CVE-2025-66511

Nextcloud Calendar app used predictable proposal participant tokens

4.8 Medium

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.

Attack profile

How easy it is to exploit

Attack vector: Network
Complexity: High
Privileges needed: None
User interaction: None

What an attacker can affect

Confidentiality: Low
Integrity: Low
Availability: None

Exploit likelihood

0.32%Up 0.3 points since Dec 6, 2025

What each source reports

CVE List Score used 4.8

Nextcloud Calendar app used predictable proposal participant tokens

Record: CVE-2025-66511
CVSS 3.1: 4.8 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
Weakness: CWE-330
Updated: 2026-11-02

EUVDEUVD-2025-201444 4.8

Record: EUVD-2025-201444
CVSS 3.1: 4.8 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
Updated: 2026-11-02

NVD 4.8

Record: CVE-2025-66511
CVSS 3.1: 4.8 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
CVSS 3.1: 6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Weakness: CWE-330
Updated: 2026-11-02

OSV 4.8

Nextcloud Calendar app used predictable proposal participant tokens

Record: CVE-2025-66511
CVSS 3.1: 4.8 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
Weakness: CWE-330
Updated: 2026-11-02

Affected products

History

History
Recorded	Changed	Sources
Nov 2, 2026	first indexed	CVE List, EUVD, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2025-66511 is a medium vulnerability (CVSS 4.8) in Nextcloud Calendar, published December 5, 2025. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.32%, the 24th percentile. No source names a fixed version.