).

Bob uploads a file with the following name

<script>alert('You have been hacked!')</script>.png

When Alice syncs her files and the client displays synced file names, the malicious code is rendered, and an alert box pops up. With more sophisticated payloads, this could be used to steal information or trick the user (phishing).

Proof-of-Concept: Demo Code

To see how this works, set up a local Nextcloud server and use the vulnerable client.

1. Create a file/folder with HTML in the name:

Code snippet (for file name)

# Python script to automate creation
malicious_name = "<b>HACKED!</b>.txt"
with open(malicious_name, "w") as f:
    f.write("test")

When synced and viewed, this will inject <b>HACKED!</b> into the Desktop client.

Recommendations

- Update ASAP: Upgrade your Nextcloud Desktop client to version 3.6.1 or later. Download here
- No Workarounds: There are no reliable workarounds. Using the client on untrusted shares before updating may put you at risk.
- Be Wary: Avoid syncing files/folders from untrusted users or public shares until you’ve updated.

References

- CVE-2022-39333 at NVD (National Vulnerability Database)
- Nextcloud Security Advisories – Original HackerOne Disclosure
- Nextcloud Desktop Client Download
- GitHub Issue / Commit Fix

Conclusion

CVE-2022-39333 is a reminder that even desktop apps can be exposed to web-style exploits. If you’re using Nextcloud Desktop, take a minute to check your version and update it to stay protected.

Have questions? Leave a comment or check the Nextcloud forums. Stay safe and keep your software up to date!

Timeline

Published on: 11/25/2022 20:15:00 UTC
Last modified on: 12/01/2022 17:39:00 UTC