Nextcloud Calendar auto-download flaw before 4.7.17/5.2.4
CVE-2025-66550 Published on December 5, 2025
Nextcloud Calendar attachments of local files are offered to downloaded
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.
Vulnerability Analysis
CVE-2025-66550 is exploitable with network access, requires user interaction and a small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality, a high impact on integrity, and no impact on availability.
Weakness Type
Improper Handling of Unexpected Data Type
The software does not handle or incorrectly handles when a particular element is not the expected type, e.g. it expects a digit (0-9) but is provided with a letter (A-Z).
Affected Versions
nextcloud security-advisories:- Version >= 5.0.0-rc.1, < 5.2.4 is affected.
- Version < 4.7.17 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.