Desktop client can be tricked into opening/executing local files when clicking a nc://open/ link

Description

Impact

After a user received a malicious file via a share and has it synced locally or the virtual filesystem enabled, being tricked into clicking a nc://open/ link will open the default editor for the file type of the shared file, which on Windows can also sometimes mean that a file depending on the type, e.g. “vbs”, is being executed.

Patches

It is recommended that the Nextcloud Desktop client is upgraded to 3.6.1

Workarounds

• Block the Nextcloud Desktop client 3.6.0 by settings the minimum.supported.desktop.version system config to 3.6.1 on the server, so new files designed to use this attack vector are not downloaded anymore. Already existing files can still be used.
• Enforce shares to be accepted by setting the sharing.force_share_accept system config to true on the server, so new files designed to use this attack vector are not downloaded anymore. Already existing shares can still be abused.
• Don’t click malicious links

References

• HackerOne
• Desktop PullRequest
• Server PullRequest

For more information

If you have any questions or comments about this advisory:

• Create a post in nextcloud/security-advisories
• Customers: Open a support ticket at support.nextcloud.com