CVE-2025-66550: Nextcloud Calendar

Medium severity, CVSS 5.7. EPSS: 0.3% chance of exploitation in the next 30 days.

Nextcloud Calendar is a calendar app for Nextcloud. Prior to 4.7.17 and 5.2.4, when a malicious user creates a calendar event with a crafted attachment that links to a download link of a file on the same Nextcloud server, the file would be downloaded without the user confirming the action. This vulnerability is fixed in 4.7.17 and 5.2.4.

Affected products

• Nextcloud Calendar: from 4.0.0, before 4.7.17 (fixed in 4.7.17); from 5.0.0, before 5.2.4 (fixed in 5.2.4)

Published 2025-12-05. Last modified 2026-06-17.