What’s Vulnerable › Vulnerabilities

CVE-2025-66546: Nextcloud Calendar app allowed booking appointments without the generated token

• Severity: low (CVSS 3.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.1% 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-639

Description

Nextcloud Calendar is a calendar app for Nextcloud. Prior to 4.7.19, 5.5.6, and 6.0.1, the calendar app allowed blindly booking appointments with a squential ID without known the appointment token. This vulnerability is fixed in 4.7.19, 5.5.6, and 6.0.1.

Affected products

• Nextcloud Calendar application: from 4.0.0 before 4.7.19; from 5.0.0 before 5.5.6; 6.0.0, 6.0.0-rc1, 6.0.0-rc2, 6.0.0-rc3, 6.0.0-rc4, 6.0.0-rc5 (+1) — fixed in 4.7.19, 5.5.6

How to fix

Upgrade to a fixed version: Nextcloud Calendar application 4.7.19, 5.5.6. Check the vendor advisory for mitigations.

References

• https://github.com/nextcloud/calendar/commit/f41650c3681fc4a4130eb883f5c0899c011326b3
• https://github.com/nextcloud/calendar/pull/7537
• https://github.com/nextcloud/security-advisories/security/advisories/GHSA-7x2j-2674-fj95
• https://hackerone.com/reports/3275810

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