What’s Vulnerable › Vulnerabilities

CVE-2025-66511: Nextcloud Calendar app used predictable proposal participant tokens

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-12-05 · updated 2026-10-18
• Weakness: CWE-330

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.

Affected products

• Nextcloud Calendar application: from 6.0.0 before 6.0.3 — fixed in 6.0.3

How to fix

Upgrade to a fixed version: Nextcloud Calendar application 6.0.3. Check the vendor advisory for mitigations.

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

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