ANCP Working Group H. Moustafa Internet-Draft France Telecom Intended status: Informational H. Tschofenig Expires: April 16, 2007 Siemens S. De Cnodder Alcatel October 13, 2006 Security Threats and Security Requirements for the Access Node Control Protocol (ANCP) draft-moustafa-ancp-security-threats-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 16, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Abstract The Access Node Control Protocol (ANCP) aims to communicate QoS-, service- and subscriber-related operations between a Network Access Server (NAS) and an Access Node (e.g., a Digital Subscriber Line Access Multiplexer (DSLAM)). The main goal of this protocol is to Moustafa, et al. Expires April 16, 2007 [Page 1] Internet-Draft ANCP Threats October 2006 configure and manage access equipments and allow them to report information to the NAS in order to enable and optimize configuration. This document investigates security threats that all ANCP nodes could encounter, developing a threat model for ANCP security aiming to decide which security functions are required at the ANCP level. Based on this, security requirements regarding the Access Node Control Protocol are defined. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. System Overview and Threat Model . . . . . . . . . . . . . . . 5 4. Objectives of Attackers . . . . . . . . . . . . . . . . . . . 7 5. Potential Attacks . . . . . . . . . . . . . . . . . . . . . . 7 5.1. Message Modification . . . . . . . . . . . . . . . . . . . 7 5.2. Replay of Signaling Messages . . . . . . . . . . . . . . . 8 5.3. Denial of Service Attacks . . . . . . . . . . . . . . . . 8 5.4. Traffic Analysis . . . . . . . . . . . . . . . . . . . . . 9 5.5. Downgrading Attack . . . . . . . . . . . . . . . . . . . . 9 5.6. Man-in-the-Middle Attack . . . . . . . . . . . . . . . . . 9 5.7. Network Snooping . . . . . . . . . . . . . . . . . . . . . 9 6. Attacks Against ANCP Defined Use Cases . . . . . . . . . . . . 10 6.1. Dynamic Access Loop Attributes . . . . . . . . . . . . . . 10 6.2. Access Loop Configuration . . . . . . . . . . . . . . . . 11 6.3. Remote Connectivity Test . . . . . . . . . . . . . . . . . 11 6.4. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 12 7. Security Requirements . . . . . . . . . . . . . . . . . . . . 12 8. Security Considerations . . . . . . . . . . . . . . . . . . . 13 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 10. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 13 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 11.1. Normative References . . . . . . . . . . . . . . . . . . . 13 11.2. Informative References . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 Moustafa, et al. Expires April 16, 2007 [Page 2] Internet-Draft ANCP Threats October 2006 Intellectual Property and Copyright Statements . . . . . . . . . . 15 Moustafa, et al. Expires April 16, 2007 [Page 3] Internet-Draft ANCP Threats October 2006 1. Introduction The Access Node Control Protocol (ANCP) aims to communicate QoS-, service- and subscriber-related operations between a Network Access Server (NAS) and an Access Node (e.g., a Digital Subscriber Line Access Multiplexer (DSLAM)). [I-D.ooghe-ancp-framework] illustrates the framework, usage scenarios and general requirements. This document focuses on description of security threats and derives security requirements from the Access Node Control Protocol. Security policy negotiation, including authentication and authorization to define the per-subscriber policy at the policy/AAA server, is out of the scope of this work. As a high-level summary, the following aspects need to be considered: Message Protection: Signaling message content can be protected against eavesdropping, modification, injection and replay while in transit. This applies both to ANCP payloads, and ANCP should also provide such protection as a service to the different service parameters between the two peers. Prevention again Impersonation: It is important that signaling messages are delivered to the correct nodes, and nowhere else. Prevention of Denial of Service Attacks: ANCP nodes and the network have finite resources (state storage, processing power, bandwidth). Exhaustion attacks against these resources and not allow ANCP nodes to be used to launch attacks on other network elements is of importance. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119], with the qualification that unless otherwise stated they apply to the design of the Access Node Control Protocol (ANCP), not its implementation or application. The relevant components are described in Section 3. Moustafa, et al. Expires April 16, 2007 [Page 4] Internet-Draft ANCP Threats October 2006 3. System Overview and Threat Model As described in [I-D.ooghe-ancp-framework] and schematically shown in Figure 1, the Access Node Control system consists of the following components: Network Access Server (NAS): A NAS provides access to a service (e.g., network access) and operates as a client of the AAA protocol. The client is responsible for passing authentication information to designated AAA servers and then acting on the response that is returned. Authentication, Authorization and Accounting (AAA) server: A AAA server is responsible for authentication users, for authorizing access to services, and for returning authorization information including configuration parameters back to the AAA client to deliver service to the user. As a consequence of service usage accounting might be enabled and information about the user's resource usage will be sent to the AAA server. Access Node (AN): The AN is a network device, usually located at a service provider central office or street cabinet, that terminates Access Loop connections from subscribers. In case the Access Loop is a Digital Subscriber Line (DSL), this is often referred to as a DSL Access Multiplexer (DSLAM). Customer Premises Equipment (CPE): A CPE is a device and associated equipment located inside a subscriber's premise and connected with a carrier's telecommunication channel(s) at the demarcation point ("demarc"). The demarc is a point established in a building or complex to separate customer equipment from the equipment of the access network provider (e.g., a telephone company). Home Gateway (HGW): The HGW connects the different Customer Premises Equipment (CPE) to the Access Node and the access network. In case of DSL, the HGW is a DSL Network Termination (NT) that could either operate as a layer 2 bridge or as a layer 3 router. In the latter case, such a device is also referred to as a Routing Gateway (RG). For the threat analysis the protocol communication between the Access Node and the NAS is important whereas the other component, such as Moustafa, et al. Expires April 16, 2007 [Page 5] Internet-Draft ANCP Threats October 2006 HGW, CPE, AAA server only play a role in the understanding of the system architecture. Note that the NAS and the AN might belong to two different administrative realms. +--------+ | AAA | | Server | +--------+ | | +-----+ +-----+ +--------+ +-----+ +----------+ | CPE |---| HGW |---| | | | | | +-----+ +-----+ | Access | | | | Internet | | Node |-----------------| NAS |---| | +-----+ +-----+ | (AN) | | | | | | CPE |---| HGW |---| | | | | | +-----+ +-----+ +--------+ +-----+ +----------+ Figure 1: System Overview In the absence of an attack, the NAS receives configuration information from the AAA server related to a CPE attempting to access the network. A number of parameters, including Quality of Service information, need to be conveyed to the Access Node in order to become effective. The Access Node Control Protocol is executed between the NAS and the AN to initiate control requests. The AN returns responses to these control requests and provides information reports. For this to happen, the following individual steps must occur: o The AN discovers the NAS. o The AN needs to start the protocol communication with the NAS to announce its presence. o The AN and the NAS perform a capability exchange. o The NAS sends requests to the AN. o The AN processes these requests, authorizes the actions and responds with the appropriate answer. In order to fulfill the commands it might be necessary for the AN to communicate with the HGW or other nodes, for example as part of a keep alive mechanism. o The AN provides status reports to the NAS. Attackers can be: o off-path, i.e., it cannot see packets between the AN and the NAS; o on-path, i.e., they can see the message exchange between the AN and the NAS. Moustafa, et al. Expires April 16, 2007 [Page 6] Internet-Draft ANCP Threats October 2006 Both off-path and on-path attackers can be: o passive, i.e., they do not participate in the network but rather listen to all transfer to obtain the maximum possible information; o active, i.e., they participate to the network and can inject falsify packets. We assume the following threat model: o An off-path adversary located at the CPE or the HGW. o An off-path adversary located on the Internet or a regional network that connects one or more NAS and associated Access Networks to Network Service Providers (NSPs) and Application Service Providers (ASPs). o An on-path adversary located at network elements between the AN and the NAS. o An adversary that took control over the NAS. o An adversary that took control over the AN. 4. Objectives of Attackers Attackers may direct their efforts either against an individual entity or against a large portion of the access network. Attacks against an individual entity fall into three classes: o attacks to disrupt the communication for individual customers o attacks to disrupt the communication of a large fraction of customers in an access network o attacks to grain profit for the attacker (e.g., by modifying the QoS settings) Attackers against the access network or a portion of it fall into three classes: o attacks to disrupt the network services o attacks to destruct the network functioning o attacks to intercept subscribers-related data to increase the QoS of a subscriber(e.g., by replaying old packets, an attacker can configure a better QoS profile on its own DSL line increasing its own benefit) 5. Potential Attacks 5.1. Message Modification This type of threat involves integrity violations, whereby an adversary modifies signaling messages (e.g., by acting as a man-in- the-middle) in order to cause unexpected network behavior. Possible actions an adversary might consider for its attack are reordering, Moustafa, et al. Expires April 16, 2007 [Page 7] Internet-Draft ANCP Threats October 2006 delaying, dropping, injecting, truncating, and otherwise modifying messages. An adversary might, for example, inject a signaling message to request allocation of QoS resources. As a consequence, other user's traffic might be impacted. 5.2. Replay of Signaling Messages This threat scenario covers the case in which an adversary eavesdrops, collects signaling messages, and replays them at a later time (or at a different place, or uses parts of them at a different place or in a different way; e.g., cut-and-paste attacks). Doing this an adversary might mount man-in-the-middle, denial of service, and theft of service attacks. 5.3. Denial of Service Attacks A number of denial of service (DoS) attacks can cause ANCP nodes to malfunction. Other attacks that could lead to DoS, such as man-in- the-middle attacks, replay attacks, and injection or modification of signaling messages, etc., are mentioned throughout this document. When state is established or certain functions are performed without requiring prior authorization there is a chance to mount denial of service attacks. An adversary can utilize this fact to transmit a large number of signaling messages to allocate state at nodes and to cause resource consumption. When the ANCP allows the AN to dynamically discovery the NAS then a man-in-the-middle vulnerablity is introduced. An adversary can use the discovery mechanisms to convince one entity to signal information to another entity, or to cause the discovery process to fail. In the first case, the signaling protocol could appear to continue correctly, except that a number of ANs might contact a single NAS or a wrong NAS. For the AN this could mean that the protocol failed for unknown reasons. Faked Error or Response Messages: An adversary may be able to inject false error or response messages as part of a DoS attack. This could be at the signaling protocol level, at the level of the specific service parameters (e.g., QoS information), or the transport layer. An adversary might cause unexpected protocol behavior or might succeed with a DoS attack. The discovery protocol, especially, exhibits vulnerabilities with regard to this threat scenario (see the above discussion on discovery). Moustafa, et al. Expires April 16, 2007 [Page 8] Internet-Draft ANCP Threats October 2006 5.4. Traffic Analysis This section covers threats whereby an adversary is able to eavesdrop on signaling messages. The signaling packets collected may allow traffic analysis or be used later to mount replay attacks. The eavesdropper might learn QoS parameters, communication patterns, policy rules for firewall traversal, policy information, application identifiers, user identities, NAT bindings, authorization objects, network configuration and performance information, and more. 5.5. Downgrading Attack Protocols may be useful in a variety of scenarios with different security and functional requirements. Different parts of a network (e.g., within a building, across a public carrier's network, or over a private microwave link) may need different levels of protection. It is often difficult to meet these (sometimes conflicting) requirements with a single mechanism or fixed set of parameters, so often a selection of mechanisms and parameters is offered. Therefore, a protocol is required to agree on certain (security) mechanisms and parameters. An insecure parameter exchange or security negotiation protocol can help an adversary to mount a downgrading attack to force selection of mechanisms weaker than those mutually desired. Thus, without binding the negotiation process to the legitimate parties and protecting it, ANCP might only be as secure as the weakest mechanism provided (e.g., weak authentication), and the benefits of defining configuration parameters and a negotiation protocol are lost. 5.6. Man-in-the-Middle Attack An adversary might claims to be an NAS or a AN to acts a man-in-the- middle to later initiate faked configuration parameters and to flood other nodes with signaling messages. The consequence can range from DoS to fraud. 5.7. Network Snooping An adversary, a sniffer, can be placed at the NAS or the AN or any other network element between the AN and the NAS capturing all traversed packets in this network portion. This can be an effective way for Traffic Analysis mentioned in Section 5.4. Adversaries can carryout Network Snooping to gather information relevant to the network and then use this information in gaining unauthorized access. This attack is also called Sniffing and it can help adversaries in other malicious purposes, as for example capturing control messages sent from the AN to the NAS announcing that a DSL line is up and containing some information related to the connected client, Moustafa, et al. Expires April 16, 2007 [Page 9] Internet-Draft ANCP Threats October 2006 indicating the client's existence at home. 6. Attacks Against ANCP Defined Use Cases ANCP is susceptible to security threats, causing disruption/ unauthorized access to network services, manipulation of the transferred data, and interference with network functions. Based on the threat model given in Section 3 and the potential attacks presented in Section 5, this section describes the possible attacks for the 4 ANCP use cases defined in [I-D.ooghe-ancp-framework]. Although ANCP protocol is not involved in the communication between the NAS and the AAA/policy server, the secure communication between the NAS and the AAA/policy server is important for ANCP security. Since this point is out-of-scope of ANCP, it is not discussed in this document. 6.1. Dynamic Access Loop Attributes This use case concerns the communication of Access Loop attributes for dynamic access line topology discovery. Since the Access Loop rate may change overtime, advertisement is beneficial to the NAS to gain knowledge about the topology of the access network for QoS scheduling. Besides data rates and Access Loop links identification, other information may also be transferred from the AN to the NAS (examples in case of DSL Access Loop are: DSL Type, Maximum achievable data rate, and maximum data rate configured for the Access Loop). This use case is thus vulnerable to a number of on-path and off-path attacks that can be either active or passive. On-path attacks can take place as follows: o Between the AN and the NAS during the Access Loop attributes transfer. These attacks may be: i) Passive, only learning these attributes. The main attacks here are caused by network snooping through capturing information about the clients'connection state and thus impacting their privacy protection, or traffic analysis that can be used in later unauthorized access. ii) Active, acting on the transferred attributes and/or injecting falsify packets. Man-in-the- middle attack is a possible major attack in such case causing Access Loop attributes transfer between a forged AN or a forged NAS which can directly cause faked attributes, message modification, and DoS through signaling replay. Off-path attacks can take place as follows: Moustafa, et al. Expires April 16, 2007 [Page 10] Internet-Draft ANCP Threats October 2006 o On the Internet affecting the Access Loop attributes sharing between the NAS and the policy server. Those attacks may be: i) Passive as eavesdropping, traffic analysis, and network snooping gaining information of the access loop attributes that can be used later in initiating replay attacks or unauthorized access to the NAS or the policy server. ii) Active as DoS through flooding the communication links to the policy server causing service disruption, and man-in-the-middle attack causing access loop configuration data retrieval from the policy server by an illegitimate NAS. 6.2. Access Loop Configuration This use case concerns the dynamic local loop line configuration through allowing the NAS to change the access loop parameters (e.g. rate) in a dynamic fashion. This allows for centralized subcriber- related service data. This dynamic configuration can be achieved for instance through profiles that are pre-configured on ANs. This use case is vulnerable to the following attacks: o Downgrading attack is possible during the update of the Access Loop configuration, where an on-path adversary on the NAS, the AN or between them can actively force other parameters than the selected ones. o DoS attacks can take place by an on-path active attacker, through replaying of the Configure Request messages. o Damaging clients' profiles at ANs can take place by on-path active hackers that gained control on the network through discovery of users information from a previously network snooping. o On-path active adversary can replay old packets related to a privileged client profile (having more services), so that to configure this profile on its own DSL line which is less privileged. In order that the attacker do not expose its identity, it may also replay packets related to the privileged client profile to configure a number of illegitimate DSL lines. o Off-path passive adversaries on the Internet can exert eaves- dropping during the Access Loop configuration retrieval by the NAS from the policy server. o Off-path active adversary on the Internet can threaten the centralized subscribers-related service data in the policy server, through for instance making subscribers records inaccessible. 6.3. Remote Connectivity Test In this use case, the NAS can carryout Remote Connectivity Test using ANCP to initiate an Access Loop test between the AN and the HGW. Thus, multiple Access Loop technologies can be supported. This use case is vulnerable to the following attacks, where most of the Moustafa, et al. Expires April 16, 2007 [Page 11] Internet-Draft ANCP Threats October 2006 attacks in this use case concern the network functionality: o On-path Man-in-the-middle attack can occur during the NAS triggering to the AN to carryout the test. An active adversary can inject falsify signals instead or can truncate the triggering. o On-path Man-in-the-middle attack can take place during the Subscriber Response message transfer from the AN to the NAS announcing the test results. o Off-path DoS attack can take place, in case of ATM based Access Loop, when the AN generates loopback cells during the Access Loop test, by an active adversary replaying these generated cells. Message truncating can also occur by an off-path active adversary, leading to service disruption due to test failures assumption. 6.4. Multicast In this use case, ANCP could be used in exchanging information between the AN and the NAS allowing the AN to perform replication inline with the policy and configuration of the subscriber. Also, this allows the NAS to follow each subscriber's multicast group memebership. Attacks that can occur in this case are mostly on-path active attacks, which are as follows: o Damaging proxy functionality in the AN, aggregation node(s) or the NAS through DoS or through signaling truncating. o DoS during the information exchange between the NAS and the AN on the subscriber's policy and multicast traffic configuration. o Man-in-the-middle attack during the multicast replication process at the AN, aggregation node(s) and the NAS that can cause modification of the multicast group memebership either for service disruption or for adversary benefit (e.g. subscriber's policy illegitimate change). 7. Security Requirements The following list represents a list of requirements motivated by the threats in Section 5: o The protocol solution MUST offer authentication of the AN to the NAS. o The protocol solution MUST offer authentication of the NAS to the AN. o The protocol solution MUST allow authorization to take place at the NAS and the AN. o The protocol solution MUST offer replay protection. o The protocol solution MUST provide data origin authentication. o The protocol solution SHOULD offer confidentiality protection. o The protocol solution MUST be robust against denial of service attacks. Moustafa, et al. Expires April 16, 2007 [Page 12] Internet-Draft ANCP Threats October 2006 o The protocol solution SHOULD provide mutual authentication between different communicating entities. o The protocol solution SHOULD distinguish the control messages from the data. o The protocol solution SHOULD provide privacy protection. 8. Security Considerations This document focuses on security threats deriving a threat model for ANCP and presenting the security requirements to be considered. 9. IANA Considerations This document does not require actions by IANA. 10. Acknowledgment The authors would like to thank Antoine Delafoy and Philippe Niger for their useful comments. 11. References 11.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", March 1997. 11.2. Informative References [I-D.ooghe-ancp-framework] Ooghe, S., "Framework and Requirements for an Access Node Control Mechanism in Broadband Multi-Service Networks", draft-ooghe-ancp-framework-00 (work in progress), June 2006. Moustafa, et al. Expires April 16, 2007 [Page 13] Internet-Draft ANCP Threats October 2006 Authors' Addresses Hassnaa Moustafa France Telecom 38-40 rue du General Leclerc Issy Les Moulineaux, 92794 Cedex 9 France Email: hassnaa.moustafa@orange-ftgroup.com Hannes Tschofenig Siemens Otto-Hahn-Ring 6 Munich, Bavaria 81739 Germany Email: Hannes.Tschofenig@siemens.com URI: http://www.tschofenig.com Stefaan De Cnodder Alcatel Copernicuslaan 50 B-2018 Antwerp, Belgium Phone: +32 3 240 85 15 Email: stefaan.de_cnodder@alcatel.be Moustafa, et al. Expires April 16, 2007 [Page 14] Internet-Draft ANCP Threats October 2006 Full Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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