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Vulnerability record · CVE-2025-66511 · published 5 December 2025

CVE-2025-66511: Nextcloud calendar vulnerability

Nextcloud · Calendar

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.

6.5 CVSS 3.1 Medium EPSS 0.30% · top 80.1% CWE-330 · CWE-330
6.5CVSS 3.1 base score
0.30%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
4References
17 Jun 2026Last modified by NVD

Description

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/nextcloud/calendar/commit/8de14ae87f321f5f09280d9895a27d54d24f33fb Patch
https://github.com/nextcloud/calendar/pull/7659 Issue Tracking
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-whm3-vv55-gf27 PatchVendor Advisory
https://hackerone.com/reports/3385434 Permissions RequiredVendor Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2025-66511), CISA KEV, FIRST EPSS (scores of 2026-09-29). This page is refreshed as NVD updates the record.