NextCloud Server and NextCloud Enterprise Server provide file storage for Nextcloud, a self-hosted productivity platform. In NextCloud Server versions 25.0.0 until 25.0.7 and 26.0.0 until 26.0.2 and Nextcloud Enterprise Server versions 21.0.0 until 21.0.9.12, 22.0.0 until 22.2.10.12, 23.0.0 until 23.0.12.7, 24.0.0 until 24.0.12.2, 25.0.0 until 25.0.7, and 26.0.0 until 26.0.2, when two server are registered as trusted servers for each other and successfully exchanged the share secrets, the malicious server could modify or delete VCards in the system addressbook on the origin server. This would impact the available and shown information in certain places, such as the user search and avatar menu. If a manipulated user modifies their own data in the personal settings the entry is fixed again.
Nextcloud Server n 25.0.7 and 26.0.2 and Nextcloud Enterprise Server 21.0.9.12, 22.2.10.12, 23.0.12.7, 24.0.12.2, 25.0.7, and 26.0.2 contain a patch for this issue. A workaround is available. Remove all trusted servers in the "Administration" > "Sharing" settings …/index.php/settings/admin/sharing. Afterwards, trigger a recreation of the local system addressbook with the following occ dav:sync-system-addressbook.
A missing check for IPv4 nested inside IPv6 in Nextcloud server < 17.0.1, < 16.0.7, and < 15.0.14 allowed a Server-Side Request Forgery (SSRF) vulnerability when subscribing to a malicious calendar URL.
A missing access control check in Nextcloud Server < 18.0.1, < 17.0.4, and < 16.0.9 causes hide-download shares to be downloadable when appending /download to the URL.
Improper authorization in Nextcloud server 17.0.0 causes leaking of previews and files when a file-drop share link is opened via the gallery app.
Improper permissions preservation in Nextcloud Server 16.0.1 causes sharees to be able to reshare with write permissions when sharing the mount point of a share they received, as a public link.
A bug in Nextcloud Server 17.0.1 causes the workflow rules to depend their behaviour on the file extension when checking file mimetypes.
Exposure of Private Information in Nextcloud Server 16.0.1 causes the server to send it's domain and user IDs to the Nextcloud Lookup Server without any further data when the Lookup server is disabled.