OOB security incident workspace has this dashboard which provides details about security incidents and their corresponding SLAs. If the requirement is to create a dashboard that has security incidents and response tasks for a given customer (since you mentioned CSM portal) then you may have to add a interactive filter on the dashboard and allow way to filter the data accordingly. Second part would be to create ACLs that would provide read access to security incident table (sn_si_incident) and response task table (sn_si_task), OOB below are ACLs on security incident Access Controls | ServiceNow
Create new set of ACLs on sn_si_incident and sn_si_task with role as snc_external with condition to limit the records based on the affected user.company or configuration item.company or any other attribute on security incident / security task that would identify the company of user who is trying to access these records.
This should provide a good start point to solution this requirement.
