The vulnerability identified as CVE-2025-66511 affects the Nextcloud Calendar app, specifically versions from 6.0.0-rc.1 up to but not including 6.0.3. The core issue is the generation of participant tokens for meeting proposals using a hash function that produces insufficiently random values, classified under CWE-330 (Use of Insufficiently Random Values). These tokens are intended to secure meeting proposal interactions, such as viewing details and submitting dates. However, because the tokens are predictable or can be computed by an attacker, unauthorized parties can generate valid tokens without authentication or user interaction. This allows them to access sensitive meeting proposal information and manipulate scheduling data. The vulnerability does not impact availability but compromises confidentiality and integrity of meeting data. The attack vector is network-based with high attack complexity, meaning the attacker must have some knowledge or capability to compute the tokens but does not require privileges or user interaction. The flaw was addressed in Nextcloud Calendar version 6.0.3 by improving the randomness of token generation, likely by switching to a cryptographically secure random number generator. No public exploits have been reported, but the vulnerability poses a risk to organizations relying on Nextcloud for calendar management.

For European organizations using Nextcloud Calendar versions prior to 6.0.3, this vulnerability could lead to unauthorized disclosure of meeting proposal details and unauthorized modification of scheduling data. This may result in privacy breaches, exposure of sensitive meeting information, and potential disruption of organizational workflows. Confidentiality and integrity of calendar data are at risk, which could affect sectors with strict data protection requirements such as finance, healthcare, and government. Although the vulnerability does not directly impact system availability, the unauthorized manipulation of meeting proposals could cause operational inefficiencies and mistrust in collaboration tools. Given the widespread adoption of Nextcloud in Europe, especially among privacy-conscious organizations and public institutions, the impact could be significant if exploited. However, the medium CVSS score and high attack complexity somewhat limit the immediacy of the threat.

European organizations should promptly update Nextcloud Calendar to version 6.0.3 or later to remediate this vulnerability. Until updates can be applied, administrators should restrict network access to the Nextcloud Calendar app to trusted users and networks to reduce exposure. Monitoring and logging access to calendar meeting proposals can help detect suspicious activities indicative of token guessing attempts. Organizations should also review their internal policies on calendar data sensitivity and consider additional access controls or encryption for sensitive meeting information. Security teams should educate users about the importance of timely software updates and verify that all Nextcloud components are kept current. Finally, organizations could implement network-level protections such as web application firewalls (WAFs) to detect and block anomalous requests targeting token generation endpoints.