Join Alexander Chigrik for an in-depth refresher on managing SQL Server user permissons.
Introduction
Permissions are the rights to
access the database objects. Permissions can be granted to a user or role to
allow that user or role to perform operations such as selection, insertion or
modification of data rows.
Each database object has an owner. By default, the owner is the creator of an
object, but the ownership can be transferred later after the object has been
created. In addition to the owner, the members of the sysadmin fixed server
roles have full permissions on all objects in all user and system databases.
There is also a public role. The public role is a special database role to
which each database user belongs. The public role contains default access
permissions for any user who can access the database. This database role cannot
be dropped, but it is strongly recommended not to grant superfluous permissions
to the public role, because each databases user has the public role's
permissions.
SQL Server 2000 provides the GRANT, DENY, and REVOKE statements to give or take
away permissions from a user or role.
Permissions Types
To perform any activity in a
database, user must have the appropriate permissions. These permissions fall
into three categories, which we call permissions types:
-
Permissions to work with data and execute procedures (object
permissions).
-
Permissions to create a database or an item in the database
(statement permissions).
-
Permissions to utilize permissions granted to predefined roles
(implied permissions).
SQL Server 2000 supports granting or revoking user rights to the following
permissions types:
Object Permissions
The object permissions are the
permissions to act on the database objects (such as tables, stored procedures
and views). They consist of the following permissions:
-
SELECT
Enables a user to select or read data from a table or view. The SELECT
permission can be applied to individual columns within a table or view, and may
be applied to user-defined functions.
-
INSERT
Enables a user to insert new data to a table or view.
DELETE
Enables a user to delete data from a table or view.
UPDATE
Enables a user to update data in a table or view. The UPDATE permission can be
applied to individual columns within a table or view, not just the entire
table.
EXECUTE
Enables a user to execute a stored procedure.
DRI (declarative referential integrity)
Enables a user to add foreign key constraints on a table.
Statement Permissions
These are the permissions to create a database or an object
in the database. These permissions are applied to the statement itself, rather
than to a specific object defined in the database. They consist of the
following permissions:
BACKUP DATABASE
The BACKUP DATABASE statement is used to back up an entire database or one or
more files or filegroups.
BACKUP LOG
The BACKUP LOG statement is used to back up the transaction log.
CREATE DATABASE
The CREATE DATABASE statement is used to create a new database and the files
used to store the database.
CREATE DEFAULT
The CREATE DEFAULT statement is used to create an object called a default.
CREATE FUNCTION
The CREATE FUNCTION statement is used to create a user-defined function, which
is a saved Transact-SQL routine that returns a value.
CREATE PROCEDURE
The CREATE PROCEDURE statement is used to create a stored procedure, which is a
saved collection of Transact-SQL statements.
CREATE RULE
The CREATE RULE statement is used to create an object called a rule.
CREATE TABLE
The CREATE TABLE statement is used to create a new table.
CREATE VIEW
The CREATE VIEW statement is used to create an object called a view.
Implied Permissions
These are the permissions
granted to the predefined roles (such as fixed server roles or fixed database
roles). For example, a member of the db_owner fixed database role has full permissions
in the database.
Managing Permissions
You can use the GRANT, DENY,
and REVOKE statements to give or take away permission from a user or role.
The GRANT statement is used to give permissions to a user or role. By using the
GRANT statement, it is possible to assign permissions to both statements as
well as objects. You can use the GRANT statement with the WITH GRANT OPTION
clause to permit the user or role receiving the permission to further
grant/revoke access to other accounts.
This example grants the SELECT permission on the authors table to Alex
GRANT SELECT ON authors TO Alex
The DENY statement is used to deny a permission from a security account in the
current database and prevents the security account from inheriting the
permission through its group or role memberships. You can use the DENY
statement to deny both statements and objects permissions.
The following example denies the user Alex SELECT permissions to the authors
table:
DENY SELECT ON authors TO Alex
The REVOKE statement is used to remove a previously granted or denied
permission from a user in the current database. You can use the REVOKE
statement to remove both statements and objects permissions. You can specify
the GRANT OPTION FOR clause with the REVOKE statement to remove the WITH GRANT
OPTION permissions. Therefore, the user will have the objects permissions, but
cannot grant the permissions to other users. Specify the CASCADE clause along
with the WITH GRANT OPTION clause, if the permissions being revoked were originally
granted using the WITH GRANT OPTION setting.
The following example revokes SELECT permissions to the authors table from the
user, Alex:
REVOKE SELECT ON authors TO Alex
Use the WITH GRANT OPTION setting very carefully, because in this case users can
grant permissions to the objects to other users and it will be more difficult
to manage security.
Do not grant the superfluous permissions to the public role, because each
database user has the public role's permissions.
Enhancing Security Using Views
Views can be used to enhance
security because permissions to access a view can be granted, denied, or
revoked, regardless of the set of permissions to access the underlying
table(s).
Views can also be used to limit the data that is available to a user. For
example, one peace of data can be accessible to users for modifications while
another piece of data can be accessible to users for query, and the rest of the
data is invisible and inaccessible.
Views can be used to restrict access to the following subsets of data:
-
A subset of the rows of a base table
-
A subset of the columns of a base table
-
A subset of combination of rows and columns of a base table
-
A subset of another view or some combination of views and tables
-
A statistical summary of data in a base table
For example, consider a scenario where you need to manage permissions to the
employee table for the Accounting, Sales, and the Tech Windows 2000 groups, and
each group has their corresponding SQL Server roles namely: the Accounting,
Sales, and Tech respectively. Let's say the employee table was created using
the following CREATE TABLE statement:
CREATE TABLE employee(
EmployeeId INT NOT NULL PRIMARY KEY,
LName VARCHAR(30) NOT NULL,
FName VARCHAR(30) NOT NULL,
Address VARCHAR(100) NOT NULL,
HireDate DATETIME NOT NULL,
Salary MONEY NOT NULL
) |
Suppose you need to assign the
Accounting role access to query the entire employee table, the Sales role to
access only the LName, FName, and HireDate columns of the employee table, and
the Tech role should not have any access either to the view or to the employee
table itself. This can be achieved using the following script:
-- grant select permission on the employee table to the Accounting role
GRANT SELECT ON employee TO Accounting
GO
-- deny select permission on the employee table from the Tech and Sales role
-- to ensure these roles do not have select permission on the employee table
DENY SELECT ON employee TO Sales, Tech
GO
-- create the view that limits access the employee table to the Sales role
CREATE VIEW employee_view
AS
SELECT LName, FName, HireDate
FROM employee
GO
-- grant select on the employee_view to the Sales group
GRANT SELECT ON employee_view TO Sales
GO
|
Using Stored Procedures to Enhance Security
Stored procedures can be used
to enhance security in much the same way as views. The permissions to execute a
stored procedure can be granted, denied, or revoked instead of granting
permissions on the underlying data objects. Stored procedures can be used to
conceal the underlying data objects too. For example, you can give a user only
the permission to execute a stored procedure and the user will not know
anything about underlying data objects. By using stored procedures, you can
also limit the data that is available to a user. You can give the users only
permission to execute a stored procedure to work with the restricted set of the
columns and rows instead of querying the entire table.
For example, the Salary and the Address columns in the employee table contain
confidential employee information and should be available only to the members
of the Accounting database role, but the rest of the columns contain
information that should be available to all database users.
This script shows how the above security task can be achieved:
-- grant select permission on the employee table to the Accounting
-- role
GRANT SELECT ON employee TO Accounting
GO
-- create the stored procedure that hides
-- Salary and the Address columns in the employee table
CREATE PROCEDURE employee_proc
AS
SELECT LName, FName, HireDate
FROM employee
GO
-- grant select permission on the employee_proc to the public role
GRANT EXECUTE ON employee_proc TO public
GO
|
Because each database user has
the public role permission, you can grant the desirable permission to the
public role if you need to grant this permission to all database users. If the
business logic of your application allows some users to update values in several
columns in the table without having permission to update anything else on the
table, you can also use a stored procedure.
For example, any members of the Accounting database role are allowed to update
the Salary column in the employee table without having permission to update
other columns. The employee table was created using the following CREATE TABLE
statement:
The following script shows how the above security task can be achieved:
-- deny UPDATE, DELETE and INSERT permissions
-- on the employee table to the Accounting role
-- to ensure this role does not have these permissions
DENY UPDATE, DELETE, INSERT ON employee TO Accounting
GO
-- create the stored procedure that modify the Salary
-- column on the employee table for the employee passed
CREATE PROCEDURE employee_proc_upd
@EmployeeID int,
@Salary money
AS
UPDATE employee
SET Salary = @Salary
WHERE EmployeeID = @EmployeeID
GO
-- grant execute on the employee_proc_upd to the Accounting role
GRANT EXECUTE ON employee_proc_upd TO Accounting
GO
|
Permissions Intersection
Under the permissions
intersection we understand the results permission, which a user will have when different
permissions were granted or revoked to this user or (and) for the roles to
which this user belong. A permissions conflict can arise, if the user is a member
of several roles with different permissions to access an object.
As we described above, permissions can be granted, revoked, or denied.
The GRANT statement removes the denied or revoked permission at the level
granted, so the denied permission at another level still applies. For example,
if you need to allow the user Alex to select the employee table and the select
permissions were denied to both user Alex and Accounting role to which Alex
belongs, you can run the following statement:
GRANT SELECT ON employee TO Alex, Accounting
A denied permission overrides all other permissions and always takes
precedence. For example, a user belongs to two roles: Accounting and
Technology. The Accounting role gives the user select, insert, delete, and update
permissions. The Technology role provides select but denies permissions for
insert, delete, and update. The result is the user only can select regardless
that the user belongs to the Accounting role. You can deny permissions to the
public role, if you need to prevent anyone from accessing an object. The
results of using the DENY statement against a database object looks like the
results of granting the 'No Access' permission to the Windows NT user account.
The REVOKE statement is used to remove a previously granted or denied
permission at the level revoked, so the same permission granted or denied at
another level still applies. For example, if you need to prevent the user Alex from
selecting the employee table and the select permissions were granted to both
user Alex and Accounting role to which Alex belongs, you can run one of the
following statements:
REVOKE SELECT ON employee FROM Alex, Accounting
DENY SELECT ON employee TO Alex
DENY SELECT ON employee TO Accounting
|
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