Sql-server – Cannot execute as the database principal because the principal “guest” does not exist

The problem is with the difference between logins and users. When you are granting the permissions you are working with a user. Only a login can have server level permissions such as sysadmin. I discussed this here if you are interested.

In the mean time I can tell you how to do what you need, however it's not the best idea in the world.

First create a seperate database, make SA the owner of that database, set the trustworthy property on for that database. Then create your usp_fts_parser stored procedure with EXECUTE AS OWNER. In this case OWNER is dbo. Also in this case dbo is SA. Because you made the database trustworthy it will allow users in it to access server level permissions. All of this means that your stored procedure will be able to act as a sysadmin. Next grant the login that you are using for the web application connect permissions to your new database. Then grant that user execute permissions on the stored procedure.

This is very important. ONLY grant the user execute permissions on the stored procedure. You have created a big security hole by turning on trustworthy with a database owned by a sysadmin so you need to be very very very careful who you give permissions to that database and what you allow them to do.

This request is very similar to, if not the same as, another question on here (DBA.StackExchange) regarding the need for very granular elevated permissions. I provided working code in my answer, and that could be used here as well:

What minimum permissions do I need to provide to a user so that it can check the status of SQL Server Agent Service?

The overall concept is the same: give the Stored Procedure itself the required permission by being associated (via the Certificate) with a Login that has the proper permission. The only difference would be that instead of granting VIEW SERVER STATE TO [MrDoStuff], you would probably just add [MrDoStuff] to the sysadmin fixed server-level role.

One difference, though, between this question and the "What minimum permissions..." question is that the other question simply needed access to a DMV and not to any code objects outside of the code being signed. It just so happens that signatures do not, by default, transfer to code called by the signed code. But there is a way of allowing this to happen: counter-sign the external code being called. Counter-signing allows code executed by signed-code to transfer the signature without actually being signed itself (and so the external code cannot be called independently). Counter signing is handled by the same ADD SIGNATURE function that was used to sign the initial Stored Procedure:

1. Restore the Certificate into the [msdb] database
2. Counter-sign the dbo.sysmail_update_account_sp Stored Procedure

Continuing the example code from that other answer, the additional steps should be:

USE [msdb];

CREATE CERTIFICATE [DoStuffCert]
    FROM FILE = 'C:\temp\ViewSqlAgentStatus.CER'
    WITH PRIVATE KEY (
        FILE = 'C:\temp\ViewSqlAgentStatus.PVK',
        DECRYPTION BY PASSWORD = 'DontStartNoneWontBeNone',
        ENCRYPTION BY PASSWORD = 'W0rdUp,Yo!'
    );

ADD COUNTER SIGNATURE TO dbo.sysmail_update_account_sp
    BY CERTIFICATE [DoStuffCert] WITH PASSWORD = 'W0rdUp,Yo!';

It is possible that if sysmail_update_account_sp executes any stored procedures, then they might also need to be counter-signed.