CVE-2025-66546: Nextcloud Calendar is a calendar app for Nextcloud

Routine

LOW 3.3UPGRADE

CVE-2025-66546 is a low-severity flaw (CVSS 3.3) affecting calendar. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	3.3 (Low)
Attack vector	Local
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.1% (top 97%)
CISA exploited list	No
Weakness	CWE-639
Published	2025-12-05
Last modified	2026-10-18

Publisher (CNA): GitHub, Inc. · Vendor: nextcloud

Last updated 2026-10-18 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2025-12-05 17:16 UTCPublished CVE record published
2026-10-18 14:10 UTCUpdated Record last modified

nextcloud calendar UPGRADE

Affected: >= 4.0.0 and < 4.7.19 · >= 5.0.0 and < 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

Fixed in: 4.7.19, 5.5.6

Safe to downgrade to: a release before 4.0.0

1. Check installed version

apt-cache policy calendar || rpm -q calendar || calendar --version

2. UPGRADE to 4.7.19 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade calendar

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade calendar

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 4.0.0 (Debian / Ubuntu)

apt-cache madison calendar sudo apt install calendar=<VERSION_BEFORE_4.0.0> sudo apt-mark hold calendar

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list calendar sudo dnf downgrade calendar-<VERSION_BEFORE_4.0.0> sudo dnf versionlock add calendar

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2025-66546?

calendar: upgrade to 4.7.19, 5.5.6. Exact commands for each OS are on this page.

Is CVE-2025-66546 being exploited?

CVE-2025-66546 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2025-66546 affect?

CVE-2025-66546 affects calendar. Severity is low (CVSS 3.3). 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

How urgent is CVE-2025-66546?

Priority 12/100. No elevated risk signals yet.

Software pages

Nextcloud vulnerabilities

Related CVEs

CVE-2025-66511 MEDIUM 4.8 UPGRADE Nextcloud Calendar is a calendar app for Nextcloud
CVE-2025-66550 MEDIUM 5.7 UPGRADE Nextcloud Calendar is a calendar app for Nextcloud
CVE-2026-29052 MEDIUM 6.1 UPGRADE The Calendar module for HumHub enables users to create one-time or recurring events, manage attendee invitatio
CVE-2026-45286 MEDIUM 4.3 UPGRADE Nextcloud is an open source content collaboration platform
CVE-2025-21035 MEDIUM 4.6 UPGRADE Improper access control in Samsung Calendar prior to version 12.5.06.5 in Android 14 and 12.6.01.12 in Android

References

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