I've got a problem, if a portal user gets a link to a specific task in odoo backend he can access the odoo backend. Is there a way to just redirect him back to the portal?

The portal user group needs read permissions for tasks so he can see the pictures posted in the <t t-call="website_mail.message_thread">

EDIT:

In the backend the portal user can see the list of tasks he's the follower but not open them or see their details.