CVE-2025-66511 is a vulnerability classified under CWE-330, indicating the use of insufficiently random values in security token generation within the Nextcloud Calendar app. Specifically, versions from 6.0.0-rc.1 up to 6.0.3 generate participant tokens for meeting proposals using a hash function rather than a cryptographically secure random number generator. This design flaw allows an attacker to compute or predict valid participant tokens, which are intended to secure access to meeting proposal details and the ability to submit dates. Because these tokens are not purely random, the entropy is insufficient, making token guessing or computation feasible. The vulnerability is remotely exploitable without authentication or user interaction but requires a higher attack complexity due to the need to reverse or predict the token generation process. Successful exploitation compromises confidentiality and integrity by exposing meeting details and allowing unauthorized modifications. The vulnerability does not impact availability. The issue was addressed and fixed in Nextcloud Calendar version 6.0.3 by improving the randomness of token generation. No known exploits have been reported in the wild as of the publication date. The CVSS v3.1 score is 4.8 (medium), reflecting the moderate impact and attack complexity.

For European organizations, this vulnerability poses a risk to the confidentiality and integrity of meeting data managed via Nextcloud Calendar. Unauthorized access to meeting proposals could lead to leakage of sensitive scheduling information, strategic plans, or personal data, potentially violating GDPR and other privacy regulations. Integrity compromise allows attackers to manipulate meeting dates, causing operational disruptions or facilitating social engineering attacks. While availability is unaffected, the breach of confidentiality and integrity can damage organizational trust and compliance posture. Organizations relying heavily on Nextcloud for collaboration, especially in sectors like government, finance, healthcare, and critical infrastructure, face increased risk. The medium severity and lack of known exploits reduce immediate urgency but do not eliminate the threat, especially as attackers may develop exploits over time.

The primary mitigation is to upgrade Nextcloud Calendar to version 6.0.3 or later, where the token generation method has been corrected to use cryptographically secure randomness. Organizations should audit their current Nextcloud Calendar deployments to identify affected versions and prioritize patching. Additionally, reviewing access logs for unusual requests to meeting proposals or unexpected date submissions can help detect exploitation attempts. Implementing network-level protections such as web application firewalls (WAFs) to monitor and restrict anomalous API calls related to calendar tokens can provide an additional defense layer. Educating users about the importance of timely software updates and monitoring Nextcloud security advisories for related vulnerabilities is also recommended. For highly sensitive environments, consider restricting external access to Nextcloud Calendar or enforcing multi-factor authentication on the broader Nextcloud platform to reduce risk exposure.