Is this SQL Procedure "injection proof"?

Most all answers and examples of SQL injection are showing some form of dynamic SQL or interpreting parameters as SQL. I haven't been able to find an example of the "correct" way. Microsoft and Oracle's documentation just shows examples of what not to do. So, I figured I should ask if this example of a stored procedure was protected against SQL injection attacks.

CREATE PROCEDURE test
    @username = varchar(30)
    @password = varchar(30)
AS
BEGIN
    SELECT *
    FROM credentials
    WHERE username = @username
    AND password = @password;
END
GO

Would this particular procedure be susceptible to SQL injections? I created the procedure and executed it with various attempts to inject SQL, such as EXEC test @password = '0; drop table credentials;', but was unable to do so. I figure I might not be doing the attack correctly.