Medium severity · published Dec 5, 2025
CVE-2025-66511
Nextcloud Calendar app used predictable proposal participant tokens
6.5
CVSS
0.3%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
Low
Integrity
Low
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.3% · higher than 22% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
SE
security-advisories · nextcloud
>= 6.0.0-rc.1, < 6.0.3 · source: vendor
CA
Calendar · Nextcloud
6.0.0 before 6.0.3 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2025-66511
CVE.org
https://www.cve.org/CVERecord?id=CVE-2025-66511
Patch
https://github.com/nextcloud/calendar/commit/8de14ae87f321f5f09280d9895a27d54d24f33fb
Issue Tracking
https://github.com/nextcloud/calendar/pull/7659
Patch
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-whm3-vv55-gf27
Permissions Required
https://hackerone.com/reports/3385434
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