The Executor of a will is a "Legal Representative" according to CRA's guide on Returns for Deceased Persons - 2019. By following those links, you come to the following: "3. Type of access you can give The list of services available to a representative is different for personal tax accounts, business tax accounts, and non-resident tax accounts. If you don't specify a level of authorization, we will assign level 1 access.
Personal There are two levels of authorizations you can give your representative on your personal tax account:
get information only (level 1) request limited changes and get information (level 2) An ordinary representative on a personal tax account cannot update your address.
A legal representative has access to all of your information. They can do all the tax-related activities that you can, including changing your direct deposit information and granting or cancelling an authorization for another representative for your personal or non-resident tax account. If you have a legal representative, go to Legal representative.
For more information, go to our List of services for representatives of individuals.
Business There are three levels of authorizations you can give your representative on your business accounts:
get information only (level 1) update and get information (level 2) delegate authority, update, and get information (level 3)"
As a result of that, it is quite clear that the Executor of an estate can in fact authorize you as a representative, and the Authorization form is the avenue for that. They must indicate their name, and their role ie; Executor (or POA if the person is still alive) above their signature.