CVE-2025-66511: Nextcloud Calendar is a calendar app for Nextcloud

Routine

MEDIUM 4.8REMOTEUPGRADE

CVE-2025-66511 is a medium-severity flaw (CVSS 4.8) 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 6.0.3, the Calendar app generates participant tokens for meeting proposals using a hash function, allowing an attacker to compute valid participant tokens, which allowed them to request details and submit dates in meeting proposals. The tokens are not purely random generated. This vulnerability is fixed in 6.0.3.

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	4.8 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 78%)
CISA exploited list	No
Weakness	CWE-330
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: >= 6.0.0 and < 6.0.3

Fixed in: 6.0.3

Safe to downgrade to: a release before 6.0.0

1. Check installed version

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

2. UPGRADE to 6.0.3 (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 6.0.0 (Debian / Ubuntu)

apt-cache madison calendar sudo apt install calendar=<VERSION_BEFORE_6.0.0> 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_6.0.0> 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-2025-66511?

calendar: upgrade to 6.0.3. Exact commands for each OS are on this page.

Is CVE-2025-66511 being exploited?

CVE-2025-66511 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-2025-66511 affect?

CVE-2025-66511 affects calendar. Severity is medium (CVSS 4.8). Nextcloud Calendar is a calendar app for Nextcloud. Prior to 6.0.3, the Calendar app generates participant tokens for meeting proposals using a hash function, allowing an attacker to compute valid participant tokens, which allowed them to r

How urgent is CVE-2025-66511?

Priority 27/100. Remotely exploitable, no login, no user action

Software pages

Nextcloud vulnerabilities

Related CVEs

CVE-2025-66550 MEDIUM 5.7 UPGRADE Nextcloud Calendar is a calendar app for Nextcloud
CVE-2025-66546 LOW 3.3 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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