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Understanding SQL Server Audit

Auditing an instance of SQL Server or a SQL Server database involves tracking and logging events that occur on the system. You can use several methods of auditing for SQL Server, as described in Auditing (Database Engine). Beginning in SQL Server 2008 Enterprise, you can also set up automatic auditing by using SQL Server Audit.

There are several levels of auditing for SQL Server, depending on government or standards requirements for your installation. SQL Server Audit provides the tools and processes you must have to enable, store, and view audits on various server and database objects.

You can record server audit action groups per-instance, and either database audit action groups or database audit actions per database. The audit event will occur every time that the auditable action is encountered.

An audit is the combination of several elements into a single package for a specific group of server actions or database actions. The components of SQL Server Audit combine to produce an output that is called an audit, just as a report definition combined with graphics and data elements produces a report.

SQL Server Audit uses Extended Events to help create an audit. For more information about Extended Events, see Introducing SQL Server Extended Events.

SQL Server Audit

The SQL Server Audit object collects a single instance of server or database-level actions and groups of actions to monitor. The audit is at the SQL Server instance level. You can have multiple audits per SQL Server instance.

When you define an audit, you specify the location for the output of the results. This is the audit destination. The audit is created in a disabled state, and does not automatically audit any actions. After the audit is enabled, the audit destination receives data from the audit.

Server Audit Specification

The Server Audit Specification object belongs to an audit. You can create one server audit specification per audit, because both are created at the SQL Server instance scope.

The server audit specification collects many server-level action groups raised by the Extended Events feature. You can include audit action groups in a server audit specification. Audit action groups are predefined groups of actions, which are atomic events occurring in the Database Engine. These actions are sent to the audit, which records them in the target.

Server-level audit action groups are described in the topic SQL Server Audit Action Groups and Actions.

Database Audit Specification

The Database Audit Specification object also belongs to a SQL Server audit. You can create one database audit specification per SQL Server database per audit.

The database audit specification collects database-level audit actions raised by the Extended Events feature. You can add either audit action groups or audit events to a database audit specification. Audit events are the atomic actions that can be audited by the SQL Server engine. Audit action groups are predefined groups of actions. Both are at the SQL Server database scope. These actions are sent to the audit, which records them in the target. Do not include server-scoped objects, such as the system views, in a user database audit specification.

Database-level audit action groups and audit actions are described in the topic SQL Server Audit Action Groups and Actions.

Target

The results of an audit are sent to a target, which can be a file, the Windows Security event log, or the Windows Application event log. (Writing to the Security log is not available on Windows XP.) Logs must be reviewed and archived periodically to make sure that the target has sufficient space to write additional records.

Important noteImportant

Any authenticated user can read and write to the Windows Application event log. The Application event log requires lower permissions than the Windows Security event log and is less secure than the Windows Security event log.

Writing to the Windows Security log requires the SQL Server service account to be added to the Generate security audits policy. By default, the Local System, Local Service, and Network Service are part of this policy. This setting can be configured by using the security policy snap-in (secpol.msc). Additionally, the Audit object access security policy must be enabled for both Success and Failure. This setting can be configured by using the security policy snap-in (secpol.msc). In Windows Vista or Windows Server 2008, you can set the more granular application generated policy from the command line by using the audit policy program (AuditPol.exe). For more information about the steps to enable writing to the Windows Security log, see How to: Write Server Audit Events to the Security Log. For more information about the Auditpol.exe program, see Knowledge Base article 921469, How to use Group Policy to configure detailed security auditing. The Windows event logs are global to the Windows operating system. For more information about the Windows event logs, see Event Viewer Overview. If you need more precise permissions on the audit, use the binary file target.

When you are saving audit information to a file, to help prevent tampering, you can restrict access to the file location in the following ways:

  • The SQL Server Service Account must have both Read and Write permission.

  • Audit Administrators typically require Read and Write permission. This assumes that the Audit Administrators are Windows accounts for administration of audit files, such as: copying them to different shares, backing them up, and so on.

  • Audit Readers that are authorized to read audit files must have Read permission.

Even when the is writing to a file, other Windows users can read the audit file if they have permission. The Database Engine does not take an exclusive lock that prevents read operations.

Because the Database Engine can access the file, SQL Server logins that have CONTROL SERVER permission can use the Database Engine to access the audit files. To record any user that is reading the audit file, define an audit on master.sys.fn_get_audit_file. This records the logins with CONTROL SERVER permission that have accessed the audit file through SQL Server.

If an Audit Administrator copies the file to a different location (for archive purposes, and so on), the ACLs on the new location should be reduced to the following permissions:

  • Audit Administrator – Read / Write

  • Audit Reader – Read

We recommend that you generate audit reports from a separate instance of SQL Server, such as an instance of SQL Server Express, to which only Audit Administrators or Audit Readers have access. By using a separate instance of the Database Engine for reporting, you can help prevent unauthorized users from obtaining access to the audit record.

You can offer additional protection against unauthorized access by encrypting the folder in which the audit file is stored by using Windows BitLocker Drive Encryption or Windows Encrypting File System.

For more information about the audit records that are written to the target, see SQL Server Audit Records.

You can use SQL Server Management Studio or Transact-SQL to define an audit. After the audit is created and enabled, the target will receive entries.

You can read the Windows event logs by using the Event Viewer utility in Windows. For file targets, you can use either the Log File Viewer in SQL Server Management Studio or the fn_get_audit_file function to read the target file.

The general process for creating and using an audit is as follows.

  1. Create an audit and define the target.

  2. Create either a server audit specification or database audit specification that maps to the audit. Enable the audit specification.

  3. Enable the audit.

  4. Read the audit events by using the Windows Event Viewer, Log File Viewer, or the fn_get_audit_file function.

The SQL Server Audit How-to Topics topic provides SQL Server Management Studio and Transact-SQL examples for using the auditing feature.

In the case of a failure during audit initiation, the server will not start. In this case, the server can be started by using the –f option at the command line.

When an audit failure causes the server to shut down or not to start because ON_FAILURE=SHUTDOWN is specified for the audit, the MSG_AUDIT_FORCED_SHUTDOWN event will be written to the log. Because the shutdown will occur on the first encounter of this setting, the event will be written one time. This event is written after the failure message for the audit causing the shutdown. An administrator can bypass audit-induced shutdowns by starting SQL Server in Single User mode using the –m flag. If you start in Single User mode, you will downgrade any audit where ON_FAILURE=SHUTDOWN is specified to run in that session as ON_FAILURE=CONTINUE. When SQL Server is started by using the –m flag, the MSG_AUDIT_SHUTDOWN_BYPASSED message will be written to the error log.

For more information about service startup options, see Using the SQL Server Service Startup Options.

Attaching a Database with an Audit Defined

Attaching a database that has an audit specification and specifies a GUID that does not exist on the server will cause an orphaned audit specification. Because an audit with a matching GUID does not exist on the server instance, no audit events will be recorded. To correct this situation, use the ALTER DATABASE AUDIT SPECIFICATION command to connect the orphaned audit specification to an existing server audit. Or, use the CREATE SERVER AUDIT command to create a new server audit with the specified GUID.

You can attach a database that has an audit specification defined on it to another edition of SQL Server that does not support SQL Server Audit, such as SQL Server Express but it will not record audit events.

Database Mirroring and SQL Server Audit

A database that has a database audit specification defined and that uses database mirroring will include the database audit specification. To work correctly on the mirrored SQL instance, the following items must be configured:

  • The mirror server must have an audit with the same GUID to enable the database audit specification to write audit records. This can be configured by using the command CREATE AUDIT WITH GUID=<GUID from source Server Audit>.

  • For binary file targets, the mirror server service account must have appropriate permissions to the location where the audit trail is being written.

  • For Windows event log targets, the security policy on the computer where the mirror server is located must allow for service account access to the security or application event log.

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