Nextcloud Server is a self hosted personal cloud system. A recipient of a share with read&share permissions could reshare the item with more permissions. It is recommended that the Nextcloud Server is upgraded to 26.0.13 or 27.1.8 or 28.0.4 and that the Nextcloud Enterprise Server is upgraded to 26.0.13 or 27.1.8 or 28.0.4.
https://hackerone.com/reports/2289425
https://github.com/nextcloud/server/pull/44339
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-jjm3-j9xh-5xmq