How severe is CVE-2025-66511?

CVE-2025-66511 has a CVSS 3.x base score of 4.8, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity low, and availability none.

Is CVE-2025-66511 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (17th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2025-66511?

CVE-2025-66511 affects Nextcloud Calendar. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2025-66511?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2025-66511 have an EU (EUVD) identifier?

Yes. CVE-2025-66511 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-201444.

When was CVE-2025-66511 published?

CVE-2025-66511 was published on 2025-12-05 and last updated on 2026-06-17.

CVE-2025-66511

CVE-2025-66511 is a medium-severity vulnerability in Nextcloud Calendar with a CVSS 3.x base score of 4.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-330.

Key facts

Severity: Medium (CVSS 3.x base score 4.8)
EPSS exploit prediction: 0% (17th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2025-201444
Weakness: CWE-330
Affected product: Nextcloud Calendar
Published: 2025-12-05
Last modified: 2026-06-17

Description

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.

Frequently asked questions

What is CVE-2025-66511?: 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.
How severe is CVE-2025-66511?: CVE-2025-66511 has a CVSS 3.x base score of 4.8, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity low, and availability none.
Is CVE-2025-66511 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (17th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2025-66511?: CVE-2025-66511 affects Nextcloud Calendar. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2025-66511?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2025-66511 have an EU (EUVD) identifier?: Yes. CVE-2025-66511 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-201444.
When was CVE-2025-66511 published?: CVE-2025-66511 was published on 2025-12-05 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:nextcloud:calendar:*:*:*:*:*:*:*:*

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