CVE-2025-66547: Nextcloud Server users can modify tags on files that do not belong to them

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-12-05 · updated 2026-10-18
Weakness: CWE-639

Description

Nextcloud Server is a self hosted personal cloud system. In Nextcloud Server and Enterprise Server prior to 31.0.1, non-privileged users can modify tags on files they should not have access to via bulk tagging. This vulnerability is fixed in 31.0.1.

Affected products

Nextcloud Server: from 31.0.0 before 31.0.1 — fixed in 31.0.1

How to fix

Upgrade to a fixed version: Nextcloud Server 31.0.1. Check the vendor advisory for mitigations.

References

https://github.com/nextcloud/security-advisories/security/advisories/GHSA-hq6c-r898-fgf2
https://github.com/nextcloud/server/commit/b44f1568f2dc97c746281d99e2342ad679e3d8a9
https://github.com/nextcloud/server/issues/51247
https://github.com/nextcloud/server/pull/51288
https://hackerone.com/reports/3040887

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