CVE-2025-66550 is a vulnerability classified under CWE-241, indicating improper handling of unexpected data types within the Nextcloud Calendar application. Specifically, in versions prior to 4.7.17 and 5.2.4, the application fails to properly validate or restrict the type of attachments linked in calendar events. A malicious user can create a calendar event containing a crafted attachment that points to a downloadable file hosted on the same Nextcloud server. When other users view or interact with this event, the linked file is automatically downloaded without requiring explicit user confirmation. This behavior arises from insufficient input validation and improper handling of attachment data types, leading to unintended automatic file downloads. The vulnerability requires the attacker to have at least low-level privileges (PR:L) and some user interaction (UI:R) but does not allow direct confidentiality breaches or denial of service. The impact primarily concerns integrity, as unauthorized files could be downloaded and potentially manipulated or executed by users. The vulnerability has a CVSS 3.1 base score of 5.7, reflecting a medium severity level. It is addressed in Nextcloud Calendar versions 4.7.17 and 5.2.4 by implementing stricter validation and user confirmation mechanisms for attachments in calendar events. No public exploits have been reported to date, but the vulnerability could be leveraged in targeted attacks to trick users into downloading malicious or unauthorized files from the server.

For European organizations, this vulnerability poses a moderate risk primarily to the integrity of data and user trust within Nextcloud environments. Organizations relying on Nextcloud Calendar for internal scheduling and collaboration may face risks of unauthorized file downloads that could lead to malware execution or data tampering if the downloaded files are maliciously crafted. Although confidentiality and availability are not directly impacted, the automatic download behavior could be exploited in social engineering or spear-phishing campaigns targeting employees. This is particularly concerning for sectors with high regulatory requirements for data integrity, such as finance, healthcare, and government agencies across Europe. Additionally, organizations with large deployments of Nextcloud, especially in countries with strong adoption of open-source collaboration tools, may see increased exposure. The absence of known exploits in the wild reduces immediate risk but does not eliminate the potential for future exploitation. Failure to patch could also lead to reputational damage if exploited in targeted attacks.

European organizations should immediately verify their Nextcloud Calendar versions and upgrade to 4.7.17 or 5.2.4 or later to remediate this vulnerability. Beyond patching, administrators should audit calendar event attachments for suspicious links and restrict calendar event creation permissions to trusted users only. Implementing network-level controls to monitor and restrict unexpected file downloads from internal servers can help detect or prevent exploitation attempts. User awareness training should emphasize caution when interacting with calendar events containing attachments or links, especially from unknown or untrusted sources. Organizations should also consider enabling logging and alerting on unusual download activities within Nextcloud environments. For enhanced security, applying application-layer firewalls or web application firewalls (WAFs) that can detect and block anomalous attachment behaviors may provide additional protection. Regular vulnerability scanning and penetration testing focused on collaboration platforms can help identify residual risks.