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Managing Trigger Security

By default, both DML and DDL triggers execute under the context of the user that calls the trigger. The caller of a trigger is the user that executes the statement that causes the trigger to run. For example, if user Mary executes a DELETE statement that causes DML trigger DML_trigMary to run, the code inside DML_trigMary executes in the context of the user privileges for Mary. This default behavior can be exploited by users who want to introduce malicious code in the database or server instance. For example, the following DDL trigger is created by user JohnDoe:

CREATE TRIGGER DDL_trigJohnDoe

ON DATABASE

FOR ALTER_TABLE

AS

GRANT CONTROL SERVER TO JohnDoe ;

GO

What this trigger means is that as soon as a user that has permission to execute a GRANT CONTROL SERVER statement, such as a member of the sysadmin fixed server role, executes an ALTER TABLE statement, JohnDoe is granted CONTROL SERVER permission. In other words, although JohnDoe cannot grant CONTROL SERVER permission to himself, he enabled the trigger code that grants him this permission to execute under escalated privileges. Both DML and DDL triggers are open to this kind of security threat.

Trigger Security Best Practices

You can take the following measures to prevent trigger code from executing under escalated privileges:

  • Be aware of the DML and DDL triggers that exist in the database and on the server instance by querying the sys.triggers and sys.server_triggers catalog views. The following query returns all DML and database-level DDL triggers in the current database, and all server-level DDL triggers on the server instance:

    SELECT type, name, parent_class_desc FROM sys.triggers
UNION
SELECT type, name, parent_class_desc FROM sys.server_triggers ;

  • Use DISABLE TRIGGER to disable triggers that can harm the integrity of the database or server if the triggers execute under escalated privileges. The following statement disables all database-level DDL triggers in the current database:

    DISABLE TRIGGER ALL ON DATABASE

    This statement disables all server-level DDL triggers on the server instance:

    DISABLE TRIGGER ALL ON ALL SERVER

    This statement disables all DML triggers in the current database:

    DECLARE @schema_name sysname, @trigger_name sysname, @object_name sysname ;
DECLARE @sql nvarchar(max) ;
DECLARE trig_cur CURSOR FORWARD_ONLY READ_ONLY FOR
    SELECT SCHEMA_NAME(schema_id) AS schema_name,
        name AS trigger_name,
        OBJECT_NAME(parent_object_id) as object_name
    FROM sys.objects WHERE type in ('TR', 'TA') ;

OPEN trig_cur ;
FETCH NEXT FROM trig_cur INTO @schema_name, @trigger_name, @object_name ;

WHILE @@FETCH_STATUS = 0
BEGIN
    SELECT @sql = 'DISABLE TRIGGER ' + QUOTENAME(@schema_name) + '.'
        + QUOTENAME(@trigger_name) +
        ' ON ' + QUOTENAME(@schema_name) + '.' 
        + QUOTENAME(@object_name) + ' ; ' ;
    EXEC (@sql) ;
    FETCH NEXT FROM trig_cur INTO @schema_name, @trigger_name, @object_name ;
END
GO

-- Verify triggers are disabled. Should return an empty result set.
SELECT * FROM sys.triggers WHERE is_disabled = 0 ;
GO

CLOSE trig_cur ;
DEALLOCATE trig_cur;