Vulnerability in Nextcloud Calendar
CVE-2025-66511
Nextcloud Calendar is a calendar app for Nextcloud. Prior to 6.0.3, the Calendar app generates participant tokens for meeting proposals using a hash function, allowing an attacker to compute valid participant tokens, which allowed them to request details and submit dates in meeting proposals. The tokens are not purely random generated. This vulnerability is fixed in 6.0.3.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 4.8 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.003 (20.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 21st percentile — 20.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: 0.000
Affected products
- Nextcloud Calendar
- Nextcloud Security-Advisories — versions >= 6.0.0-rc.1, < 6.0.3
Weakness classification (CWE)
References
- github.com/nextcloud/security-advisories/security/advisories/GHSA-whm3-vv55-gf27 (Patch, Vendor Advisory)
- github.com/nextcloud/calendar/pull/7659 (Issue Tracking)
- github.com/nextcloud/calendar/commit/8de14ae87f321f5f09280d9895a27d54d24f33fb (Patch)
- hackerone.com/reports/3385434 (Permissions Required, Vendor Advisory)
Frequently asked questions
- What is CVE-2025-66511?
- CVE-2025-66511 is a medium-severity vulnerability in Nextcloud Calendar, classified under Use of Insufficiently Random Values. CVSS score: 4.8/10. Published 2025-12-05.
- How severe is CVE-2025-66511?
- Medium severity. CVSS v3 base score is 4.8 out of 10.