4. Handoff Vulnerabilities A handoff occurs when an EAP peer moves to a new authenticator. Several mechanisms have been proposed for reducing handoff latency within networks utilizing EAP. These include: EAP pre-authentication In EAP pre-authentication, an EAP peer pre-establishes EAP keying material with an authenticator prior to arrival. EAP pre-authentication only affects the timing of EAP authentication, but does not shorten or eliminate EAP (phase 1a) or AAA (phase 1b) exchanges; Discovery (phase 0) and Secure Association Protocol (phase 2) exchanges occur as described in Section 1.3. As a result, the primary benefit is to enable EAP authentication to be removed from the handoff critical path, thereby reducing latency. Use of EAP pre-authentication within IEEE 802.11 is described in [IEEE-802.11] and [8021XPreAuth].
Proactive key distribution In proactive key distribution, keying material and authorizations are transported from the backend authentication server to a candidate authenticator in advance of a handoff. As a result, EAP (phase 1a) is not needed, but the Discovery (phase 0), and Secure Association Protocol exchanges (phase 2) are still necessary. Within the AAA exchange (phase 1b), authorization and key distribution functions are typically supported, but not authentication. Proactive key distribution is described in [MishraPro], [IEEE-03-084], and [HANDOFF]. Key caching Caching of EAP keying material enables an EAP peer to re-attach to an authenticator without requiring EAP (phase 1a) or AAA (phase 1b) exchanges. However, Discovery (phase 0) and Secure Association Protocol (phase 2) exchanges are still needed. Use of key caching within IEEE 802.11 is described in [IEEE-802.11]. Context transfer In context transfer schemes, keying material and authorizations are transferred between a previous authenticator and a new authenticator. This can occur in response to a handoff request by the EAP peer, or in advance, as in proactive key distribution. As a result, EAP (phase 1a) is eliminated, but not the Discovery (phase 0) or Secure Association Protocol exchanges (phase 2). If a secure channel can be established between the new and previous authenticator without assistance from the backend authentication server, then the AAA exchange (phase 1b) can be eliminated; otherwise, it is still needed, although it can be shortened. Context transfer protocols are described in [IEEE-802.11F] (now deprecated) and "Context Transfer Protocol (CXTP)" [RFC4067]. "Fast Authentication Methods for Handovers between IEEE 802.11 Wireless LANs" [Bargh] analyzes fast handoff techniques, including context transfer mechanisms.
Token distribution In token distribution schemes, the EAP peer is provided with a credential, subsequently enabling it to authenticate with one or more additional authenticators. During the subsequent authentications, EAP (phase 1a) is eliminated or shortened; the Discovery (phase 0) and Secure Association Protocol exchanges (phase 2) still occur, although the latter can be shortened. If the token includes authorizations and can be validated by an authenticator without assistance from the backend authentication server, then the AAA exchange (phase 1b) can be eliminated; otherwise, it is still needed, although it can be shortened. Token-based schemes, initially proposed in early versions of IEEE 802.11i [IEEE-802.11i], are described in [Token], [Tokenk], and [SHORT-TERM]. The sections that follow discuss the security vulnerabilities introduced by the above schemes. 4.1. EAP Pre-Authentication EAP pre-authentication differs from a normal EAP conversation primarily with respect to the lower-layer encapsulation. For example, in [IEEE-802.11], EAP pre-authentication frames utilize a distinct Ethertype, but otherwise conforms to the encapsulation described in [IEEE-802.1X]. As a result, an EAP pre-authentication conversation differs little from the model described in Section 1.3, other than the introduction of a delay between phase 1 and phase 2. EAP pre-authentication relies on lower-layer mechanisms for discovery of candidate authenticators. Where discovery can provide information on candidate authenticators outside the immediate listening range, and the peer can determine whether it already possesses valid EAP keying material with candidate authenticators, the peer can avoid unnecessary EAP pre-authentications and can establish EAP keying material well in advance, regardless of the coverage overlap between authenticators. However, if the peer can only discover candidate authenticators within listening range and cannot determine whether it can reuse existing EAP keying material, then it is possible that the peer will not be able to complete EAP pre-authentication prior to connectivity loss or that it can pre-authenticate multiple times with the same authenticator, increasing backend authentication server load. Since a peer can complete EAP pre-authentication with an authenticator without eventually attaching to it, it is possible that phase 2 will not occur. In this case, an Accounting-Request signifying the start of service will not be sent, or will only be sent with a substantial delay after the completion of authentication.
As noted in "IEEE 802.1X RADIUS Usage Guidelines" [RFC3580], the AAA exchange resulting from EAP pre-authentication differs little from an ordinary exchange described in "RADIUS Support for EAP" [RFC3579]. For example, since in IEEE 802.11 [IEEE-802.11] an Association exchange does not occur prior to EAP pre-authentication, the SSID is not known by the authenticator at authentication time, so that an Access-Request cannot include the SSID within the Called-Station-Id attribute as described in [RFC3580] Section 3.20. Since only the absence of an SSID in the Called-Station-Id attribute distinguishes an EAP pre-authentication attempt, if the authenticator does not always include the SSID for a normal EAP authentication attempt, it is possible that the backend authentication server will not be able to determine whether a session constitutes an EAP pre-authentication attempt, potentially resulting in authorization or accounting problems. Where the number of simultaneous sessions is limited, the backend authentication server can refuse to authorize a valid EAP pre-authentication attempt or can enable the peer to engage in more simultaneous sessions than they are authorized for. Where EAP pre-authentication occurs with an authenticator which the peer never attaches to, it is possible that the backend accounting server will not be able to determine whether the absence of an Accounting-Request was due to packet loss or a session that never started. In order to enable pre-authentication requests to be handled more reliably, it is RECOMMENDED that AAA protocols explicitly identify EAP pre-authentication. In order to suppress unnecessary EAP pre-authentication exchanges, it is RECOMMENDED that authenticators unambiguously identify themselves as described in Section 2.3. 4.2. Proactive Key Distribution In proactive key distribution schemes, the backend authentication server transports keying material and authorizations to an authenticator in advance of the arrival of the peer. The authenticators selected to receive the transported key material are selected based on past patterns of peer movement between authenticators known as the "neighbor graph". In order to reduce handoff latency, proactive key distribution schemes typically only demonstrate proof of possession of transported keying material between the EAP peer and authenticator. During a handoff, the backend authentication server is not provided with proof that the peer successfully authenticated to an authenticator; instead, the authenticator generates a stream of accounting messages without a corresponding set of authentication exchanges. As described in [MishraPro], knowledge of the neighbor graph can be established via static configuration or analysis of authentication exchanges. In
order to prevent corruption of the neighbor graph, new neighbor graph entries can only be created as the result of a successful EAP exchange, and accounting packets with no corresponding authentication exchange need to be verified to correspond to neighbor graph entries (e.g., corresponding to handoffs between neighbors). In order to prevent compromise of one authenticator from resulting in compromise of other authenticators, cryptographic separation needs to be maintained between the keying material transported to each authenticator. However, even where cryptographic separation is maintained, an attacker compromising an authenticator can still disrupt the operation of other authenticators. As noted in [RFC3579] Section 4.3.7, in the absence of spoofing detection within the AAA infrastructure, it is possible for EAP authenticators to impersonate each other. By forging NAS identification attributes within authentication messages, an attacker compromising one authenticator could corrupt the neighbor graph, tricking the backend authentication server into transporting keying material to arbitrary authenticators. While this would not enable recovery of EAP keying material without breaking fundamental cryptographic assumptions, it could enable subsequent fraudulent accounting messages, or allow an attacker to disrupt service by increasing load on the backend authentication server or thrashing the authenticator key cache. Since proactive key distribution requires the distribution of derived keying material to candidate authenticators, the effectiveness of this scheme depends on the ability of backend authentication server to anticipate the movement of the EAP peer. Since proactive key distribution relies on backend authentication server knowledge of the neighbor graph, it is most applicable to intra-domain handoff scenarios. However, in inter-domain handoff, where there can be many authenticators, peers can frequently connect to authenticators that have not been previously encountered, making it difficult for the backend authentication server to derive a complete neighbor graph. Since proactive key distribution schemes typically require introduction of server-initiated messages as described in [RFC5176] and [HANDOFF], security issues described in [RFC5176] Section 6 are applicable, including authorization (Section 6.1) and replay detection (Section 6.3) problems.
4.3. AAA Bypass Fast handoff techniques that enable elimination of the AAA exchange (phase 1b) differ fundamentally from typical network access scenarios (dial-up, wired LAN, etc.) that include user authentication as well as authorization for the offered service. Where the AAA exchange (phase 1b) is omitted, authorizations and keying material are not provided by the backend authentication server, and as a result, they need to be supplied by other means. This section describes some of the implications. 4.3.1. Key Transport Where transported keying material is not supplied by the backend authentication server, it needs to be provided by another party authorized to access that keying material. As noted in Section 1.5, only the EAP peer, authenticator, and server are authorized to possess transported keying material. Since EAP peers do not trust each other, if the backend authentication server does not supply transported keying material to a new authenticator, it can only be provided by a previous authenticator. As noted in Section 1.5, the goal of the EAP conversation is to derive session keys known only to the peer and the authenticator. If keying material is replicated between a previous authenticator and a new authenticator, then the previous authenticator can possess session keys used between the peer and new authenticator. Also, the new authenticator can possess session keys used between the peer and the previous authenticator. If a one-way function is used to derive the keying material to be transported to the new authenticator, then the new authenticator cannot possess previous session keys without breaking a fundamental cryptographic assumption. 4.3.2. Authorization As a part of the authentication process, the backend authentication server determines the user's authorization profile and transmits the authorizations to the authenticator along with the transported keying material. Typically, the profile is determined based on the user identity, but a certificate presented by the user can also provide authorization information. The backend authentication server is responsible for making a user authorization decision, which requires answering the following questions:
(a) Is this a legitimate user of this network? (b) Is the user allowed to access this network? (c) Is the user permitted to access this network on this day and at this time? (d) Is the user within the concurrent session limit? (e) Are there any fraud, credit limit, or other concerns that could lead to access denial? (f) If access is to be granted, what are the service parameters (mandatory tunneling, bandwidth, filters, and so on) to be provisioned for the user? While the authorization decision is, in principle, simple, the distributed decision making process can add complexity. Where brokers or proxies are involved, all of the AAA entities in the chain from the authenticator to the home backend authentication server are involved in the decision. For example, a broker can deny access even if the home backend authentication server would allow it, or a proxy can add authorizations (e.g., bandwidth limits). Decisions can be based on static policy definitions and profiles as well as dynamic state (e.g., time of day or concurrent session limits). In addition to the Accept/Reject decisions made by AAA entities, service parameters or constraints can be communicated to the authenticator. The criteria for Accept/Reject decisions or the reasons for choosing particular authorizations are typically not communicated to the authenticator, only the final result is. As a result, the authenticator has no way to know on what the decision was based. Was a set of authorization parameters sent because this service is always provided to the user, or was the decision based on the time of day and the capabilities of the authenticator? 4.3.3. Correctness When the AAA exchange (phase 1b) is bypassed, several challenges arise in ensuring correct authorization: Theft of service Bypassing the AAA exchange (phase 1b) SHOULD NOT enable a user to extend their network access or gain access to services they are not entitled to.
Consideration of network-wide state Handoff techniques SHOULD NOT render the backend authentication server incapable of keeping track of network-wide state. For example, a backend authentication server can need to keep track of simultaneous user sessions. Elevation of privilege Backend authentication servers often perform conditional evaluation, in which the authorizations returned in an Access-Accept message are contingent on the authenticator or on dynamic state such as the time of day. In this situation, bypassing the AAA exchange could enable unauthorized access unless the restrictions are explicitly encoded within the authorizations provided by the backend authentication server. A handoff mechanism that provides proper authorization is said to be "correct". One condition for correctness is as follows: For a handoff to be "correct" it MUST establish on the new authenticator the same authorizations as would have been created had the new authenticator completed a AAA conversation with the backend authentication server. A properly designed handoff scheme will only succeed if it is "correct" in this way. If a successful handoff would establish "incorrect" authorizations, it is preferable for it to fail. Where the supported services differ between authenticators, a handoff that bypasses the backend authentication server is likely to fail. Section 1.1 of [RFC2865] states: A authenticator that does not implement a given service MUST NOT implement the RADIUS attributes for that service. For example, a authenticator that is unable to offer ARAP service MUST NOT implement the RADIUS attributes for ARAP. A authenticator MUST treat a RADIUS access-accept authorizing an unavailable service as an access-reject instead. This behavior applies to attributes that are known, but not implemented. For attributes that are unknown, Section 5 of [RFC2865] states: A RADIUS server MAY ignore Attributes with an unknown Type. A RADIUS client MAY ignore Attributes with an unknown Type. In order to perform a correct handoff, if a new authenticator is provided with RADIUS authorizations for a known but unavailable service, then it MUST process these authorizations the same way it would handle a RADIUS Access-Accept requesting an unavailable
service; this MUST cause the handoff to fail. However, if a new authenticator is provided with authorizations including unknown attributes, then these attributes MAY be ignored. The definition of a "known but unsupported service" MUST encompass requests for unavailable security services. This includes vendor-specific attributes related to security, such as those described in [RFC2548]. Although it can seem somewhat counter-intuitive, failure is indeed the "correct" result where a known but unsupported service is requested. Presumably, a correctly configured backend authentication server would not request that an authenticator provide a service that it does not implement. This implies that if the new authenticator were to complete a AAA conversation, it would be likely to receive different service instructions. Failure of the handoff is the desired result since it will cause the new authenticator to go back to the backend server in order to receive the appropriate service definition. Handoff mechanisms that bypass the backend authentication server are most likely to be successful when employed in a homogeneous deployment within a single administrative domain. In a heterogeneous deployment, the backend authentication server can return different authorizations depending on the authenticator making the request in order to make sure that the requested service is consistent with the authenticator capabilities. Where a backend authentication server would send different authorizations to the new authenticator than were sent to a previous authenticator, transferring authorizations between the previous authenticator and the new authenticator will result in incorrect authorization. Virtual LAN (VLAN) support is defined in [IEEE-802.1Q]; RADIUS support for dynamic VLANs is described in [RFC3580] and [RFC4675]. If some authenticators support dynamic VLANs while others do not, then attributes present in the Access-Request (such as the NAS-Port-Type, NAS-IP-Address, NAS-IPv6-Address, and NAS-Identifier) could be examined by the backend authentication server to determine when VLAN attributes will be returned, and if so, which ones. However, if the backend authenticator is bypassed, then a handoff occurring between authenticators supporting different VLAN capabilities could result in a user obtaining access to an unauthorized VLAN (e.g., a user with access to a guest VLAN being given unrestricted access to the network).
Similarly, it is preferable for a handoff between an authenticator providing confidentiality and another that does not to fail, since if the handoff were successful, the user would be moved from a secure to an insecure channel without permission from the backend authentication server. 5. Security Considerations The EAP threat model is described in [RFC3748] Section 7.1. The security properties of EAP methods (known as "security claims") are described in [RFC3748] Section 7.2.1. EAP method requirements for applications such as Wireless LAN authentication are described in [RFC4017]. The RADIUS threat model is described in [RFC3579] Section 4.1, and responses to these threats are described in [RFC3579], Sections 4.2 and 4.3. However, in addition to threats against EAP and AAA, there are other system level threats. In developing the threat model, it is assumed that: All traffic is visible to the attacker. The attacker can alter, forge, or replay messages. The attacker can reroute messages to another principal. The attacker can be a principal or an outsider. The attacker can compromise any key that is sufficiently old. Threats arising from these assumptions include: (a) An attacker can compromise or steal an EAP peer or authenticator, in an attempt to gain access to other EAP peers or authenticators or to obtain long-term secrets. (b) An attacker can attempt a downgrade attack in order to exploit known weaknesses in an authentication method or cryptographic algorithm. (c) An attacker can try to modify or spoof packets, including Discovery or Secure Association Protocol frames, EAP or AAA packets. (d) An attacker can attempt to induce an EAP peer, authenticator, or server to disclose keying material to an unauthorized party, or utilize keying material outside the context that it was intended for. (e) An attacker can alter, forge, or replay packets.
(f) An attacker can cause an EAP peer, authenticator, or server to reuse a stale key. Use of stale keys can also occur unintentionally. For example, a poorly implemented backend authentication server can provide stale keying material to an authenticator, or a poorly implemented authenticator can reuse nonces. (g) An authenticated attacker can attempt to obtain elevated privilege in order to access information that it does not have rights to. (h) An attacker can attempt a man-in-the-middle attack in order to gain access to the network. (i) An attacker can compromise an EAP authenticator in an effort to commit fraud. For example, a compromised authenticator can provide incorrect information to the EAP peer and/or server via out-of-band mechanisms (such as via a AAA or lower-layer protocol). This includes impersonating another authenticator, or providing inconsistent information to the peer and EAP server. (j) An attacker can launch a denial-of-service attack against the EAP peer, authenticator, or backend authentication server. In order to address these threats, [RFC4962] Section 3 describes required and recommended security properties. The sections that follow analyze the compliance of EAP methods, AAA protocols, and Secure Association Protocols with those guidelines. 5.1. Peer and Authenticator Compromise Requirement: In the event that an authenticator is compromised or stolen, an attacker can gain access to the network through that authenticator, or can obtain the credentials needed for the authenticator/AAA client to communicate with one or more backend authentication servers. Similarly, if a peer is compromised or stolen, an attacker can obtain credentials needed to communicate with one or more authenticators. A mandatory requirement from [RFC4962] Section 3: Prevent the Domino effect Compromise of a single peer MUST NOT compromise keying material held by any other peer within the system, including session keys and long-term keys. Likewise, compromise of a single authenticator MUST NOT compromise keying material held by any other authenticator within the system. In the context of a key
hierarchy, this means that the compromise of one node in the key hierarchy must not disclose the information necessary to compromise other branches in the key hierarchy. Obviously, the compromise of the root of the key hierarchy will compromise all of the keys; however, a compromise in one branch MUST NOT result in the compromise of other branches. There are many implications of this requirement; however, two implications deserve highlighting. First, the scope of the keying material must be defined and understood by all parties that communicate with a party that holds that keying material. Second, a party that holds keying material in a key hierarchy must not share that keying material with parties that are associated with other branches in the key hierarchy. Group keys are an obvious exception. Since all members of the group have a copy of the same key, compromise of any one of the group members will result in the disclosure of the group key. Some of the implications of the requirement are as follows: Key Sharing In order to be able to determine whether keying material has been shared, it is necessary for the identity of the EAP authenticator (NAS-Identifier) to be defined and understood by all parties that communicate with it. EAP lower-layer specifications such as [IEEE-802.11], [IEEE-802.16e], [IEEE-802.1X], IKEv2 [RFC4306], and PPP [RFC1661] do not involve key sharing. AAA Credential Sharing AAA credentials (such as RADIUS shared secrets, IPsec pre-shared keys or certificates) MUST NOT be shared between AAA clients, since if one AAA client were compromised, this would enable an attacker to impersonate other AAA clients to the backend authentication server, or even to impersonate a backend authentication server to other AAA clients. Compromise of Long-Term Credentials An attacker obtaining keying material (such as TSKs, TEKs, or the MSK) MUST NOT be able to obtain long-term user credentials such as pre-shared keys, passwords, or private-keys without breaking a fundamental cryptographic assumption. The mandatory requirements of [RFC4017] Section 2.2 include generation of EAP keying material, capability to generate EAP keying material with 128 bits of effective strength, resistance to dictionary attacks, shared state equivalence, and protection against man-in-the-middle attacks.
5.2. Cryptographic Negotiation Mandatory requirements from [RFC4962] Section 3: Cryptographic algorithm independent The AAA key management protocol MUST be cryptographic algorithm independent. However, an EAP method MAY depend on a specific cryptographic algorithm. The ability to negotiate the use of a particular cryptographic algorithm provides resilience against compromise of a particular cryptographic algorithm. Algorithm independence is also REQUIRED with a Secure Association Protocol if one is defined. This is usually accomplished by including an algorithm identifier and parameters in the protocol, and by specifying the algorithm requirements in the protocol specification. While highly desirable, the ability to negotiate key derivation functions (KDFs) is not required. For interoperability, at least one suite of mandatory-to-implement algorithms MUST be selected. Note that without protection by IPsec as described in [RFC3579] Section 4.2, RADIUS [RFC2865] does not meet this requirement, since the integrity protection algorithm cannot be negotiated. This requirement does not mean that a protocol must support both public-key and symmetric-key cryptographic algorithms. It means that the protocol needs to be structured in such a way that multiple public-key algorithms can be used whenever a public-key algorithm is employed. Likewise, it means that the protocol needs to be structured in such a way that multiple symmetric-key algorithms can be used whenever a symmetric-key algorithm is employed. Confirm ciphersuite selection The selection of the "best" ciphersuite SHOULD be securely confirmed. The mechanism SHOULD detect attempted roll-back attacks. EAP methods satisfying [RFC4017] Section 2.2 mandatory requirements and AAA protocols utilizing transmission-layer security are capable of addressing downgrade attacks. [RFC3748] Section 7.2.1 describes the "protected ciphersuite negotiation" security claim that refers to the ability of an EAP method to negotiate the ciphersuite used to protect the EAP method conversation, as well as to integrity protect the ciphersuite negotiation. [RFC4017] Section 2.2 requires EAP methods satisfying this security claim. Since TLS v1.2 [RFC5246] and IKEv2 [RFC4306] support negotiation of Key Derivation Functions (KDFs), EAP methods based on TLS or IKEv2 will, if properly designed,
inherit this capability. However, negotiation of KDFs is not required by RFC 4962 [RFC4962], and EAP methods based on neither TLS nor IKEv2 typically do not support KDF negotiation. When AAA protocols utilize TLS [RFC5246] or IPsec [RFC4301] for transmission layer security, they can leverage the cryptographic algorithm negotiation support provided by IKEv2 [RFC4306] or TLS [RFC5246]. RADIUS [RFC3579] by itself does not support cryptographic algorithm negotiation and relies on MD5 for integrity protection, authentication, and confidentiality. Given the known weaknesses in MD5 [MD5Collision], this is undesirable, and can be addressed via use of RADIUS over IPsec, as described in [RFC3579] Section 4.2. To ensure against downgrade attacks within lower-layer protocols, algorithm independence is REQUIRED with lower layers using EAP for key derivation. For interoperability, at least one suite of mandatory-to-implement algorithms MUST be selected. Lower-layer protocols supporting EAP for key derivation SHOULD also support secure ciphersuite negotiation as well as KDF negotiation. As described in [RFC1968], PPP ECP does not support secure ciphersuite negotiation. While [IEEE-802.16e] and [IEEE-802.11] support ciphersuite negotiation for protection of data, they do not support negotiation of the cryptographic primitives used within the Secure Association Protocol, such as message integrity checks (MICs) and KDFs. 5.3. Confidentiality and Authentication Mandatory requirements from [RFC4962] Section 3: Authenticate all parties Each party in the AAA key management protocol MUST be authenticated to the other parties with whom they communicate. Authentication mechanisms MUST maintain the confidentiality of any secret values used in the authentication process. When a secure association protocol is used to establish session keys, the parties involved in the secure association protocol MUST identify themselves using identities that are meaningful in the lower-layer protocol environment that will employ the session keys. In this situation, the authenticator and peer may be known by different identifiers in the AAA protocol environment and the lower-layer protocol environment, making authorization decisions difficult without a clear key scope. If the lower-layer identifier of the
peer will be used to make authorization decisions, then the pair of identifiers associated with the peer MUST be authorized by the authenticator and/or the AAA server. AAA protocols, such as RADIUS [RFC2865] and Diameter [RFC3588], provide a mechanism for the identification of AAA clients; since the EAP authenticator and AAA client are always co-resident, this mechanism is applicable to the identification of EAP authenticators. When multiple base stations and a "controller" (such as a WLAN switch) comprise a single EAP authenticator, the "base station identity" is not relevant; the EAP method conversation takes place between the EAP peer and the EAP server. Also, many base stations can share the same authenticator identity. The authenticator identity is important in the AAA protocol exchange and the secure association protocol conversation. Authentication mechanisms MUST NOT employ plaintext passwords. Passwords may be used provided that they are not sent to another party without confidentiality protection. Keying material confidentiality and integrity While preserving algorithm independence, confidentiality and integrity of all keying material MUST be maintained. Conformance to these requirements is analyzed in the sections that follow. 5.3.1. Spoofing Per-packet authentication and integrity protection provides protection against spoofing attacks. Diameter [RFC3588] provides support for per-packet authentication and integrity protection via use of IPsec or TLS. RADIUS/EAP [RFC3579] provides for per-packet authentication and integrity protection via use of the Message-Authenticator Attribute. [RFC3748] Section 7.2.1 describes the "integrity protection" security claim and [RFC4017] Section 2.2 requires EAP methods supporting this claim. In order to prevent forgery of Secure Association Protocol frames, per-frame authentication and integrity protection is RECOMMENDED on all messages. IKEv2 [RFC4306] supports per-frame integrity
protection and authentication, as does the Secure Association Protocol defined in [IEEE-802.16e]. [IEEE-802.11] supports per-frame integrity protection and authentication on all messages within the 4-way handshake except the first message. An attack leveraging this omission is described in [Analysis]. 5.3.2. Impersonation Both RADIUS [RFC2865] and Diameter [RFC3588] implementations are potentially vulnerable to a rogue authenticator impersonating another authenticator. While both protocols support mutual authentication between the AAA client/authenticator and the backend authentication server, the security mechanisms vary. In RADIUS, the shared secret used for authentication is determined by the source address of the RADIUS packet. However, when RADIUS Access-Requests are forwarded by a proxy, the NAS-IP-Address, NAS-Identifier, or NAS-IPv6-Address attributes received by the RADIUS server will not correspond to the source address. As noted in [RFC3579] Section 4.3.7, if the first-hop proxy does not check the NAS identification attributes against the source address in the Access-Request packet, it is possible for a rogue authenticator to forge NAS-IP-Address [RFC2865], NAS-IPv6-Address [RFC3162], or NAS-Identifier [RFC2865] attributes in order to impersonate another authenticator; attributes such as the Called-Station-Id [RFC2865] and Calling-Station-Id [RFC2865] can be forged as well. Among other things, this can result in messages (and transported keying material) being sent to the wrong authenticator. While [RFC3588] requires use of the Route-Record AVP, this utilizes Fully Qualified Domain Names (FQDNs), so that impersonation detection requires DNS A, AAAA, and PTR Resource Records (RRs) to be properly configured. As a result, Diameter is as vulnerable to this attack as RADIUS, if not more so. [RFC3579] Section 4.3.7 recommends mechanisms for impersonation detection; to prevent access to keying material by proxies without a "need to know", it is necessary to allow the backend authentication server to communicate with the authenticator directly, such as via the redirect functionality supported in [RFC3588]. 5.3.3. Channel Binding It is possible for a compromised or poorly implemented EAP authenticator to communicate incorrect information to the EAP peer and/or server. This can enable an authenticator to impersonate another authenticator or communicate incorrect information via out-of-band mechanisms (such as via AAA or the lower layer).
Where EAP is used in pass-through mode, the EAP peer does not verify the identity of the pass-through authenticator within the EAP conversation. Within the Secure Association Protocol, the EAP peer and authenticator only demonstrate mutual possession of the transported keying material; they do not mutually authenticate. This creates a potential security vulnerability, described in [RFC3748] Section 7.15. As described in [RFC3579] Section 4.3.7, it is possible for a first-hop AAA proxy to detect a AAA client attempting to impersonate another authenticator. However, it is possible for a pass-through authenticator acting as a AAA client to provide correct information to the backend authentication server while communicating misleading information to the EAP peer via the lower layer. For example, a compromised authenticator can utilize another authenticator's Called-Station-Id or NAS-Identifier in communicating with the EAP peer via the lower layer. Also, a pass-through authenticator acting as a AAA client can provide an incorrect peer Calling-Station-Id [RFC2865] [RFC3580] to the backend authentication server via the AAA protocol. As noted in [RFC3748] Section 7.15, this vulnerability can be addressed by EAP methods that support a protected exchange of channel properties such as endpoint identifiers, including (but not limited to): Called-Station-Id [RFC2865] [RFC3580], Calling-Station-Id [RFC2865] [RFC3580], NAS-Identifier [RFC2865], NAS-IP-Address [RFC2865], and NAS-IPv6-Address [RFC3162]. Using such a protected exchange, it is possible to match the channel properties provided by the authenticator via out-of-band mechanisms against those exchanged within the EAP method. Typically, the EAP method imports channel binding parameters from the lower layer on the peer, and transmits them securely to the EAP server, which exports them to the lower layer or AAA layer. However, transport can occur from EAP server to peer, or can be bi-directional. On the side of the exchange (peer or server) where channel binding is verified, the lower layer or AAA layer passes the result of the verification (TRUE or FALSE) up to the EAP method. While the verification can be done either by the peer or the server, typically only the server has the knowledge to determine the correctness of the values, as opposed to merely verifying their equality. For further discussion, see [EAP-SERVICE]. It is also possible to perform channel binding without transporting data over EAP, as described in [EAP-CHANNEL]. In this approach the EAP method includes channel binding parameters in the calculation of exported EAP keying material, making it impossible for the peer and
authenticator to complete the Secure Association Protocol if there is a mismatch in the channel binding parameters. However, this approach can only be applied where methods generating EAP keying material are used along with lower layers that utilize EAP keying material. For example, this mechanism would not enable verification of channel binding on wired IEEE 802 networks using [IEEE-802.1X]. 5.3.4. Mutual Authentication [RFC3748] Section 7.2.1 describes the "mutual authentication" and "dictionary attack resistance" claims, and [RFC4017] requires EAP methods satisfying these claims. EAP methods complying with [RFC4017] therefore provide for mutual authentication between the EAP peer and server. [RFC3748] Section 7.2.1 also describes the "Cryptographic binding" security claim, and [RFC4017] Section 2.2 requires support for this claim. As described in [EAP-BINDING], EAP method sequences and compound authentication mechanisms can be subject to man-in-the-middle attacks. When such attacks are successfully carried out, the attacker acts as an intermediary between a victim and a legitimate authenticator. This allows the attacker to authenticate successfully to the authenticator, as well as to obtain access to the network. In order to prevent these attacks, [EAP-BINDING] recommends derivation of a compound key by which the EAP peer and server can prove that they have participated in the entire EAP exchange. Since the compound key MUST NOT be known to an attacker posing as an authenticator, and yet must be derived from EAP keying material, it MAY be desirable to derive the compound key from a portion of the EMSK. Where this is done, in order to provide proper key hygiene, it is RECOMMENDED that the compound key used for man-in-the-middle protection be cryptographically separate from other keys derived from the EMSK. Diameter [RFC3588] provides for per-packet authentication and integrity protection via IPsec or TLS, and RADIUS/EAP [RFC3579] also provides for per-packet authentication and integrity protection. Where the authenticator/AAA client and backend authentication server communicate directly and credible key wrap is used (see Section 3.8), this ensures that the AAA Key Transport (phase 1b) achieves its security objectives: mutually authenticating the AAA client/authenticator and backend authentication server and providing transported keying material to the EAP authenticator and to no other party.
[RFC2607] Section 7 describes the security issues occurring when the authenticator/AAA client and backend authentication server do not communicate directly. Where a AAA intermediary is present (such as a RADIUS proxy or a Diameter agent), and data object security is not used, transported keying material can be recovered by an attacker in control of the intermediary. As discussed in Section 2.1, unless the TSKs are derived independently from EAP keying material (as in IKEv2), possession of transported keying material enables decryption of data traffic sent between the peer and the authenticator to whom the keying material was transported. It also allows the AAA intermediary to impersonate the authenticator or the peer. Since the peer does not authenticate to a AAA intermediary, it has no ability to determine whether it is authentic or authorized to obtain keying material. However, as long as transported keying material or keys derived from it are only utilized by a single authenticator, compromise of the transported keying material does not enable an attacker to impersonate the peer to another authenticator. Vulnerability to compromise of a AAA intermediary can be mitigated by implementation of redirect functionality, as described in [RFC3588] and [RFC4072]. The Secure Association Protocol does not provide for mutual authentication between the EAP peer and authenticator, only mutual proof of possession of transported keying material. In order for the peer to verify the identity of the authenticator, mutual proof of possession needs to be combined with impersonation prevention and channel binding. Impersonation prevention (described in Section 5.3.2) enables the backend authentication server to determine that the transported keying material has been provided to the correct authenticator. When utilized along with impersonation prevention, channel binding (described in Section 5.3.3) enables the EAP peer to verify that the EAP server has authorized the authenticator to possess the transported keying material. Completion of the Secure Association Protocol exchange demonstrates that the EAP peer and the authenticator possess the transported keying material. 5.4. Key Binding Mandatory requirement from [RFC4962] Section 3: Bind key to its context Keying material MUST be bound to the appropriate context. The context includes the following: o The manner in which the keying material is expected to be used.
o The other parties that are expected to have access to the keying material. o The expected lifetime of the keying material. Lifetime of a child key SHOULD NOT be greater than the lifetime of its parent in the key hierarchy. Any party with legitimate access to keying material can determine its context. In addition, the protocol MUST ensure that all parties with legitimate access to keying material have the same context for the keying material. This requires that the parties are properly identified and authenticated, so that all of the parties that have access to the keying material can be determined. The context will include the peer and NAS identities in more than one form. One (or more) name form is needed to identify these parties in the authentication exchange and the AAA protocol. Another name form may be needed to identify these parties within the lower layer that will employ the session key. Within EAP, exported keying material (MSK, EMSK,IV) is bound to the Peer-Id(s) and Server-Id(s), which are exported along with the keying material. However, not all EAP methods support authenticated server identities (see Appendix A). Within the AAA protocol, transported keying material is destined for the EAP authenticator identified by the NAS-Identifier Attribute within the request, and is for use by the EAP peer identified by the Peer-Id(s), User-Name [RFC2865], or Chargeable User Identity (CUI) [RFC4372] attributes. The maximum lifetime of the transported keying material can be provided, as discussed in Section 3.5.1. Key usage restrictions can also be included as described in Section 3.2. Key lifetime issues are discussed in Sections 3.3, 3.4, and 3.5.