CVE-2025-66511: calendar — Fix

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.

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CVSS v3.1 Score

4.8

MEDIUM

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

Weakness Type (CWE)

CWE-330 CWE-330

Affected Products

Vendor	Product	Versions
nextcloud	calendar	6.0.0 — 6.0.3

References

Advisories & Patches

https://github.com/nextcloud/calendar/commit/8de14ae87f321f5f09280d9895a27d54d24f33fb https://github.com/nextcloud/security-advisories/security/advisories/GHSA-whm3-vv55-gf27 https://hackerone.com/reports/3385434

Other References

https://github.com/nextcloud/calendar/pull/7659

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. It has a CVSS v3.1 base score of 4.8 (MEDIUM).

How severe is CVE-2025-66511? +

CVE-2025-66511 has a CVSS v3.1 score of 4.8 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance.

What products are affected by CVE-2025-66511? +

CVE-2025-66511 affects products from nextcloud, specifically: calendar. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-66511? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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