What’s Vulnerable › Vulnerabilities

CVE-2025-66550: Nextcloud Calendar attachments of local files are offered to downloaded

• Severity: medium (CVSS 5.7 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-241

Description

Nextcloud Calendar is a calendar app for Nextcloud. Prior to 4.7.17 and 5.2.4, when a malicious user creates a calendar event with a crafted attachment that links to a download link of a file on the same Nextcloud server, the file would be downloaded without the user confirming the action. This vulnerability is fixed in 4.7.17 and 5.2.4.

Affected products

• Nextcloud Calendar application: from 4.0.0 before 4.7.17; from 5.0.0 before 5.2.4 — fixed in 4.7.17, 5.2.4

How to fix

Upgrade to a fixed version: Nextcloud Calendar application 4.7.17, 5.2.4. Check the vendor advisory for mitigations.

References

• https://github.com/nextcloud/calendar/commit/63a6c398db01391eb9fd5297a0d4c3d6e614f769
• https://github.com/nextcloud/calendar/pull/6971
• https://github.com/nextcloud/security-advisories/security/advisories/GHSA-f29c-ppmv-8mcv
• https://hackerone.com/reports/3112033

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