Stored programs and views are defined prior to use and, when
referenced, execute within a security context that determines
their privileges. These privileges are controlled by their
DEFINER attribute, and, if there is one, their
SQL SECURITY characteristic.
All stored programs (procedures, functions, triggers, and events)
and views can have a DEFINER attribute that
names a MySQL account. If the DEFINER attribute
is omitted from a stored program or view definition, the default
account is the user who creates the object.
In addition, stored routines (procedures and functions) and views
can have a SQL SECURITY characteristic with a
value of DEFINER or INVOKER
to specify whether the object executes in definer or invoker
context. If the SQL SECURITY characteristic is
omitted, 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 program or view that executes in definer security context executes with the privileges of the account named by its
DEFINERattribute. These privileges may be entirely different from those of the invoking user. The invoker must have appropriate privileges to reference the object (for example,EXECUTEto call a stored procedure orSELECTto select from a view), but when the object executes, the invoker's privileges are ignored and only theDEFINERaccount privileges matter. If this account has few privileges, the object is correspondingly limited in the operations it can perform. If theDEFINERaccount is highly privileged (such as arootaccount), 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
DEFINERattribute can be specified but has no effect for objects that execute in invoker context.
Consider the following stored procedure:
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 in the DEFINER attribute. This
account must have the EXECUTE
privilege for p1 as well as the
UPDATE privilege for the table
t1. 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;
p2, unlike p1, executes in
INVOKER security context. The
DEFINER attribute is irrelevant and
p2 executes with the privileges of the invoking
user. p2 fails if the invoker lacks the
EXECUTE privilege for
p2 or the UPDATE
privilege for the table t1.
MySQL uses the following rules to control which accounts a user
can specify in an object DEFINER attribute:
You can specify a
DEFINERvalue other than your own account only if you have theSUPERprivilege.If you do not have the
SUPERprivilege, the only legal user value is your own account, either specified literally or by usingCURRENT_USER. You cannot set the definer to some other account.
To minimize the risk potential for stored program and view creation and use, follow these guidelines:
For a stored routine or view, use
SQL SECURITY INVOKERin 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 programs or views while using an account that has the
SUPERprivilege, specify an explicitDEFINERattribute that names an account possessing only the privileges required for the operations performed by the object. Specify a highly privilegedDEFINERaccount only when absolutely necessary.Administrators can prevent users from specifying highly privileged
DEFINERaccounts by not granting them theSUPERprivilege.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 reference to these objects by not granting unauthorized users particular privileges:
However, no such control exists for triggers because users do not reference them directly. A trigger always executes in
DEFINERcontext and is activated by access to the table with which it is associated, even ordinary table accesses by users with no special privileges. If theDEFINERaccount is highly privileged, the trigger can perform sensitive or dangerous operations. This remains true if theSUPERandTRIGGERprivileges needed to create the trigger are revoked from the account of the user who created it. Administrators should be especially careful about granting users that combination of privileges.