Nextcloud Server before 9.0.52 & ownCloud Server before 9.0.4 are vulnerable to a content-spoofing attack in the files app. The location bar in the files app was not verifying the passed parameters. An attacker could craft an invalid link to a fake directory structure and use this to display an attacker-controlled error message to the user.
https://github.com/owncloud/core/commit/c92c234059f8b1dc7d53122985ec0d398895a2cf
https://github.com/nextcloud/server/commit/dea8e29289a1b99d5e889627c2e377887f4f2983
https://github.com/nextcloud/server/commit/8aa0832bd449c44ec300da4189bd8ed4e036140c
https://github.com/nextcloud/server/commit/2da43e3751576bbc838f238a09955c4dcdebee8e