Cyber Resilience

CVE-2025-66550

Nextcloud Calendar 4.0.0 – 4.7.17

Published
05 December 2025
Modified
10 December 2025
Patch / advisory
View advisory
CVSS Score v3.1 5.7
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N
EPSS Score 0.0033 26th percentile
Risk Priority 44 floored blend · peak EPSS

Summary

CVE-2025-66550 is a medium-severity Improper Handling of Unexpected Data Type (CWE-241) vulnerability in Nextcloud Calendar. Its CVSS base score is 5.7 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 26th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to SI-10 (Information Input Validation) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

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…

more

file would be downloaded without the user confirming the action. This vulnerability is fixed in 4.7.17 and 5.2.4.

CWE(s)
CWE-241

Related Threats

MITRE ATT&CK Enterprise Techniques

T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
T1210 Exploitation of Remote Services Lateral Movement
Adversaries may exploit remote services to gain unauthorized access to internal systems once inside of a network.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2024-37316Same product: Nextcloud Calendar
CVE-2025-66546Same product: Nextcloud Calendar
CVE-2023-33183Same product: Nextcloud Calendar
CVE-2023-48308Same product: Nextcloud Calendar
CVE-2023-45150Same product: Nextcloud Calendar
CVE-2025-66511Same product: Nextcloud Calendar
CVE-2026-45286Same product: Nextcloud Calendar
CVE-2023-33182Same vendor: Nextcloud
CVE-2024-9423Shared CWE-241
CVE-2024-25966Shared CWE-241

Affected Assets

nextcloud
calendar
4.0.0 — 4.7.17 · 5.0.0 — 5.2.4

Mitigating Controls

Mitigating Controls (NIST 800-53 r5) AI

SI-10 Information Input Validation good match

Input validation explicitly checks that supplied data matches expected types and rejects mismatches before they reach processing logic.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require proper input validation and type checking to prevent this weakness.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing can detect type-handling flaws but does not itself implement the preventive control.

A.8.25 Secure development life cycle partial match
prevents

Secure development lifecycle includes input validation and type checking that directly mitigates improper handling of unexpected data types.

A.8.26 Application security requirements partial match
prevents

Application security requirements typically mandate strong input validation and type enforcement to prevent malformed data processing.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure system architecture principles encourage robust input handling but do not specifically address data-type validation.

A.8.28 Secure coding partial match
prevents

Secure coding standards explicitly require type checking and input sanitization, directly addressing improper handling of unexpected data types.

References