CVE-2025-66511

Name: CVE-2025-66511
Creator: NVD / NIST
Published: 2025-12-05T17:16:04.797+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.5/10EPSS 0.25%

Last modified Jun 17, 2026

CVE-2025-66511 is a medium-severity vulnerability rated 6.5/10 on the CVSS scale. 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. EPSS estimates a 0.25% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

6.5/10

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

EPSS Probability

0.25%

15.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-330

Affected Software

Vendor	Product	Versions
Nextcloud	Calendar	>= 6.0.0, < 6.0.3

References

https://github.com/nextcloud/calendar/commit/8de14ae87f321f5f09280d9895a27d54d24f33fbPatch
https://github.com/nextcloud/calendar/pull/7659Issue Tracking
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-whm3-vv55-gf27Patch, Vendor Advisory
https://hackerone.com/reports/3385434Permissions Required, Vendor Advisory

Timeline

Published: Dec 5, 2025
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2025-66511?

How severe is CVE-2025-66511?

CVE-2025-66511 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 0.25% probability of exploitation in the next 30 days.

How do I fix CVE-2025-66511?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2025-66511?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST