CVE-2025-66550

Unknown Unknown - Not Provided

BaseFortify

Publication date: 2025-12-05

Last updated on: 2025-12-10

Assigner: GitHub, Inc.

Description

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.

CVSS Scores

EPSS Scores

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Meta Information

Published

2025-12-05

Last Modified

2025-12-10

Generated

2026-07-06

AI Q&A

2025-12-05

EPSS Evaluated

2026-07-05

NVD

CVE-2025-66550

EUVD

EUVD-2025-201443

Affected Vendors & Products

Vendor	Product	Version / Range
nextcloud	calendar	From 4.0.0 (inc) to 4.7.17 (exc)
nextcloud	calendar	From 5.0.0 (inc) to 5.2.4 (exc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-241	The product 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).

Attack-Flow Graph

Executive Summary

This vulnerability in Nextcloud Calendar allows a malicious user to create a calendar event with a specially crafted attachment that links to a file on the same Nextcloud server. When another user views this event, the linked file is downloaded automatically without the user's confirmation. This behavior occurs in versions prior to 4.7.17 and 5.2.4 and has been fixed in those versions.

Impact Analysis

The vulnerability can lead to unauthorized downloading of files from the Nextcloud server without user consent, potentially exposing sensitive or private data. This could result in information leakage or unauthorized access to internal files, impacting data confidentiality and user trust.

Mitigation Strategies

Update Nextcloud Calendar to version 4.7.17 or 5.2.4 or later, as these versions contain the fix for this vulnerability.

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