27 Repository Resource Security

How the ACL-Based Security Mechanism Works

ACLs in Oracle XML DB are XML schema-based resources, stored and managed in Oracle XML DB. Each resource in Oracle XML DB Repository is protected by an ACL. Before a user performs an operation on a resource, the user privileges on the resource are checked. The set of privileges checked depends on the operation to be performed.

Some ACLs are supplied with Oracle XML DB. There is only one ACL, the bootstrap ACL, located at /sys/acls/bootstrap_acl.xml in Oracle XML DB Repository, that is self-protected; that is, it is protected by its own contents. This ACL, supplied with Oracle XML DB, grants READ privilege to all users. The bootstrap ACL also grants FULL ACCESS to roles XDBADMIN (the Oracle XML DB ADMIN) and DBA. Role XDBADMIN is particularly useful for users who must register global XML schemas.

Other ACLs supplied with Oracle XML DB include the following. Each is protected by the bootstrap ACL.

• all_all_acl.xml Grants all privileges to all users

• all_owner_acl.xml Grants all privileges to the owner of the resource

• ro_all_acl.xml Grants read privileges to all users

All ACLs must conform to the Oracle XML DB ACL XML schema, which is located in the repository at /sys/schemas/PUBLIC/xmlns.oracle.com/xdb/acl.xsd.

All ACLs are stored in table XDB$ACL, which is owned by database user XDB. This is an XML schema-based XMLType table. Each row in this table (and therefore each ACL) has a system-generated object identifier (OID) that can be accessed as a column named OBJECT_ID.

Each resource has a property named ACLOID. The ACLOID stores the OID of the ACL that protects the resource. As mentioned, an ACL is itself a resource. Hence, the XMLRef property of an ACL resource, for example, /sys/acls/all_all_acl.xml, is a REF to the row in table XDB$ACL that contains the content of the ACL. These two properties form the link between table XDB$RESOURCE, which stores Oracle XML DB resources, and table XDB$ACL.

Principal

A principal is an entity that may be granted access control privileges to an Oracle XML DB resource. Oracle XML DB supports the following as principals:

• Database users

• Database roles. A database role can be understood as a group; for example, the DBA role represents the group of all database administrators.

• LDAP users and groups. For details on using LDAP principals see "Integrating Oracle XML DB with LDAP".

The special principal, dav:owner, corresponds to the owner of the resource being secured. The owner of the resource is one of the properties of the resource. Use of the dav:owner principal facilitates greater ACL sharing between users, because the owner of the document often has special rights. See Also "Access Privileges".

Privilege

A privilege is a particular right that can be granted or denied to a principal. Oracle XML DB has a set of system-defined rights (such as READ or UPDATE) that can be referenced in any ACL. Privileges can be granted or denied to the principal dav:owner, that represents the owner of the document, regardless of who the owner is. Privileges can be one of the following:

• Aggregate (containing other privileges)

• Atomic (which cannot be subdivided)

Aggregate privileges are a naming convenience to simplify usability when the number of privileges becomes large, as well as to promote inter operability between ACL clients. Please see "Access Privileges" for the list of atomic and aggregate privileges that can be used in ACLs.

The set of privileges granted to a principal controls the ability of that principal to perform a given operation or method on an Oracle XML DB resource. For example, if the principal HR wants to perform the read operation on a given resource, then the read privileges must be granted to HR prior to the read operation. Therefore, privileges control how users can operate on resources.

Access Control Entry (ACE)

An access control entry (ACE) is an element (ace) that serves as an entry in an ACL that grants or denies access to a particular principal (database user). An ACL consists of a sequence of ACEs, where ordering is relevant. See "ACL Evaluation Rules".

An Oracle XML DB ACE either grants or denies privileges for a principal. An ace element has the following:

• Operation: Either grant or deny.

• Principal: A valid principal (element principal).

• Privileges: A set of privileges to be granted or denied for a particular principal (element privilege).

• Principal Format (optional): The format of the principal. An LDAP distinguished name (DN), a short name (database user/role or LDAP nickname), or an LDAP GUID. The default value is short name. If the principal name matches both a database user and an LDAP nickname, it is assumed to refer to the LDAP nickname.

• Collection (optional): A BOOLEAN attribute that specifies whether the principal is a collection of users (LDAP group or database role) or a single user (LDAP or database user).

Access Control List (ACL)

An access control list (ACL) is a list of access control entries (ACEs). It defines access control for a resource.

The following example shows entries in an ACL:

<acl description="myacl"
     xmlns="http://xmlns.oracle.com/xdb/acl.xsd"
     xmlns:dav="DAV:"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://xmlns.oracle.com/xdb/acl.xsd
                         http://xmlns.oracle.com/xdb/acl.xsd">
  <ace>
    <grant>true</grant>
    <principal>dav:owner</principal>
    <privilege>
      <dav:all/>
    </privilege>
  </ace>
</acl>

In this ACL there is only one ACE. The ACE grants all privileges to the owner of the resource.

ACL Evaluation Rules

Privileges are checked before a user is allowed to access a resource. This is done by evaluating the resource ACL for the current user. To evaluate an ACL, the database collects the ACEs in the ACL that apply to the user logged into the current database session. The list of currently active roles for the user is maintained as part of the session, and it is used to match ACEs, which specify roles as principals, with the current users.

If one ACE grants a certain privilege to the current user and another ACE denies that privilege to the user, then a conflict arises. There are two possible ways to manage conflicts among ACEs for the same principal.

• The default behavior, termed ace-order, is to use only the first ACE that occurs for a given principal; additional ACEs for that principal have no effect. In this case, ACE order is relevant.

• You can, however, configure the database to use an alternate behavior, deny-trumps-grant. In this case, an ACE with child deny for a given principal denies permission to that principal, whether or not there are other ACEs for that principal that have a grant child. In this case, deny always takes precedence over grant, and ACE order is irrelevant.

You configure ACL evaluation behavior by setting configuration parameter acl-evaluation-method, in configuration file xdbconfig.xml, to either ace-order or deny-trumps-grant. The default configuration file specifies ace-method, but the default value for element acl-evaluation-method, used when no method is given, is deny-trumps-grant.

Atomic Privileges

Table 27-1 lists the atomic privileges supported by Oracle XML DB. Note that Oracle XML DB supports all WebDAV privileges. Privilege names are used as XML element names. Privileges with a dav: prefix are part of the WebDAV namespace. Other privileges are part of the Oracle XML DB ACL namespace: http://xmlns.oracle.com/xdb/acl.xsd.

Because you can directly access the XMLType storage for ACLs, the XML structure is part of the client interface. Hence, ACLs can be manipulated using the XMLType APIs.

Aggregate Privileges

Table 27-2 lists the aggregate privileges defined by Oracle XML DB, along with the atomic privileges of which they are composed.

Table 27-3 shows the privileges required for some common operations on resources in Oracle XML DB Repository. Column Privileges Required assumes that you already have the resolve privilege on container C and all its parent containers, up to the root of the hierarchy.

Creating an ACL Using DBMS_XDB.createResource

Example 27-1 creates an ACL as file resource /TESTUSER/acl1.xml. If applied to a resource, this ACL grants all privileges to the owner of a resource.

DECLARE
  b BOOLEAN;
BEGIN
  b := DBMS_XDB.createFolder('/TESTUSER');
  b := DBMS_XDB.createResource(
         '/TESTUSER/acl1.xml', 
         '<acl description="myacl"
               xmlns="http://xmlns.oracle.com/xdb/acl.xsd"
               xmlns:dav="DAV:"
               xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
               xsi:schemaLocation="http://xmlns.oracle.com/xdb/acl.xsd
                                   http://xmlns.oracle.com/xdb/acl.xsd">
            <ace>
              <grant>true</grant>
              <principal>dav:owner</principal>
              <privilege>
                <dav:all/>
              </privilege>
            </ace>
          </acl>');
END;

Retrieving an ACL Document, Given its Repository Path

Example 27-2 shows how to retrieve an ACL document, given its location in Oracle XML DB Repository.

SELECT a.OBJECT_VALUE FROM RESOURCE_VIEW r, XDB.XDB$ACL a
  WHERE ref(a) = extractValue(r.RES, '/Resource/XMLRef')
    AND equals_path(r.RES, '/TESTUSER/acl1.xml') = 1;

OBJECT_VALUE
--------------------------------------------------------------------------------
<acl description="myacl" xmlns="http://xmlns.oracle.com/xdb/acl.xsd" xmlns:dav="
DAV:" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="
http://xmlns.oracle.com/xdb/acl.xsd                                    http://xm
lns.oracle.com/xdb/acl.xsd" shared="true">
  <ace>
    <grant>true</grant>
    <principal>dav:owner</principal>
    <privilege>
      <dav:all/>
    </privilege>
  </ace>
</acl>
 
1 row selected.

Setting the ACL of a Resource

DECLARE
  b BOOLEAN;
BEGIN
  b := DBMS_XDB.createResource('/TESTUSER/po1.xml', 'Hello');
END;
/
 
CALL DBMS_XDB.setACL('/TESTUSER/po1.xml', '/TESTUSER/acl1.xml');

Deleting an ACL

Example 27-4 illustrates how to delete an ACL using procedure DBMS_XDB.deleteResource.

CALL DBMS_XDB.deleteResource('/TESTUSER/acl1.xml');

Updating an ACL

This can be done using standard methods for updating resources. In particular, since an ACL is an XML document, SQL function updateXML and related XML-updating functions can be used to manipulate ACLs. Oracle XML DB ACLs are cached, for fast evaluation. When a transaction that updates an ACL is committed, the modified ACL is picked up by existing database sessions, after the timeout specified in the Oracle XML DB configuration file, /xdbconfig.xml. The XPath location for this timeout parameter is /xdbconfig/sysconfig/acl-max-age; the value is expressed in seconds. Sessions initiated after the ACL is modified use the new ACL without any delay.If an ACL resource is updated with non-ACL content, the same rules apply as for deletion. Thus, if any resource is being protected by an ACL that is being updated, you must first change the ACL.

You can use FTP or WebDAV to update an ACL. For more details on how to use these protocols, see Chapter 28, "Using Protocols to Access the Repository". You can update an ACL or an access control entry (ACE) using RESOURCE_VIEW.

UPDATE RESOURCE_VIEW r
  SET r.RES =
        updateXML(
          r.RES,
          '/r:Resource/r:Contents/a:acl',
          '<acl description="myacl"
                xmlns="http://xmlns.oracle.com/xdb/acl.xsd"
                xmlns:dav="DAV:"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:schemaLocation="http://xmlns.oracle.com/xdb/acl.xsd
                                    http://xmlns.oracle.com/xdb/acl.xsd">
             <ace>
               <grant>true</grant>
               <principal>TESTUSER</principal>
               <privilege>
                 <dav:all/>
               </privilege>
             </ace>
           </acl>',
          'xmlns:r="http://xmlns.oracle.com/xdb/XDBResource.xsd"
           xmlns:a="http://xmlns.oracle.com/xdb/acl.xsd"')
  WHERE equals_path(r.RES, '/TESTUSER/acl1.xml') = 1;

UPDATE RESOURCE_VIEW r
  SET r.RES =
        appendChildXML(
          r.RES,
          '/r:Resource/r:Contents/a:acl',
          XMLType('<ace xmlns="http://xmlns.oracle.com/xdb/acl.xsd">
                     <grant>true</grant>
                     <principal>HR</principal>
                     <privilege>
                       <read-properties/>
                       <read-contents/>
                     </privilege>
                   </ace>'),
                  'xmlns:r="http://xmlns.oracle.com/xdb/XDBResource.xsd"
                   xmlns:a="http://xmlns.oracle.com/xdb/acl.xsd"')
  WHERE equals_path(r.RES, '/TESTUSER/acl1.xml') = 1;

UPDATE RESOURCE_VIEW r
  SET r.RES =
        deleteXML(r.RES,
                  '/r:Resource/r:Contents/a:acl/a:ace[1]',
                  'xmlns:r="http://xmlns.oracle.com/xdb/XDBResource.xsd"
                   xmlns:a="http://xmlns.oracle.com/xdb/acl.xsd"')
  WHERE equals_path(r.RES, '/TESTUSER/acl1.xml') = 1;

Retrieving the ACL Document that Protects a Given Resource

Example 26-2 illustrates how to use function DBMS_XDB.getACLDocument to retrieve the ACL document that protects a given resource.

SELECT DBMS_XDB.getACLDocument('/TESTUSER/po1.xml').getCLOBVal() FROM DUAL;
 
DBMS_XDB.GETACLDOCUMENT('/TESTUSER/PO1.XML').GETCLOBVAL()
--------------------------------------------------------------------------------
<acl description="myacl" xmlns="http://xmlns.oracle.com/xdb/acl.xsd" xmlns:dav="
DAV:" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="
http://xmlns.oracle.com/xdb/acl.xsd                                     http://x
mlns.oracle.com/xdb/acl.xsd">
  <ace>
    <grant>true</grant>
    <principal>TESTUSER</principal>
    <privilege>
      <dav:all/>
    </privilege>
  </ace>
  <ace xmlns="http://xmlns.oracle.com/xdb/acl.xsd">
    <grant>true</grant>
    <principal>HR</principal>
    <privilege>
      <read-properties/>
      <read-contents/>
    </privilege>
  </ace>
</acl>
 
1 row selected.

Retrieving Privileges Granted to the Current User for a Particular Resource

Example 27-9 illustrates how to retrieve privileges granted to the current user using function DBMS_XDB.getPrivileges.

SELECT DBMS_XDB.getPrivileges('/TESTUSER/po1.xml').getCLOBVal() FROM DUAL;
 
DBMS_XDB.GETPRIVILEGES('/TESTUSER/PO1.XML').GETCLOBVAL()
--------------------------------------------------------------------------------
<privilege xmlns="http://xmlns.oracle.com/xdb/acl.xsd" xmlns:xsi="http://www.w3.
org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.oracle.com/xdb/acl
.xsd http://xmlns.oracle.com/xdb/acl.xsd DAV: http://xmlns.oracle.com/xdb/dav.xs
d" xmlns:xdbacl="http://xmlns.oracle.com/xdb/acl.xsd" xmlns:dav="DAV:">
  <dav:take-ownership/>
  <dav:execute/>
  <dav:unlock/>
  <read-properties/>
  <dav:lock/>
  <read-contents/>
  <dav:read-current-user-privilege-set/>
  <write-config/>
  <dav:write-content/>
  <resolve/>
  <dav:write-properties/>
  <unlink-from/>
  <unlink/>
  <read-acl/>
  <write-acl-ref/>
  <update-acl/>
  <link/>
</privilege>

1 row selected.

Checking if the Current User Has Privileges on a Resource

Example 27-10 illustrates how to use function DBMS_XDB.checkPrivileges to check if the current user has a given set of privileges on a resource. This function returns a nonzero value if the user has the privileges.

SELECT DBMS_XDB.checkPrivileges(
         '/TESTUSER/po1.xml',
         XMLType('<privilege
                      xmlns="http://xmlns.oracle.com/xdb/acl.xsd"
                      xmlns:dav="DAV:"
                      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                      xsi:schemaLocation="http://xmlns.oracle.com/xdb/acl.xsd
                                          http://xmlns.oracle.com/xdb/acl.xsd">
                    <read-contents/>
                    <read-properties/>
                  </privilege>'))
  FROM DUAL;
 
DBMS_XDB.CHECKPRIVILEGES('/TESTUSER/PO1.XML',
---------------------------------------------
                                            1
 
1 row selected.

Checking if the Current User Has Privileges With the ACL and Resource Owner

Function DBMS_XDB.ACLCheckPrivileges is typically used by applications that must perform ACL evaluation on their own, before allowing a user to perform an operation.

CONNECT sh/sh

SELECT DBMS_XDB.ACLCheckPrivileges(
         '/TESTUSER/acl1.xml',
         'TESTUSER',
         XMLType('<privilege
                      xmlns="http://xmlns.oracle.com/xdb/acl.xsd"
                      xmlns:dav="DAV:"
                      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                      xsi:schemaLocation="http://xmlns.oracle.com/xdb/acl.xsd
                                          http://xmlns.oracle.com/xdb/acl.xsd">
                    <read-contents/>
                    <read-properties/>
                  </privilege>'))
  FROM DUAL;
 
DBMS_XDB.ACLCHECKPRIVILEGES('/TESTUSER/ACL1.XML','TESTUSER',
------------------------------------------------------------
                                                           0
 
1 row selected.

Retrieving the Path of the ACL that Protects a Given Resource

Example 27-12 retrieves the path of the ACL that protects a given resource, by using a RESOURCE_VIEW query. The query uses the fact that the XMLRef and ACLOID elements of the resource form the link between an ACL and a resource.

SELECT a.ANY_PATH
  FROM RESOURCE_VIEW a
  WHERE extractValue(a.RES, '/Resource/XMLRef')
        = make_ref(XDB.XDB$ACL,
                   (SELECT extractValue(r.RES, '/Resource/ACLOID')
                      FROM RESOURCE_VIEW r
                      WHERE equals_path(r.RES, '/TESTUSER/po1.xml') = 1));
 
ANY_PATH
------------------
/TESTUSER/acl1.xml
 
1 row selected.

Retrieving the Paths of All Resources Protected by a Given ACL

Example 27-13 retrieves the paths of all resources protected by a given ACL.

SELECT r.ANY_PATH
  FROM RESOURCE_VIEW r
  WHERE make_ref(XDB.XDB$ACL, extractValue(r.RES, '/Resource/ACLOID'))
        = (SELECT extractValue(a.RES, '/Resource/XMLRef')
             FROM RESOURCE_VIEW a
             WHERE equals_path(a.RES, '/TESTUSER/acl1.xml') = 1);
 
ANY_PATH
-----------------
/TESTUSER/po1.xml
 
1 row selected.

Finding Information about Access Control Lists

You can use data dictionary views to find information about existing access control lists. Table 27-4 shows the data dictionary views for ACLs.

Checking Privilege Assignments

When a database user tries to access a service, the access control list (ACL) for the service is checked to determine the access privileges for the user. Access control entries (ACEs) contained in the ACL are used to grant or deny access permissions to a user or role.

Use PL/SQL package DBMS_NETWORK_ACL_ADMIN to check privilege assignments that might affect permissions of the user to access a network host. Function check_privilege checks ACLs to determine the access of a given user to a particular network resource. It checks whether a specified privilege is granted or denied to the user.

If you are a database administrator, you can use view DBA_NETWORK_ACLS to determine the ACL assignments for the network hosts or domains. You can then use view DBA_NETWORK_ACL_PRIVILEGES to determine if each ACL grants (GRANTED), denies (DENIED), or does not apply to (NULL) the access privilege of the user.