Nextcloud: URI scheme bypass in mail app lead to HTML content spoof and opener control

Bug When we load a HTML mail from mailbox via api, etc http://nextcloud/index.php/apps/mail/accounts//folders/SU5CT1g=/messages//html Our content will be passed to HTML Purifier to strip malicious XSS patterns. After that, an filter will apply to transform acceptable URI schemes http, https, ftp,...