CVE-2025-66550 Nextcloud Calendar attachments of local files are offered to downloaded
A malicious user can force the download of local server files via crafted calendar event attachments, bypassing user confirmation.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- None listed
What is CVE-2025-66550?
CVE-2025-66550 is a medium-severity vulnerability in nextcloud security-advisories, published on 5 Dec 2025. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Official 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.
Weakness: CWE-241
Which versions are affected by CVE-2025-66550?
It affects 1 product.
Products include Nextcloud Calendar.
How do I fix CVE-2025-66550?
No fix has been published for CVE-2025-66550 yet. Limit exposure of the affected software until one is available.
Is CVE-2025-66550 being exploited?
There is no known exploitation of CVE-2025-66550 in the wild and no public exploit code.
Other vulnerabilities in nextcloud security-advisories
- CVE-2022-24838 Command Injection in Appointment Emails for Nextcloud Calendar
- CVE-2025-66511 Nextcloud Calendar app used predictable proposal participant tokens
- CVE-2018-3763 Cross-site scripting in Nextcloud Calendar application
- CVE-2024-37316 Nextcloud Calendar's event create can create attachments that link to other websites
- CVE-2026-45286 Nextcloud: Calendar app leaked user identifiers via attendee suggestion endpoint
Sources
Also known as GHSA-f29c-ppmv-8mcv.