Symptoms

Adding user to a SP group with Full Control will allow the user to access the K2 Designer for SharePoint 2010 even though the user is not a member of a K2 Workflow Designers's SharePoint group.
 

Diagnoses

The issue has been confirmed as a known issue. This issue only occurs with SP group with Full control permission.
 

Resolution

This item will be fixed in future release(s) of the K2 product. Please use the workaround below.
i) Find the K2 WebDesigner application from your SharePoint site in IIS. On my vm, I have a SP web application (Portal). For example IIS site andgt Portal andgt _layouts andgt WebDesigner application. Click Authorization Rules.
ii) In here, I can set to specified users (Specified Uses field), AD user in this format, i.e. Domain_nameuser_name
iii) When I launch K2D4SP with a different account, I get the prompt windows credentials. Only the account in IIS authorization rule can access it.