23.6 Stored Object Access Control

Stored programs (procedures, functions, triggers, and events) and views are defined prior to use and, when referenced, execute within a security context that determines their privileges. The privileges applicable to execution of a stored object are controlled by its DEFINER attribute and SQL SECURITY characteristic.

• The DEFINER Attribute

• The SQL SECURITY Characteristic

• Examples

• Orphan Stored Objects

• Risk-Minimization Guidelines

The DEFINER Attribute

A stored object definition can include a DEFINER attribute that names a MySQL account. If a definition omits the DEFINER attribute, the default object definer is the user who creates it.

The following rules determine which accounts you can specify as the DEFINER attribute for a stored object:

• If you have the SUPER privilege, you can specify any account as the DEFINER attribute. If the account does not exist, a warning is generated.

• Otherwise, the only permitted account is your own, specified either literally or as CURRENT_USER or CURRENT_USER(). You cannot set the definer to any other account.

Creating a stored object with a nonexistent DEFINER account creates an orphan object, which may have negative consequences; see Orphan Stored Objects.

The SQL SECURITY Characteristic

For stored routines (procedures and functions) and views, the object definition can include an SQL SECURITY characteristic with a value of DEFINER or INVOKER to specify whether the object executes in definer or invoker context. If the definition omits the SQL SECURITY characteristic, the default is definer context.

Triggers and events have no SQL SECURITY characteristic and always execute in definer context. The server invokes these objects automatically as necessary, so there is no invoking user.

Definer and invoker security contexts differ as follows:

• A stored object that executes in definer security context executes with the privileges of the account named by its DEFINER attribute. These privileges may be entirely different from those of the invoking user. The invoker must have appropriate privileges to reference the object (for example, EXECUTE to call a stored procedure or SELECT to select from a view), but during object execution, the invoker's privileges are ignored and only the DEFINER account privileges matter. If the DEFINER account has few privileges, the object is correspondingly limited in the operations it can perform. If the DEFINER account is highly privileged (such as an administrative account), the object can perform powerful operations no matter who invokes it.

• A stored routine or view that executes in invoker security context can perform only operations for which the invoker has privileges. The DEFINER attribute has no effect on object execution.

Examples

Consider the following stored procedure, which is declared with SQL SECURITY DEFINER to execute in definer security context:

CREATE DEFINER = 'admin'@'localhost' PROCEDURE p1()
SQL SECURITY DEFINER
BEGIN
  UPDATE t1 SET counter = counter + 1;
END;

Any user who has the EXECUTE privilege for p1 can invoke it with a CALL statement. However, when p1 executes, it does so in definer security context and thus executes with the privileges of 'admin'@'localhost', the account named as its DEFINER attribute. This account must have the EXECUTE privilege for p1 as well as the UPDATE privilege for the table t1 referenced within the object body. Otherwise, the procedure fails.

Now consider this stored procedure, which is identical to p1 except that its SQL SECURITY characteristic is INVOKER:

CREATE DEFINER = 'admin'@'localhost' PROCEDURE p2()
SQL SECURITY INVOKER
BEGIN
  UPDATE t1 SET counter = counter + 1;
END;

Unlike p1, p2 executes in invoker security context and thus with the privileges of the invoking user regardless of the DEFINER attribute value. p2 fails if the invoker lacks the EXECUTE privilege for p2 or the UPDATE privilege for the table t1.

Orphan Stored Objects

An orphan stored object is one for which its DEFINER attribute names a nonexistent account:

• An orphan stored object can be created by specifying a nonexistent DEFINER account at object-creation time.

• An existing stored object can become orphaned through execution of a DROP USER statement that drops the object DEFINER account, or a RENAME USER statement that renames the object DEFINER account.

An orphan stored object may be problematic in these ways:

• Because the DEFINER account does not exist, the object may not work as expected if it executes in definer security context:

  • For a stored routine, an error occurs at routine execution time if the SQL SECURITY value is DEFINER but the definer account does not exist.

  • For a trigger, it is not a good idea for trigger activation to occur until the account actually does exist. Otherwise, the behavior with respect to privilege checking is undefined.

  • For an event, an error occurs at event execution time if the account does not exist.

  • For a view, an error occurs when the view is referenced if the SQL SECURITY value is DEFINER but the definer account does not exist.

• The object may present a security risk if the nonexistent DEFINER account is subsequently re-created for a purpose unrelated to the object. In this case, the account “adopts” the object and, with the appropriate privileges, is able to execute it even if that is not intended.

To obtain information about the accounts used as stored object definers in a MySQL installation, query the INFORMATION_SCHEMA.

This query identifies which INFORMATION_SCHEMA tables describe objects that have a DEFINER attribute:

mysql> SELECT TABLE_SCHEMA, TABLE_NAME FROM INFORMATION_SCHEMA.COLUMNS
       WHERE COLUMN_NAME = 'DEFINER';
+--------------------+------------+
| TABLE_SCHEMA       | TABLE_NAME |
+--------------------+------------+
| information_schema | EVENTS     |
| information_schema | ROUTINES   |
| information_schema | TRIGGERS   |
| information_schema | VIEWS      |
+--------------------+------------+

The result tells you which tables to query to discover which stored object DEFINER values exist and which objects have a particular DEFINER value:

• To identify which DEFINER values exist in each table, use these queries:

  SELECT DISTINCT DEFINER FROM INFORMATION_SCHEMA.EVENTS;
  SELECT DISTINCT DEFINER FROM INFORMATION_SCHEMA.ROUTINES;
  SELECT DISTINCT DEFINER FROM INFORMATION_SCHEMA.TRIGGERS;
  SELECT DISTINCT DEFINER FROM INFORMATION_SCHEMA.VIEWS;

  The query results are significant for any account displayed as follows:

  • If the account exists, dropping or renaming it causes stored objects to become orphaned. If you plan to drop or rename the account, consider first dropping its associated stored objects or redefining them to have a different definer.

  • If the account does not exist, creating it causes it to adopt currently orphaned stored objects. If you plan to create the account, consider whether the orphaned objects should be associated with it. If not, redefine them to have a different definer.

  To redefine an object with a different definer, you can use ALTER EVENT or ALTER VIEW to directly modify the DEFINER account of events and views. For stored procedures and functions and for triggers, you must drop the object and re-create it with a different DEFINER account

• To identify which objects have a given DEFINER account, use these queries, substituting the account of interest for user_name@host_name:

  SELECT EVENT_SCHEMA, EVENT_NAME FROM INFORMATION_SCHEMA.EVENTS
  WHERE DEFINER = 'user_name@host_name';
  SELECT ROUTINE_SCHEMA, ROUTINE_NAME, ROUTINE_TYPE
  FROM INFORMATION_SCHEMA.ROUTINES
  WHERE DEFINER = 'user_name@host_name';
  SELECT TRIGGER_SCHEMA, TRIGGER_NAME FROM INFORMATION_SCHEMA.TRIGGERS
  WHERE DEFINER = 'user_name@host_name';
  SELECT TABLE_SCHEMA, TABLE_NAME FROM INFORMATION_SCHEMA.VIEWS
  WHERE DEFINER = 'user_name@host_name';

  For the ROUTINES table, the query includes the ROUTINE_TYPE column so that output rows distinguish whether the DEFINER is for a stored procedure or stored function.

  If the account you are searching for does not exist, any objects displayed by those queries are orphan objects.

Risk-Minimization Guidelines

To minimize the risk potential for stored object creation and use, follow these guidelines:

• Do not create orphan stored objects; that is, objects for which the DEFINER attribute names a nonexistent account. Do not cause stored objects to become orphaned by dropping or renaming an account named by the DEFINER attribute of any existing object.

• For a stored routine or view, use SQL SECURITY INVOKER in the object definition when possible so that it can be used only by users with permissions appropriate for the operations performed by the object.

• If you create definer-context stored objects while using an account that has the SUPER privilege, specify an explicit DEFINER attribute that names an account possessing only the privileges required for the operations performed by the object. Specify a highly privileged DEFINER account only when absolutely necessary.

• Administrators can prevent users from creating stored objects that specify highly privileged DEFINER accounts by not granting them the SUPER privilege.

• Definer-context objects should be written keeping in mind that they may be able to access data for which the invoking user has no privileges. In some cases, you can prevent references to these objects by not granting unauthorized users particular privileges:

  • A stored routine cannot be referenced by a user who does not have the EXECUTE privilege for it.

  • A view cannot be referenced by a user who does not have the appropriate privilege for it (SELECT to select from it, INSERT to insert into it, and so forth).

  However, no such control exists for triggers and events because they always execute in definer context. The server invokes these objects automatically as necessary, and users do not reference them directly:

  • A trigger is activated by access to the table with which it is associated, even ordinary table accesses by users with no special privileges.

  • An event is executed by the server on a scheduled basis.

  In both cases, if the DEFINER account is highly privileged, the object may be able to perform sensitive or dangerous operations. This remains true if the privileges needed to create the object are revoked from the account of the user who created it. Administrators should be especially careful about granting users object-creation privileges.