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Metadata Visibility Configuration

  • Artykuł

In earlier versions of SQL Server, metadata for all objects in a database is visible to members of the public role. This means that any user that is logged on to an instance of SQL Server can view metadata for every object in the server, even those objects on which a user has no rights.

In SQL Server 2005, the visibility of metadata is limited to securables that a user either owns or on which the user has been granted some permission. For example, the following query returns a row if the user has been granted a permission such as SELECT or INSERT on the table myTable.

SELECT name, object_id
FROM sys.tables
WHERE name = 'myTable';
GO

However, if the user does not have any permission on myTable, the query returns an empty result set.

Scope and Impact of Metadata Visibility Configuration

Metadata visibility configuration only applies to the following securables.

Catalog views

SQL Server 2005 Database Engine sp_help stored procedures

Metadata exposing built-in functions

Information schema views

Compatibility views

Extended properties

Metadata visibility configuration does not apply to the following securables.

Log shipping system tables

SQL Server Agent system tables

Database maintenance plan system tables

Backup system tables

Replication system tables

Replication and SQL Server Agent sp_help stored procedures

Limited metadata accessibility means the following:

  • Applications that assume public metadata access will break.
  • Queries on system views might only return a subset of rows, or sometimes an empty result set.
  • Metadata-emitting, built-in functions such as OBJECTPROPERTYEX may return NULL.
  • The Database Engine sp_help stored procedures might return only a subset of rows, or NULL.

SQL modules, such as stored procedures and triggers, run under the security context of the caller and, therefore, have limited metadata accessibility. For example, in the following code, when the stored procedure tries to access metadata for the table myTable on which the caller has no rights, an empty result set is returned. In earlier releases of SQL Server, a row is returned.

CREATE PROCEDURE assumes_caller_can_access_metadata
BEGIN
SELECT name, id 
FROM sysobjects 
WHERE name = 'myTable';
END;
GO

To allow callers to view metadata, you can grant the callers VIEW DEFINITION permission at an appropriate scope: object level, database level or server level. Therefore, in the previous example, if the caller has VIEW DEFINITION permission on myTable, the stored procedure returns a row. For more information, see GRANT (Transact-SQL) and GRANT Database Permissions (Transact-SQL).

You can also modify the stored procedure so that it executes under the credentials of the owner. When the procedure owner and the table owner are the same owner, ownership chaining applies, and the security context of the procedure owner enables access to the metadata for myTable. Under this scenario, the following code returns a row of metadata to the caller.

Note

The following example uses the sys.objects catalog view instead of the sys.sysobjects compatibility view.

CREATE PROCEDURE does_not_assume_caller_can_access_metadata
WITH EXECUTE AS OWNER
AS
BEGIN
SELECT name, id
FROM sys.objects 
WHERE name = 'myTable' 
END;
GO

Note

You can use EXECUTE AS to temporarily switch to the security context of the caller. For more information, see EXECUTE AS (Transact-SQL).

For more information about ownership chaining, see Ownership Chains.

Benefits and Limits of Metadata Visibility Configuration

Metadata visibility configuration can play an important role in your overall security plan. However, there are cases in which a skilled and determined user can force the disclosure of some metadata. We recommend that you deploy metadata permissions as one of many defenses-in-depth.

It is theoretically possible to force the emission of metadata in error messages by manipulating the order of predicate evaluation in queries. The possibility of such trial-and-error attacks is not specific to SQL Server. It is implied by the associative and commutative transformations permitted in relational algebra. You can mitigate this risk by limiting the information returned in error messages. To further restrict the visibility of metadata in this way, you can start the server with trace flag 3625. This trace flag limits the amount of information shown in error messages. In turn, this helps to prevent forced disclosures. The tradeoff is that error messages will be terse and might be difficult to use for debugging purposes. For more information, see Using the SQL Server Service Startup Options and Trace Flags (Transact-SQL).

The following metadata is not subject to forced disclosure:

  • The value stored in the provider_string column of sys.servers. A user that does not have ALTER ANY LINKED SERVER permission will see a NULL value in this column.

  • Source definition of a user-defined object such as a stored procedure or trigger. The source code is visible only when one of the following is true:

    • The user has VIEW DEFINITION permission on the object.
    • The user has not been denied VIEW DEFINITION permission on the object and has CONTROL, ALTER, or TAKE OWNERSHIP permission on the object. All other users will see NULL.

  • The definition columns found in the following catalog views:

    sys.all_sql_modules

    sys.sql_modules

    sys.server_sql_modules

    sys.check_constraints

    sys.default_constraints

    sys.computed_columns

    sys.numbered_procedures

  • The ctext column in the syscomments compatibility view.

  • The output of the sp_helptext procedure.

  • The following columns in the information schema views:

    INFORMATION_SCHEMA.CHECK_CONSTRAINTS.CHECK_CLAUSE

    INFORMATION_SCHEMA.COLUMNS.COLUMN_DEFAULT

    INFORMATION_SCHEMA.DOMAINS.DOMAIN_DEFAULT

    INFORMATION_SCHEMA.ROUTINE_COLUMNS.COLUMN_DEFAULT

    INFORMATION_SCHEMA.ROUTINES.ROUTINE_DEFINITION

    INFORMATION_SCHEMA.VIEWS.VIEW_DEFINITION

  • OBJECT_DEFINITION() function

  • The value stored in the password_hash column of sys.sql_logins. A user that does not have CONTROL SERVER permission will see a NULL value in this column.

Note

The SQL definitions of built-in system procedures and functions are publicly visible through the sys.system_sql_modules catalog view, the sp_helptext stored procedure, and the OBJECT_DEFINITION() function.

General Principles of Metadata Visibility

The following are some general principles to consider regarding metadata visibility:

  • Fixed roles implicit permissions
  • Scope of permissions
  • Precedence of DENY
  • Visibility of subcomponent metadata

Fixed Roles and Implicit Permissions

In SQL Server 2005, the metadata that can be accessed by fixed roles depends upon their corresponding implicit permissions.

Scope of Permissions

Permissions at one scope imply the ability to see metadata at that scope and at all enclosed scopes. For example, SELECT permission on a schema implies that the grantee has SELECT permission on all securables that are contained by that schema. The granting of SELECT permission on a schema therefore enables a user to see the metadata of the schema and also all tables, views, functions, procedures, queues, synonyms, types, and XML schema collections within it. For more information about scopes, see Permissions Hierarchy.

Precedence of DENY

DENY typically takes precedence over other permissions. For example, if a database user is granted EXECUTE permission on a schema but has been denied EXECUTE permission on a stored procedure in that schema, the user cannot view the metadata for that stored procedure.

Additionally, if a user is denied EXECUTE permission on a schema but has been granted EXECUTE permission on a stored procedure in that schema, the user cannot view the metadata for that stored procedure.

For another example, if a user has been granted and denied EXECUTE permission on a stored procedure, which is possible through your various role memberships, DENY takes precedence and the user cannot view the metadata of the stored procedure.

Visibility of Subcomponent Metadata

The visibility of subcomponents, such as indexes, check constraints, and triggers is determined by permissions on the parent. These subcomponents do not have grantable permissions. For example, if a user has been granted some permission on a table, the user can view the metadata for the tables, columns, indexes, check constraints, triggers, and other such subcomponents.

Metadata That Is Accessible to All Database Users

Some metadata must be accessible to all users in a specific database. For example, filegroups do not have conferrable permissions; therefore, a user cannot be granted permission to view the metadata of a filegroup. However, any user that can create a table must be able to access filegroup metadata to use the ON filegroup or TEXTIMAGE_ON filegroup clauses of the CREATE TABLE statement.

The metadata that is returned by the DB_ID() and DB_NAME() functions is visible to all users.

The following table lists the catalog views that are visible to the public role.

sys.partition_functions

sys.partition_range_values

sys.partition_schemes

sys.data_spaces

sys.filegroups

sys.destination_data_spaces

sys.database_files

sys.allocation_units

sys.partitions

sys.messages

sys.schemas

sys.configurations

sys.sql_dependencies

sys.type_assembly_usages

sys.parameter_type_usages

sys.column_type_usages

See Also

Tasks

Troubleshooting Metadata Visibility
Troubleshooting Metadata Visibility of Distributed Partitioned Views

Concepts

Permissions of Fixed Database Roles
VIEW ANY DATABASE Permission
VIEW DEFINITION Permission
Viewing Object Definitions

Other Resources

GRANT (Transact-SQL)
DENY (Transact-SQL)
REVOKE (Transact-SQL)
EXECUTE AS Clause (Transact-SQL)
Catalog Views (Transact-SQL)
Compatibility Views (Transact-SQL)

Help and Information

Getting SQL Server 2005 Assistance