Using Authentication, Authorization, and Accounting services to Dynamically Provision View-based Access Control Model User-to-Group Mappings

This memo specifies a way to dynamically provision selected View-Based Access Control Model (VACM) Management Information Base (MIB) objects, based on information received from an Authentication, Authorization, and Accounting (AAA) service, such as RADIUS and . It reduces the need for security administrators to manually update VACM configurations due to user churn, allowing a centralized AAA service to provide the information associating a given user with the access control policy (known as a "group" in VACM) governing that user's access to management information. This memo requires no changes to the Abstract Service Interface for the Access Control Subsystem, and requires no changes to the Elements of Procedure for VACM. It provides a MIB module that reflects the information provided by the AAA service, along with elements of procedure for maintaining that information and performing corresponding updates to VACM MIB data. The reader is expected to be familiar with , , , and their supporting specifications.

For a detailed overview of the documents that describe the current Internet-Standard Management Framework, please refer to section 7 of RFC 3410 . Managed objects are accessed via a virtual information store, termed the Management Information Base or MIB. MIB objects are generally accessed through the Simple Network Management Protocol (SNMP). Objects in the MIB are defined using the mechanisms defined in the Structure of Management Information (SMI). This memo specifies a MIB module that is compliant to the SMIv2, which is described in STD 58, RFC 2578 , STD 58, RFC 2579 and STD 58, RFC 2580 .

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 .

There are two use cases for AAA support of management access via SNMP. These are (a) service authorization and (b) access control authorization. The former is discussed in detail in . The latter is the subject of this memo. The use case assumption here is that roles within an organization, which are reflected in VACM as groups, naming access control policies, change infrequently, while the users assigned to those roles change much more frequently. This memo describes how the user-to-role (group) mapping can be delegated to the RADIUS server, avoiding the need to re-provision managed devices as users are added, deleted, or assigned new roles in an organization. This memo assumes that the detailed access control policies are pre-configured in VACM, and does not attempt to address the question of how the policy associated with a given role is put in place. The only additional information obtained from the AAA service is the mapping of the authenticated user's identifier to a specific role (or "group" in VACM terminology) in the access control policy. Dynamic user authorization for MIB database access control, as defined herein, is limited to mapping the authenticated user to a group, which in turn is mapped to whatever access control policies are already in place in VACM. The SNMP architecture maintains strong modularity and separation of concerns, separating user identity (authentication) from user database access rights (authorization). RADIUS, on the other hand, allows for no such separation of authorization from authentication. Consequently, the approach here is to leverage existing RADIUS usage for identifying a principal, documented in , along with the RADIUS Management-Policy-Id Attribute . A unique identifier is needed for each AAA-authorized "session", corresponding to a communication channel, such as a transport session, for which a principal has been AAA-authenticated and which is authorized to offer SNMP service. How these identifiers are assigned is implementation-dependent. When a RADIUS Management-Policy-Id Attribute (or equivalent) is bound to such a session and principal authentication, this binding provides sufficient information to compute dynamic updates to VACM. How this information is communicated within an implementation is implementation dependent; this memo is only concerned with externally observable behavior. The key concept here is that what we will informally call a "AAA binding" binds: a communications channel an authenticated principal service authorization an access control policy name Some of the binding is done via other specifications. A transport model, such as the Secure Shell Transport Model , provides a binding between 1) and 2) and 3), providing a securityName. In turn, provides a binding between (1+2+3) and 4). This document extends that further, to create a binding between (1+2+3+4) and the local (VACM MIB) definition of the named policy, called a group in VACM.

Though this memo was motivated to support the use of specific Transport Models, such as the Secure Shell Transport Model , it MAY be used with other implementation environments satisfying these requirements: use an AAA service for sign-on service and data access authorization; provide an indication of the start of a session for a particular authenticated principal, identified using an SNMP securityName , and provide the corresponding value to be used to identify a VACM group to be used, based on information provided by the AAA service in use; provide an indication of the end of the need for being able to make access decisions for a particular authenticated principal, as at the end of a session, whether due to disconnection, termination due to timeout, or any other reason. Likewise, although this memo specifically refers to RADIUS, it MAY be used with other AAA services satisfying these requirements: the service provides information semantically equivalent to the RADIUS Management-Policy-Id Attribute , which corresponds to the name of a VACM group; the service provides an authenticated principal identifier (e.g., the RADIUS User-Name Attribute ) which can be transformed to an equivalent principal identifier in the form of a securityName .

This MIB module makes use of the SnmpAdminString and SnmpSecurityModel textual conventions.

This MIB module defines a single table, the vacmAaaSecurityToGroupTable. This table is indexed by the integer assigned to each security model, the protocol-independent securityName corresponding to a principal, and the unique identifier of a session.

This MIB module has a close operational relationship with the SNMP-VIEW-BASED-ACM-MIB (more commonly known as the "VACM MIB") from . It also relies on IMPORTS from several other modules.

Although the MIB module defined here has a close relationship with the VACM MIB's vacmSecurityToGroupTable, it in no way changes the elements of procedure for VACM, nor does it affect any other tables defined in VACM. See the elements of procedure (below) for details of how the contents of the vacmSecurityToGroupTable are affected by this MIB module.

This MIB module employs definitions from , and .

This MIB module contains REFERENCE clauses making reference to , , and .

The following elements of procedure are formulated in terms of two types of events: an indication of the establishment of a session, and an indication that one has ended. These can result in the creation of entries in the vacmAaaSecurityToGroupTable, which can in turn trigger creation, update, or deletion of entries in the vacmSecurityToGroupTable. There are various possible implementation-dependent error cases not spelled out here, such as running out of memory. By their nature, recovery in such cases will be implementation-dependent. Implementors are advised to consider fail-safe strategies, e.g., prematurely terminating access in preference to erroneously perpetuating access.

These procedures assume that a transport model, such as , coordinates session establishment with AAA authentication and authorization. They rely on the receipt by the AAA client of the RADIUS Management-Policy-Id Attribute (or its equivalent) from the RADIUS Access-Accept message (or equivalent). They also assume that the User-Name from the RADIUS Access-Request message (or equivalent) corresponds to a securityName . To ensure correct processing of SNMP PDUs, the handling of the indication of the establishment of a session in accordance with the elements of procedure below MUST be completed before the isAccessAllowed() abstract service interface is invoked for any SNMP PDUs from that session. If a session termination indication occurs before all invocations of the isAccessAllowed() abstract service interface have completed for all SNMP PDUs from that session, those remaining invocations MAY result in denial of access.

Four pieces of information are needed to process the session establishment indication: the SnmpSecurityModel needed as an index into the vacmSecurityToGroupTable; the RADIUS User-Name Attribute; a session identifier, as a unique, definitive identifier of the session that the AAA authorization is tied to; the RADIUS Management-Policy-Id Attribute. All four of these pieces of information are REQUIRED. In particular, if either the User-Name or Management-Policy-Id is absent, invalid, or a zero-length string, no further processing of the session establishment indication is undertaken. As noted in , the above text refers specifically to RADIUS attributes. Other AAA services can be substituted, but the requirements imposed on the User-Name and the Management-Policy-Id-Attribute MUST be satisfied using the equivalent fields for those services.

Whenever an indication arrives that a new session has been established, determine whether a corresponding entry exists in the vacmAaaSecurityToGroupTable. If one does not, create a new row with the columns populated as follows: vacmAaaSecurityModel = value of SnmpSecurityModel corresponding to the security model in use; vacmAaaSecurityName = RADIUS User-Name Attribute or equivalent, the securityName that will be used in invocations of the isAccessAllowed() abstract service interface ; vacmAaaSessionID = session identifier, unique across all open sessions of all of this SNMP engine's transport models; vacmAaaGroupName = RADIUS Management-Policy-Id Attribute or equivalent. Otherwise, if the row already exists, update the vacmAaaGroupName with the the RADIUS Management-Policy-Id Attribute or equivalent supplied.

Whenever an entry is created in the vacmAaaSecurityToGroupTable, the vacmSecurityToGroupTable is examined to determine whether a corresponding entry exists there, using the value of vacmAaaSecurityModel for vacmSecurityModel, and the value of vacmAaaSecurityName for vacmSecurityName. If no corresponding entry exists, create one, using the vacmAaaGroupName of the newly created entry to fill in vacmGroupName, using a value of "volatile" for the row's StorageType, and a value of "active" for its RowStatus.

Whenever the value of an instance of vacmAaaGroupName is updated, if a corresponding entry exists in the vacmSecurityToGroupTable, and that entry's StorageType is "volatile" and its RowStatus is "active", update the value of vacmGroupName with the value from vacmAaaGroupName. If a corresponding entry already exists in the vacmSecurityToGroupTable, and that row's StorageType is anything other than "volatile", or its RowStatus is anything other than "active", then that instance of vacmGroupName MUST NOT be modified. The operational assumption here is that if the row's StorageType is "volatile", then this entry was probably dynamically created; an entry created by a security administrator would not normally be given a StorageType of "volatile". If value being provided by RADIUS (or other AAA service) is the same as what is already there, this is a no-op. If the value is different, the new information is understood as a more recent role (group) assignment for the user, which should supersede the one currently held there. The structure of the vacmSecurityToGroupTable makes it impossible for a (vacmSecurityModel, vacmSecurityName) tuple to map to more than one group.

Whenever a RADIUS (or other AAA) authenticated session ends for any reason, an indication is provided. This indication MUST provide means of determining the SnmpSecurityModel, and an identifier for the transport session tied to the AAA authorization. The manner in which this occurs is implementation dependent.

Entries in the vacmAaaSecurityToGroupTable MUST NOT persist across system reboots. When a session has been terminated, the vacmAaaSecurityToGroupTable is searched for a corresponding entry. A "matching" entry is any entry for which the SnmpSecurityModel and session ID match the information associated with the session termination indication. Any matching entries are deleted. It is possible that no entries will match; this is not an error, and no special processing is required in this case.

Whenever the last remaining row bearing a particular (vacmAaaSecurityModel, vacmAaaSecurityName) pair is deleted from the vacmAaaSecurityToGroupTable, the vacmSecurityToGroupTable is examined for a corresponding row. If one exists, and if its StorageType is "volatile" and its RowStatus is "active", that row MUST be deleted as well. The mechanism to accomplish this task is implementation-dependent.

The algorithms in this memo make heuristic use of the StorageType of entries in the vacmSecurityToGroupTable to distinguish those provisioned by a security administrator (which would presumably not be configured as "volatile") from those dynamically generated. In making this distinction, it assumes that those entries explicitly provisioned by a security administrator and given a non-"volatile" status are not to be dynamically overridden. Furthermore, it assumes that any active entries with "volatile" status can be treated as dynamic, and deleted or updated as needed. Users of this memo need to be aware of this operational assumption, which, while reasonable, is not necessarily universally valid. For example, this situation could also occur if the SNMP security administrator had mistakenly created these non-volatile entries in error. The design of VACM ensures that if an unknown policy (group name) is used in the vacmSecurityToGroupTable, no access is granted. A consequence of this is that no matter what information is provided by the AAA server, no user can gain SNMP access rights not already granted to some group through the VACM configuration.

In order to ensure that the access control policy ultimately applied as a result of the mechanisms described here is indeed the intended policy for a given principal using a particular security model, care needs to be applied in the mapping of the authenticated user (principal) identity to the securityName used to make the access control decision. Broadly speaking, there are two approaches to ensure consistency of identity: Entries for the vacmSecurityToGroupTable corresponding to a given security model are created only through the operation of the procedures described in this memo. A consequence of this would be that all such entries would have been created using the RADIUS User-Name (or other AAA-authenticated identity) and RADIUS Management-Policy-Id Attribute (or equivalent). Administrative policy allows a matching pre-configured entry to exist in the vacmSecurityToGroupTable, i.e., an entry with the corresponding vacmSecurityModel and with a vacmSecurityName matching the authenticated principal's RADIUS User-Name. In this case, administrative policy also needs to ensure consistency of identity between each authenticated principal's RADIUS User-Name and the administratively configured vacmSecurityName in the vacmSecurityToGroupTable row entries for that particular security model. In the later case, inconsistent re-use of the same name for different entities or individuals (principals) can cause the incorrect access control policy to be applied for the authenticated principal, depending on whether the policy configured using SNMP, or the policy applied using the procedures of this memo, is the intended policy. This may result in greater or lesser access rights than the administrative policy intended. Inadvertent mis-identification in such cases may be undetectable by the SNMP engine or other software elements of the managed entity.

There are no management objects defined in this MIB module that have a MAX-ACCESS clause of read-write and/or read-create. So, if this MIB module is implemented correctly, then there is no risk that an intruder can alter or create any management objects of this MIB module via direct SNMP SET operations. Some of the readable objects in this MIB module (including some objects with a MAX-ACCESS of not-accessible, whose values are exposed as a result access to indexed objects) may be considered sensitive or vulnerable in some network environments. It is thus important to control even GET and/or NOTIFY access to these objects and possibly to even encrypt the values of these objects when sending them over the network via SNMP. These are the tables and objects and their sensitivity/vulnerability: vacmAaaSecurityToGroupTable - the entire table is potentially sensitive, since walking the table will reveal user names, security models in use, session identifiers, and group names; vacmAaaSecurityModel - though not-accessible, this is exposed as an index of vacmAaaGroupName; vacmAaaSecurityName - though not-accessible, this is exposed as an index of vacmAaaGroupName; vacmAaaSessionID - though not-accessible, this is exposed as an index of vacmAaaGroupName; vacmAaaGroupName - since this identifies a security policy and associates it with a particular user, this is potentially sensitive. SNMP versions prior to SNMPv3 did not include adequate security. Even if the network itself is secure (for example by using IPsec), even then, there is no control as to who on the secure network is allowed to access and GET/SET (read/change/create/delete) the objects in this MIB module. It is RECOMMENDED that implementers consider the security features as provided by the SNMPv3 framework (see , section 8), including full support for the SNMPv3 cryptographic mechanisms (for authentication and privacy). Further, deployment of SNMP versions prior to SNMPv3 is NOT RECOMMENDED. Instead, it is RECOMMENDED to deploy SNMPv3 and to enable cryptographic security. It is then a customer/operator responsibility to ensure that the SNMP entity giving access to an instance of this MIB module is properly configured to give access to the objects only to those principals (users) that have legitimate rights to indeed GET or SET (change/create/delete) them.

The MIB module in this document uses the following IANA-assigned OBJECT IDENTIFIER value recorded in the SMI Numbers registry:

Editor's Note (to be removed prior to publication): the IANA is requested to assign a value for "XXX" under the 'mib-2' subtree and to record the assignment in the SMI Numbers registry. When the assignment has been made, the RFC Editor is asked to replace "XXX" (here and in the MIB module) with the assigned value and to remove this note.

The following participants from the isms working group contributed to the development of this document: Andrew Donati David Harrington Jeffrey Hutzelman Jürgen Schönwälder Tom Petch Wes Hardaker During the IESG review additional comments were received from: Adrian Farrel Amanda Baber Dan Romescanu David Kessens Francis Dupont Glenn Keeni Jari Arkko Joel Jaeggli Magnus Nyström Mike Heard Robert Story Russ Housley Sean Turner Tim Polk