MySQL :: Security in MySQL :: 4.14 Proxy Users

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Security in MySQL / Access Control and Account Management / Proxy Users

4.14 Proxy Users

The MySQL server authenticates client connections using authentication plugins. The plugin that authenticates a given connection may request that the connecting (external) user be treated as a different user for privilege-checking purposes. This enables the external user to be a proxy for the second user; that is, to assume the privileges of the second user:

The external user is a “proxy user” (a user who can impersonate or become known as another user).
The second user is a “proxied user” (a user whose identity and privileges can be assumed by a proxy user).

This section describes how the proxy user capability works. For general information about authentication plugins, see Section 4.13, “Pluggable Authentication”. For information about specific plugins, see Section 6.1, “Authentication Plugins”. For information about writing authentication plugins that support proxy users, see Implementing Proxy User Support in Authentication Plugins.

Requirements for Proxy User Support

For proxying to occur for a given authentication plugin, these conditions must be satisfied:

Proxying must be supported, either by the plugin itself, or by the MySQL server on behalf of the plugin. In the latter case, server support may need to be enabled explicitly; see Server Support for Proxy User Mapping.
The account for the external proxy user must be set up to be authenticated by the plugin. Use the CREATE USER statement to associate an account with an authentication plugin, or ALTER USER to change its plugin.
The account for the proxied user must exist and be granted the privileges to be assumed by the proxy user. Use the CREATE USER and GRANT statements for this.
Normally, the proxied user is configured so that it can be used only in proxying scenaries and not for direct logins.
The proxy user account must have the PROXY privilege for the proxied account. Use the GRANT statement for this.
For a client connecting to the proxy account to be treated as a proxy user, the authentication plugin must return a user name different from the client user name, to indicate the user name of the proxied account that defines the privileges to be assumed by the proxy user.
Alternatively, for plugins that are provided proxy mapping by the server, the proxied user is determined from the PROXY privilege held by the proxy user.

The proxy mechanism permits mapping only the external client user name to the proxied user name. There is no provision for mapping host names:

When a client connects to the server, the server determines the proper account based on the user name passed by the client program and the host from which the client connects.
If that account is a proxy account, the server attempts to determine the appropriate proxied account by finding a match for a proxied account using the user name returned by the authentication plugin and the host name of the proxy account. The host name in the proxied account is ignored.

Simple Proxy User Example

Consider the following account definitions:

-- create proxy account
CREATE USER 'employee_ext'@'localhost'
  IDENTIFIED WITH my_auth_plugin
  AS 'my_auth_string';
-- create proxied account and grant its privileges;
-- use mysql_no_login plugin to prevent direct login
CREATE USER 'employee'@'localhost'
  IDENTIFIED WITH mysql_no_login;
GRANT ALL
  ON employees.*
  TO 'employee'@'localhost';
-- grant to proxy account the
-- PROXY privilege for proxied account
GRANT PROXY
  ON 'employee'@'localhost'
  TO 'employee_ext'@'localhost';

When a client connects as employee_ext from the local host, MySQL uses the plugin named my_auth_plugin to perform authentication. Suppose that my_auth_plugin returns a user name of employee to the server, based on the content of 'my_auth_string' and perhaps by consulting some external authentication system. The name employee differs from employee_ext, so returning employee serves as a request to the server to treat the employee_ext external user, for purposes of privilege checking, as the employee local user.

In this case, employee_ext is the proxy user and employee is the proxied user.

The server verifies that proxy authentication for employee is possible for the employee_ext user by checking whether employee_ext (the proxy user) has the PROXY privilege for employee (the proxied user). If this privilege has not been granted, an error occurs. Otherwise, employee_ext assumes the privileges of employee. The server checks statements executed during the client session by employee_ext against the privileges granted to employee. In this case, employee_ext can access tables in the employees database.

The proxied account, employee, uses the mysql_no_login authentication plugin to prevent clients from using the account to log in directly. (This assumes that the plugin is installed. For instructions, see Section 6.1.10, “No-Login Pluggable Authentication”.) For alternative methods of protecting proxied accounts against direct use, see Preventing Direct Login to Proxied Accounts.

When proxying occurs, the USER() and CURRENT_USER() functions can be used to see the difference between the connecting user (the proxy user) and the account whose privileges apply during the current session (the proxied user). For the example just described, those functions return these values:

mysql> SELECT USER(), CURRENT_USER();
+------------------------+--------------------+
| USER()                 | CURRENT_USER()     |
+------------------------+--------------------+
| employee_ext@localhost | employee@localhost |
+------------------------+--------------------+

In the CREATE USER statement that creates the proxy user account, the IDENTIFIED WITH clause that names the proxy-supporting authentication plugin is optionally followed by an AS 'auth_string' clause specifying a string that the server passes to the plugin when the user connects. If present, the string provides information that helps the plugin determine how to map the proxy (external) client user name to a proxied user name. It is up to each plugin whether it requires the AS clause. If so, the format of the authentication string depends on how the plugin intends to use it. Consult the documentation for a given plugin for information about the authentication string values it accepts.

Preventing Direct Login to Proxied Accounts

Proxied accounts generally are intended to be used only by means of proxy accounts. That is, clients connect using a proxy account, then are mapped onto and assume the privileges of the appropriate proxied user.

There are multiple ways to ensure that a proxied account cannot be used directly:

Associate the account with the mysql_no_login authentication plugin. In this case, the account cannot be used for direct logins under any circumstances. This assumes that the plugin is installed. For instructions, see Section 6.1.10, “No-Login Pluggable Authentication”.
Include the ACCOUNT LOCK option when you create the account. See CREATE USER Statement. With this method, also include a password so that if the account is unlocked later, it cannot be accessed with no password. (If the validate_password plugin is enabled, it does not permit creating an account without a password, even if the account is locked. See Section 6.3, “The Password Validation Plugin”.)
Create the account with a password but do not tell anyone else the password. If you do not let anyone know the password for the account, clients cannot use it to connect directly to the MySQL server.

Granting and Revoking the PROXY Privilege

The PROXY privilege is needed to enable an external user to connect as and have the privileges of another user. To grant this privilege, use the GRANT statement. For example:

GRANT PROXY ON 'proxied_user' TO 'proxy_user';

The statement creates a row in the mysql.proxies_priv grant table.

At connect time, proxy_user must represent a valid externally authenticated MySQL user, and proxied_user must represent a valid locally authenticated user. Otherwise, the connection attempt fails.

The corresponding REVOKE syntax is:

REVOKE PROXY ON 'proxied_user' FROM 'proxy_user';

MySQL GRANT and REVOKE syntax extensions work as usual. Examples:

-- grant PROXY to multiple accounts
GRANT PROXY ON 'a' TO 'b', 'c', 'd';
-- revoke PROXY from multiple accounts
REVOKE PROXY ON 'a' FROM 'b', 'c', 'd';
-- grant PROXY to an account and enable the account to grant
-- PROXY to the proxied account
GRANT PROXY ON 'a' TO 'd' WITH GRANT OPTION;
-- grant PROXY to default proxy account
GRANT PROXY ON 'a' TO ''@'';

The PROXY privilege can be granted in these cases:

By a user that has GRANT PROXY ... WITH GRANT OPTION for proxied_user.
By proxied_user for itself: The value of USER() must exactly match CURRENT_USER() and proxied_user, for both the user name and host name parts of the account name.

The initial root account created during MySQL installation has the PROXY ... WITH GRANT OPTION privilege for ''@'', that is, for all users and all hosts. This enables root to set up proxy users, as well as to delegate to other accounts the authority to set up proxy users. For example, root can do this:

CREATE USER 'admin'@'localhost'
  IDENTIFIED BY 'admin_password';
GRANT PROXY
  ON ''@''
  TO 'admin'@'localhost'
  WITH GRANT OPTION;

Those statements create an admin user that can manage all GRANT PROXY mappings. For example, admin can do this:

GRANT PROXY ON sally TO joe;

Default Proxy Users

To specify that some or all users should connect using a given authentication plugin, create a “blank” MySQL account with an empty user name and host name (''@''), associate it with that plugin, and let the plugin return the real authenticated user name (if different from the blank user). Suppose that there exists a plugin named ldap_auth that implements LDAP authentication and maps connecting users onto either a developer or manager account. To set up proxying of users onto these accounts, use the following statements:

-- create default proxy account
CREATE USER ''@''
  IDENTIFIED WITH ldap_auth
  AS 'O=Oracle, OU=MySQL';
-- create proxied accounts; use
-- mysql_no_login plugin to prevent direct login
CREATE USER 'developer'@'localhost'
  IDENTIFIED WITH mysql_no_login;
CREATE USER 'manager'@'localhost'
  IDENTIFIED WITH mysql_no_login;
-- grant to default proxy account the
-- PROXY privilege for proxied accounts
GRANT PROXY
  ON 'manager'@'localhost'
  TO ''@'';
GRANT PROXY
  ON 'developer'@'localhost'
  TO ''@'';

Now assume that a client connects as follows:

$> mysql --user=myuser --password ...
Enter password: myuser_password

The server does not find myuser defined as a MySQL user, but because there is a blank user account (''@'') that matches the client user name and host name, the server authenticates the client against that account: The server invokes the ldap_auth authentication plugin and passes myuser and myuser_password to it as the user name and password.

If the ldap_auth plugin finds in the LDAP directory that myuser_password is not the correct password for myuser, authentication fails and the server rejects the connection.

If the password is correct and ldap_auth finds that myuser is a developer, it returns the user name developer to the MySQL server, rather than myuser. Returning a user name different from the client user name of myuser signals to the server that it should treat myuser as a proxy. The server verifies that ''@'' can authenticate as developer (because ''@'' has the PROXY privilege to do so) and accepts the connection. The session proceeds with myuser having the privileges of the developer proxied user. (These privileges should be set up by the DBA using GRANT statements, not shown.) The USER() and CURRENT_USER() functions return these values:

mysql> SELECT USER(), CURRENT_USER();
+------------------+---------------------+
| USER()           | CURRENT_USER()      |
+------------------+---------------------+
| myuser@localhost | developer@localhost |
+------------------+---------------------+

If the plugin instead finds in the LDAP directory that myuser is a manager, it returns manager as the user name and the session proceeds with myuser having the privileges of the manager proxied user.

mysql> SELECT USER(), CURRENT_USER();
+------------------+-------------------+
| USER()           | CURRENT_USER()    |
+------------------+-------------------+
| myuser@localhost | manager@localhost |
+------------------+-------------------+

For simplicity, external authentication cannot be multilevel: Neither the credentials for developer nor those for manager are taken into account in the preceding example. However, they are still used if a client tries to connect and authenticate directly as the developer or manager account, which is why those proxied accounts should be protected against direct login (see Preventing Direct Login to Proxied Accounts).

Default Proxy User and Anonymous User Conflicts

If you intend to create a default proxy user, check for other existing “match any user” accounts that take precedence over the default proxy user because they can prevent that user from working as intended.

In the preceding discussion, the default proxy user account has '' in the host part, which matches any host. If you set up a default proxy user, take care to also check whether nonproxy accounts exist with the same user part and '%' in the host part, because '%' also matches any host, but has precedence over '' by the rules that the server uses to sort account rows internally (see Section 4.5, “Access Control, Stage 1: Connection Verification”).

Suppose that a MySQL installation includes these two accounts:

-- create default proxy account
CREATE USER ''@''
  IDENTIFIED WITH some_plugin
  AS 'some_auth_string';
-- create anonymous account
CREATE USER ''@'%'
  IDENTIFIED BY 'anon_user_password';

The first account (''@'') is intended as the default proxy user, used to authenticate connections for users who do not otherwise match a more-specific account. The second account (''@'%') is an anonymous-user account, which might have been created, for example, to enable users without their own account to connect anonymously.

Both accounts have the same user part (''), which matches any user. And each account has a host part that matches any host. Nevertheless, there is a priority in account matching for connection attempts because the matching rules sort a host of '%' ahead of ''. For accounts that do not match any more-specific account, the server attempts to authenticate them against ''@'%' (the anonymous user) rather than ''@'' (the default proxy user). As a result, the default proxy account is never used.

To avoid this problem, use one of the following strategies:

Remove the anonymous account so that it does not conflict with the default proxy user.
Use a more-specific default proxy user that matches ahead of the anonymous user. For example, to permit only localhost proxy connections, use ''@'localhost':
```
CREATE USER ''@'localhost'
  IDENTIFIED WITH some_plugin
  AS 'some_auth_string';
```
In addition, modify any GRANT PROXY statements to name ''@'localhost' rather than ''@'' as the proxy user.
Be aware that this strategy prevents anonymous-user connections from localhost.
Use a named default account rather than an anonymous default account. For an example of this technique, consult the instructions for using the authentication_windows plugin. See Section 6.1.8, “Windows Pluggable Authentication”.
Create multiple proxy users, one for local connections and one for “everything else” (remote connections). This can be useful particularly when local users should have different privileges from remote users.
Create the proxy users:
```
-- create proxy user for local connections
CREATE USER ''@'localhost'
  IDENTIFIED WITH some_plugin
  AS 'some_auth_string';
-- create proxy user for remote connections
CREATE USER ''@'%'
  IDENTIFIED WITH some_plugin
  AS 'some_auth_string';
```
Create the proxied users:
```
-- create proxied user for local connections
CREATE USER 'developer'@'localhost'
  IDENTIFIED WITH mysql_no_login;
-- create proxied user for remote connections
CREATE USER 'developer'@'%'
  IDENTIFIED WITH mysql_no_login;
```
Grant to each proxy account the PROXY privilege for the corresponding proxied account:
```
GRANT PROXY
  ON 'developer'@'localhost'
  TO ''@'localhost';
GRANT PROXY
  ON 'developer'@'%'
  TO ''@'%';
```
Finally, grant appropriate privileges to the local and remote proxied users (not shown).
Assume that the some_plugin/'some_auth_string' combination causes some_plugin to map the client user name to developer. Local connections match the ''@'localhost' proxy user, which maps to the 'developer'@'localhost' proxied user. Remote connections match the ''@'%' proxy user, which maps to the 'developer'@'%' proxied user.

Server Support for Proxy User Mapping

Some authentication plugins implement proxy user mapping for themselves (for example, the PAM and Windows authentication plugins). Other authentication plugins do not support proxy users by default. Of these, some can request that the MySQL server itself map proxy users according to granted proxy privileges: mysql_native_password, sha256_password. If the check_proxy_users system variable is enabled, the server performs proxy user mapping for any authentication plugins that make such a request:

By default, check_proxy_users is disabled, so the server performs no proxy user mapping even for authentication plugins that request server support for proxy users.
If check_proxy_users is enabled, it may also be necessary to enable a plugin-specific system variable to take advantage of server proxy user mapping support:
- For the mysql_native_password plugin, enable mysql_native_password_proxy_users.
- For the sha256_password plugin, enable sha256_password_proxy_users.

For example, to enable all the preceding capabilities, start the server with these lines in the my.cnf file:

[mysqld]
check_proxy_users=ON
mysql_native_password_proxy_users=ON
sha256_password_proxy_users=ON

Assuming that the relevant system variables have been enabled, create the proxy user as usual using CREATE USER, then grant it the PROXY privilege to a single other account to be treated as the proxied user. When the server receives a successful connection request for the proxy user, it finds that the user has the PROXY privilege and uses it to determine the proper proxied user.

-- create proxy account
CREATE USER 'proxy_user'@'localhost'
  IDENTIFIED WITH mysql_native_password
  BY 'password';
-- create proxied account and grant its privileges;
-- use mysql_no_login plugin to prevent direct login
CREATE USER 'proxied_user'@'localhost'
  IDENTIFIED WITH mysql_no_login;
-- grant privileges to proxied account
GRANT ...
  ON ...
  TO 'proxied_user'@'localhost';
-- grant to proxy account the
-- PROXY privilege for proxied account
GRANT PROXY
  ON 'proxied_user'@'localhost'
  TO 'proxy_user'@'localhost';

To use the proxy account, connect to the server using its name and password:

$> mysql -u proxy_user -p
Enter password: (enter proxy_user password here)

Authentication succeeds, the server finds that proxy_user has the PROXY privilege for proxied_user, and the session proceeds with proxy_user having the privileges of proxied_user.

Proxy user mapping performed by the server is subject to these restrictions:

The server does not proxy to or from an anonymous user, even if the associated PROXY privilege is granted.
When a single account has been granted proxy privileges for more than one proxied account, server proxy user mapping is nondeterministic. Therefore, granting to a single account proxy privileges for multiple proxied accounts is discouraged.

Proxy User System Variables

Two system variables help trace the proxy login process:

proxy_user: This value is NULL if proxying is not used. Otherwise, it indicates the proxy user account. For example, if a client authenticates through the ''@'' proxy account, this variable is set as follows:
```
mysql> SELECT @@proxy_user;
+--------------+
| @@proxy_user |
+--------------+
| ''@''        |
+--------------+
```
external_user: Sometimes the authentication plugin may use an external user to authenticate to the MySQL server. For example, when using Windows native authentication, a plugin that authenticates using the windows API does not need the login ID passed to it. However, it still uses a Windows user ID to authenticate. The plugin may return this external user ID (or the first 512 UTF-8 bytes of it) to the server using the external_user read-only session variable. If the plugin does not set this variable, its value is NULL.

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