Nextcloud Server before 12.0.8 and 13.0.3 suffer from improper authentication on the OAuth2 token endpoint. Missing checks potentially allowed handing out new tokens in case the OAuth2 client was partly compromised.

Nextcloud Server before 11.0.7 and 12.0.5 suffers from an Authorization Bypass Through User-Controlled Key vulnerability. A missing ownership check allowed logged-in users to change the scope of app passwords of other users. Note that the app passwords themselves where neither disclosed nor could the error be misused to identify as another user.

Nextcloud Server before 10.0.4 and 11.0.2 are vulnerable to disclosure of calendar and addressbook names to other logged-in users. Note that no actual content of the calendar and addressbook has been disclosed.

Nextcloud Server before 11.0.3 is vulnerable to disclosure of valid share tokens for public calendars due to a logical error. Thus granting an attacker potentially access to publicly shared calendars without knowing the share token.

Nextcloud Server before 9.0.58 and 10.0.5 and 11.0.3 are shipping a vulnerable JavaScript library for sanitizing untrusted user-input which suffered from a XSS vulnerability caused by a behaviour change in Safari 10.1 and 10.2. Note that Nextcloud employs a strict Content-Security-Policy preventing exploitation of this XSS issue on modern web browsers.

Nextcloud Server before 11.0.3 is vulnerable to an improper session handling allowed an application specific password without permission to the files access to the users file.

Nextcloud Server before 9.0.58 and 10.0.5 and 11.0.3 are vulnerable to an inadequate escaping of error messages leading to XSS vulnerabilities in multiple components.

Nextcloud Server before 11.0.3 is vulnerable to an inadequate escaping leading to a XSS vulnerability in the search module. To be exploitable a user has to write or paste malicious content into the search dialogue.