Nextcloud is an open source content collaboration platform. From versions 0.3.0 to before 3.1.0, 5.0.0 to before 5.1.0, and 6.0.0 to before 6.4.0, a missing signature verification in User OIDC allowed a malicious ID4me authority to identify as any user. This issue has been patched in versions 3.1.0, 4.1.0, 5.1.0, 6.4.0 and 8.3.0.
https://hackerone.com/reports/3489490
https://github.com/nextcloud/user_oidc/pull/1285
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-qqgv-fqwp-mjpp