The Nextcloud Calendar application, a widely used tool for managing schedules and coordinating events within the Nextcloud platform, has been identified with a notable security flaw. This vulnerability, cataloged as CVE-2025-66511, stems from an insecure method used to generate unique participant tokens for meeting proposals.
In simpler terms, when a meeting proposal is created in affected versions of Nextcloud Calendar, the special codes (tokens) given to participants were not truly random. Instead, they were generated using a predictable hash function. This predictability meant that a malicious actor could potentially guess or compute valid tokens. With these compromised tokens, an attacker could then bypass security measures to view private meeting details or even submit false date suggestions to meeting proposals without proper authorization. This kind of unauthorized access could compromise the privacy of your calendar events and disrupt organizational planning.
CVE Details
Product: Nextcloud Calendar
Published Date: December 5, 2025
Last Modified: December 10, 2025
Severity: Medium (CVSS Score 4.8)
Status: Analyzed
Affected Products
This vulnerability impacts all versions of the Nextcloud Calendar application prior to version 6.0.3. It is crucial for users and administrators to verify their current version and take immediate action if they are running an older, unpatched release.
Current Status
The security community has thoroughly analyzed and confirmed the existence and nature of this vulnerability. This means the problem has been understood, and the necessary steps to address it have been clearly identified.
Severity Level
Rated as “Medium” with a CVSS score of 4.8, CVE-2025-66511 presents a moderate risk. While it doesn’t allow an attacker to completely take over your Nextcloud system, the implications of unauthorized access to meeting proposals are significant. Attackers could potentially gather sensitive information about upcoming events, attendees, and discussions. Furthermore, the ability to submit arbitrary dates could lead to confusion, misinformation, or even sabotage of organizational activities, impacting productivity and trust. The core of the problem lies in the insufficient randomness of the generated tokens, making them susceptible to educated guesses or computational attacks rather than requiring sophisticated exploitation techniques.
Possible Solutions
The good news is that Nextcloud has already released a fix for this vulnerability. The solution is straightforward: upgrade your Nextcloud Calendar application to version 6.0.3 or a newer release. This updated version incorporates a robust method for generating participant tokens, utilizing cryptographically secure random bytes. This ensures that tokens are truly unpredictable, eliminating the possibility of attackers computing valid tokens and maintaining the integrity and confidentiality of your meeting proposals. Regularly updating all software, especially security-critical applications like Nextcloud, is a cornerstone of a strong cybersecurity posture.
References
https://github.com/nextcloud/calendar/commit/8de14ae87f321f5f09280d9895a27d54d24f33fb
https://github.com/nextcloud/calendar/pull/7659
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-whm3-vv55-gf27
https://hackerone.com/reports/3385434
