Sharepoint Visitors Tasks

Question

We need to give visitors users the ability to confirm completing their assigned tasks, without giving them owner, contribute or design permissions...

Visitor can only confirm completing his tasks not editing or modifying them.

Answer 1

Considering by default that users need contribute access or some version of contribute in order to act on the data within SharePoint, how do you propose they modify the data?

"completing a task" is still a subset of a CRUD operation, requiring some form of contribute permissions. That being said, there are always workarounds. Have you given any thought to a custom form which allows the user to update the completed and % fields?