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Execute As User = 'User1' doesn't work in the Azure SQL DB master database. User1 account created in the master database and in a user database. Execute As User works in the user database and in the master database, receives the error: Msg 15517, Level 16, State 1, Line 7 - Cannot execute as the database principal because the principal "user1" does not exist, this type of principal cannot be impersonated, or you do not have permission. Why doesn't Execute As work in the master database in Azure SQL DB?
My guess is that a database principal in the master database for Azure SQL Database is more akin to a login. That is, it is there to permit users to log in and be authenticated. And as a side effect of this, EXECUTE AS is broken.
What are you trying to do with EXECUTE AS in master?
We're testing permissions on a Create Database command that only runs in master db to copy a database for automation.
In on-prem SQL Server you would need EXECUTE AS LOGIN to test this, since CREATE DATABASE requires a server-level permission. EXECUTE AS LOGIN is not supported in Azure SQL Database at all.
Seems like you will need to test by making a real login.