RFC 3748
Extensible Authentication Protocol (EAP)
Pages: 67
Proposed Standard
→ Errata
Obsoletes: 2284
Updated by: 5247 7057
Proposed Standard
→ Errata
Obsoletes: 2284
Updated by: 5247 7057
Part 3 of 3 – Pages 42 to 67
7. Security Considerations
This section defines a generic threat model as well as the EAP method security claims mitigating those threats. It is expected that the generic threat model and corresponding security claims will used to define EAP method requirements for use in specific environments. An example of such a requirements analysis is provided in [IEEE-802.11i-req]. A security claims section is required in EAP method specifications, so that EAP methods can be evaluated against the requirements.7.1. Threat Model
EAP was developed for use with PPP [RFC1661] and was later adapted for use in wired IEEE 802 networks [IEEE-802] in [IEEE-802.1X]. Subsequently, EAP has been proposed for use on wireless LAN networks and over the Internet. In all these situations, it is possible for an attacker to gain access to links over which EAP packets are transmitted. For example, attacks on telephone infrastructure are documented in [DECEPTION]. An attacker with access to the link may carry out a number of attacks, including: [1] An attacker may try to discover user identities by snooping authentication traffic. [2] An attacker may try to modify or spoof EAP packets. [3] An attacker may launch denial of service attacks by spoofing lower layer indications or Success/Failure packets, by replaying EAP packets, or by generating packets with overlapping Identifiers. [4] An attacker may attempt to recover the pass-phrase by mounting an offline dictionary attack. [5] An attacker may attempt to convince the peer to connect to an untrusted network by mounting a man-in-the-middle attack. [6] An attacker may attempt to disrupt the EAP negotiation in order cause a weak authentication method to be selected. [7] An attacker may attempt to recover keys by taking advantage of weak key derivation techniques used within EAP methods.
[8] An attacker may attempt to take advantage of weak ciphersuites
subsequently used after the EAP conversation is complete.
[9] An attacker may attempt to perform downgrading attacks on lower
layer ciphersuite negotiation in order to ensure that a weaker
ciphersuite is used subsequently to EAP authentication.
[10] An attacker acting as an authenticator may 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.
Depending on the lower layer, these attacks may be carried out
without requiring physical proximity. Where EAP is used over
wireless networks, EAP packets may be forwarded by authenticators
(e.g., pre-authentication) so that the attacker need not be within
the coverage area of an authenticator in order to carry out an attack
on it or its peers. Where EAP is used over the Internet, attacks may
be carried out at an even greater distance.
7.2. Security Claims
In order to clearly articulate the security provided by an EAP
method, EAP method specifications MUST include a Security Claims
section, including the following declarations:
[a] Mechanism. This is a statement of the authentication technology:
certificates, pre-shared keys, passwords, token cards, etc.
[b] Security claims. This is a statement of the claimed security
properties of the method, using terms defined in Section 7.2.1:
mutual authentication, integrity protection, replay protection,
confidentiality, key derivation, dictionary attack resistance,
fast reconnect, cryptographic binding. The Security Claims
section of an EAP method specification SHOULD provide
justification for the claims that are made. This can be
accomplished by including a proof in an Appendix, or including a
reference to a proof.
[c] Key strength. If the method derives keys, then the effective key
strength MUST be estimated. This estimate is meant for potential
users of the method to determine if the keys produced are strong
enough for the intended application.
The effective key strength SHOULD be stated as a number of bits,
defined as follows: If the effective key strength is N bits, the
best currently known methods to recover the key (with non-
negligible probability) require, on average, an effort comparable
to 2^(N-1) operations of a typical block cipher. The statement
SHOULD be accompanied by a short rationale, explaining how this
number was derived. This explanation SHOULD include the
parameters required to achieve the stated key strength based on
current knowledge of the algorithms.
(Note: Although it is difficult to define what "comparable
effort" and "typical block cipher" exactly mean, reasonable
approximations are sufficient here. Refer to e.g. [SILVERMAN]
for more discussion.)
The key strength depends on the methods used to derive the keys.
For instance, if keys are derived from a shared secret (such as a
password or a long-term secret), and possibly some public
information such as nonces, the effective key strength is limited
by the strength of the long-term secret (assuming that the
derivation procedure is computationally simple). To take another
example, when using public key algorithms, the strength of the
symmetric key depends on the strength of the public keys used.
[d] Description of key hierarchy. EAP methods deriving keys MUST
either provide a reference to a key hierarchy specification, or
describe how Master Session Keys (MSKs) and Extended Master
Session Keys (EMSKs) are to be derived.
[e] Indication of vulnerabilities. In addition to the security
claims that are made, the specification MUST indicate which of
the security claims detailed in Section 7.2.1 are NOT being made.
7.2.1. Security Claims Terminology for EAP Methods
These terms are used to describe the security properties of EAP
methods:
Protected ciphersuite negotiation
This refers to the ability of an EAP method to negotiate the
ciphersuite used to protect the EAP conversation, as well as to
integrity protect the negotiation. It does not refer to the
ability to negotiate the ciphersuite used to protect data.
Mutual authentication
This refers to an EAP method in which, within an interlocked
exchange, the authenticator authenticates the peer and the peer
authenticates the authenticator. Two independent one-way methods,
running in opposite directions do not provide mutual
authentication as defined here.
Integrity protection
This refers to providing data origin authentication and protection
against unauthorized modification of information for EAP packets
(including EAP Requests and Responses). When making this claim, a
method specification MUST describe the EAP packets and fields
within the EAP packet that are protected.
Replay protection
This refers to protection against replay of an EAP method or its
messages, including success and failure result indications.
Confidentiality
This refers to encryption of EAP messages, including EAP Requests
and Responses, and success and failure result indications. A
method making this claim MUST support identity protection (see
Section 7.3).
Key derivation
This refers to the ability of the EAP method to derive exportable
keying material, such as the Master Session Key (MSK), and
Extended Master Session Key (EMSK). The MSK is used only for
further key derivation, not directly for protection of the EAP
conversation or subsequent data. Use of the EMSK is reserved.
Key strength
If the effective key strength is N bits, the best currently known
methods to recover the key (with non-negligible probability)
require, on average, an effort comparable to 2^(N-1) operations of
a typical block cipher.
Dictionary attack resistance
Where password authentication is used, passwords are commonly
selected from a small set (as compared to a set of N-bit keys),
which raises a concern about dictionary attacks. A method may be
said to provide protection against dictionary attacks if, when it
uses a password as a secret, the method does not allow an offline
attack that has a work factor based on the number of passwords in
an attacker's dictionary.
Fast reconnect
The ability, in the case where a security association has been
previously established, to create a new or refreshed security
association more efficiently or in a smaller number of round-
trips.
Cryptographic binding
The demonstration of the EAP peer to the EAP server that a single
entity has acted as the EAP peer for all methods executed within a
tunnel method. Binding MAY also imply that the EAP server
demonstrates to the peer that a single entity has acted as the EAP
server for all methods executed within a tunnel method. If
executed correctly, binding serves to mitigate man-in-the-middle
vulnerabilities.
Session independence
The demonstration that passive attacks (such as capture of the EAP
conversation) or active attacks (including compromise of the MSK
or EMSK) does not enable compromise of subsequent or prior MSKs or
EMSKs.
Fragmentation
This refers to whether an EAP method supports fragmentation and
reassembly. As noted in Section 3.1, EAP methods should support
fragmentation and reassembly if EAP packets can exceed the minimum
MTU of 1020 octets.
Channel binding
The communication within an EAP method of integrity-protected
channel properties such as endpoint identifiers which can be
compared to values communicated via out of band mechanisms (such
as via a AAA or lower layer protocol).
Note: This list of security claims is not exhaustive. Additional
properties, such as additional denial-of-service protection, may be
relevant as well.
7.3. Identity Protection
An Identity exchange is optional within the EAP conversation.
Therefore, it is possible to omit the Identity exchange entirely, or
to use a method-specific identity exchange once a protected channel
has been established.
However, where roaming is supported as described in [RFC2607], it may
be necessary to locate the appropriate backend authentication server
before the authentication conversation can proceed. The realm
portion of the Network Access Identifier (NAI) [RFC2486] is typically
included within the EAP-Response/Identity in order to enable the authentication exchange to be routed to the appropriate backend authentication server. Therefore, while the peer-name portion of the NAI may be omitted in the EAP-Response/Identity where proxies or relays are present, the realm portion may be required. It is possible for the identity in the identity response to be different from the identity authenticated by the EAP method. This may be intentional in the case of identity privacy. An EAP method SHOULD use the authenticated identity when making access control decisions.7.4. Man-in-the-Middle Attacks
Where EAP is tunneled within another protocol that omits peer authentication, there exists a potential vulnerability to a man-in- the-middle attack. For details, see [BINDING] and [MITM]. As noted in Section 2.1, EAP does not permit untunneled sequences of authentication methods. Were a sequence of EAP authentication methods to be permitted, the peer might not have proof that a single entity has acted as the authenticator for all EAP methods within the sequence. For example, an authenticator might terminate one EAP method, then forward the next method in the sequence to another party without the peer's knowledge or consent. Similarly, the authenticator might not have proof that a single entity has acted as the peer for all EAP methods within the sequence. Tunneling EAP within another protocol enables an attack by a rogue EAP authenticator tunneling EAP to a legitimate server. Where the tunneling protocol is used for key establishment but does not require peer authentication, an attacker convincing a legitimate peer to connect to it will be able to tunnel EAP packets to a legitimate server, successfully authenticating and obtaining the key. This allows the attacker to successfully establish itself as a man-in- the-middle, gaining access to the network, as well as the ability to decrypt data traffic between the legitimate peer and server. This attack may be mitigated by the following measures: [a] Requiring mutual authentication within EAP tunneling mechanisms. [b] Requiring cryptographic binding between the EAP tunneling protocol and the tunneled EAP methods. Where cryptographic binding is supported, a mechanism is also needed to protect against downgrade attacks that would bypass it. For further details on cryptographic binding, see [BINDING].
[c] Limiting the EAP methods authorized for use without protection,
based on peer and authenticator policy.
[d] Avoiding the use of tunnels when a single, strong method is
available.
7.5. Packet Modification Attacks
While EAP methods may support per-packet data origin authentication,
integrity, and replay protection, support is not provided within the
EAP layer.
Since the Identifier is only a single octet, it is easy to guess,
allowing an attacker to successfully inject or replay EAP packets.
An attacker may also modify EAP headers (Code, Identifier, Length,
Type) within EAP packets where the header is unprotected. This could
cause packets to be inappropriately discarded or misinterpreted.
To protect EAP packets against modification, spoofing, or replay,
methods supporting protected ciphersuite negotiation, mutual
authentication, and key derivation, as well as integrity and replay
protection, are recommended. See Section 7.2.1 for definitions of
these security claims.
Method-specific MICs may be used to provide protection. If a per-
packet MIC is employed within an EAP method, then peers,
authentication servers, and authenticators not operating in pass-
through mode MUST validate the MIC. MIC validation failures SHOULD
be logged. Whether a MIC validation failure is considered a fatal
error or not is determined by the EAP method specification.
It is RECOMMENDED that methods providing integrity protection of EAP
packets include coverage of all the EAP header fields, including the
Code, Identifier, Length, Type, and Type-Data fields.
Since EAP messages of Types Identity, Notification, and Nak do not
include their own MIC, it may be desirable for the EAP method MIC to
cover information contained within these messages, as well as the
header of each EAP message.
To provide protection, EAP also may be encapsulated within a
protected channel created by protocols such as ISAKMP [RFC2408], as
is done in [IKEv2] or within TLS [RFC2246]. However, as noted in
Section 7.4, EAP tunneling may result in a man-in-the-middle
vulnerability.
Existing EAP methods define message integrity checks (MICs) that cover more than one EAP packet. For example, EAP-TLS [RFC2716] defines a MIC over a TLS record that could be split into multiple fragments; within the FINISHED message, the MIC is computed over previous messages. Where the MIC covers more than one EAP packet, a MIC validation failure is typically considered a fatal error. Within EAP-TLS [RFC2716], a MIC validation failure is treated as a fatal error, since that is what is specified in TLS [RFC2246]. However, it is also possible to develop EAP methods that support per-packet MICs, and respond to verification failures by silently discarding the offending packet. In this document, descriptions of EAP message handling assume that per-packet MIC validation, where it occurs, is effectively performed as though it occurs before sending any responses or changing the state of the host which received the packet.7.6. Dictionary Attacks
Password authentication algorithms such as EAP-MD5, MS-CHAPv1 [RFC2433], and Kerberos V [RFC1510] are known to be vulnerable to dictionary attacks. MS-CHAPv1 vulnerabilities are documented in [PPTPv1]; MS-CHAPv2 vulnerabilities are documented in [PPTPv2]; Kerberos vulnerabilities are described in [KRBATTACK], [KRBLIM], and [KERB4WEAK]. In order to protect against dictionary attacks, authentication methods resistant to dictionary attacks (as defined in Section 7.2.1) are recommended. If an authentication algorithm is used that is known to be vulnerable to dictionary attacks, then the conversation may be tunneled within a protected channel in order to provide additional protection. However, as noted in Section 7.4, EAP tunneling may result in a man- in-the-middle vulnerability, and therefore dictionary attack resistant methods are preferred.7.7. Connection to an Untrusted Network
With EAP methods supporting one-way authentication, such as EAP-MD5, the peer does not authenticate the authenticator, making the peer vulnerable to attack by a rogue authenticator. Methods supporting mutual authentication (as defined in Section 7.2.1) address this vulnerability. In EAP there is no requirement that authentication be full duplex or that the same protocol be used in both directions. It is perfectly
acceptable for different protocols to be used in each direction. This will, of course, depend on the specific protocols negotiated. However, in general, completing a single unitary mutual authentication is preferable to two one-way authentications, one in each direction. This is because separate authentications that are not bound cryptographically so as to demonstrate they are part of the same session are subject to man-in-the-middle attacks, as discussed in Section 7.4.7.8. Negotiation Attacks
In a negotiation attack, the attacker attempts to convince the peer and authenticator to negotiate a less secure EAP method. EAP does not provide protection for Nak Response packets, although it is possible for a method to include coverage of Nak Responses within a method-specific MIC. Within or associated with each authenticator, it is not anticipated that a particular named peer will support a choice of methods. This would make the peer vulnerable to attacks that negotiate the least secure method from among a set. Instead, for each named peer, there SHOULD be an indication of exactly one method used to authenticate that peer name. If a peer needs to make use of different authentication methods under different circumstances, then distinct identities SHOULD be employed, each of which identifies exactly one authentication method.7.9. Implementation Idiosyncrasies
The interaction of EAP with lower layers such as PPP and IEEE 802 are highly implementation dependent. For example, upon failure of authentication, some PPP implementations do not terminate the link, instead limiting traffic in Network-Layer Protocols to a filtered subset, which in turn allows the peer the opportunity to update secrets or send mail to the network administrator indicating a problem. Similarly, while an authentication failure will result in denied access to the controlled port in [IEEE-802.1X], limited traffic may be permitted on the uncontrolled port. In EAP there is no provision for retries of failed authentication. However, in PPP the LCP state machine can renegotiate the authentication protocol at any time, thus allowing a new attempt. Similarly, in IEEE 802.1X the Supplicant or Authenticator can re- authenticate at any time. It is recommended that any counters used for authentication failure not be reset until after successful authentication, or subsequent termination of the failed link.
7.10. Key Derivation
It is possible for the peer and EAP server to mutually authenticate and derive keys. In order to provide keying material for use in a subsequently negotiated ciphersuite, an EAP method supporting key derivation MUST export a Master Session Key (MSK) of at least 64 octets, and an Extended Master Session Key (EMSK) of at least 64 octets. EAP Methods deriving keys MUST provide for mutual authentication between the EAP peer and the EAP Server. The MSK and EMSK MUST NOT be used directly to protect data; however, they are of sufficient size to enable derivation of a AAA-Key subsequently used to derive Transient Session Keys (TSKs) for use with the selected ciphersuite. Each ciphersuite is responsible for specifying how to derive the TSKs from the AAA-Key. The AAA-Key is derived from the keying material exported by the EAP method (MSK and EMSK). This derivation occurs on the AAA server. In many existing protocols that use EAP, the AAA-Key and MSK are equivalent, but more complicated mechanisms are possible (see [KEYFRAME] for details). EAP methods SHOULD ensure the freshness of the MSK and EMSK, even in cases where one party may not have a high quality random number generator. A RECOMMENDED method is for each party to provide a nonce of at least 128 bits, used in the derivation of the MSK and EMSK. EAP methods export the MSK and EMSK, but not Transient Session Keys so as to allow EAP methods to be ciphersuite and media independent. Keying material exported by EAP methods MUST be independent of the ciphersuite negotiated to protect data. Depending on the lower layer, EAP methods may run before or after ciphersuite negotiation, so that the selected ciphersuite may not be known to the EAP method. By providing keying material usable with any ciphersuite, EAP methods can used with a wide range of ciphersuites and media. In order to preserve algorithm independence, EAP methods deriving keys SHOULD support (and document) the protected negotiation of the ciphersuite used to protect the EAP conversation between the peer and server. This is distinct from the ciphersuite negotiated between the peer and authenticator, used to protect data. The strength of Transient Session Keys (TSKs) used to protect data is ultimately dependent on the strength of keys generated by the EAP method. If an EAP method cannot produce keying material of sufficient strength, then the TSKs may be subject to a brute force
attack. In order to enable deployments requiring strong keys, EAP methods supporting key derivation SHOULD be capable of generating an MSK and EMSK, each with an effective key strength of at least 128 bits. Methods supporting key derivation MUST demonstrate cryptographic separation between the MSK and EMSK branches of the EAP key hierarchy. Without violating a fundamental cryptographic assumption (such as the non-invertibility of a one-way function), an attacker recovering the MSK or EMSK MUST NOT be able to recover the other quantity with a level of effort less than brute force. Non-overlapping substrings of the MSK MUST be cryptographically separate from each other, as defined in Section 7.2.1. That is, knowledge of one substring MUST NOT help in recovering some other substring without breaking some hard cryptographic assumption. This is required because some existing ciphersuites form TSKs by simply splitting the AAA-Key to pieces of appropriate length. Likewise, non-overlapping substrings of the EMSK MUST be cryptographically separate from each other, and from substrings of the MSK. The EMSK is reserved for future use and MUST remain on the EAP peer and EAP server where it is derived; it MUST NOT be transported to, or shared with, additional parties, or used to derive any other keys. (This restriction will be relaxed in a future document that specifies how the EMSK can be used.) Since EAP does not provide for explicit key lifetime negotiation, EAP peers, authenticators, and authentication servers MUST be prepared for situations in which one of the parties discards the key state, which remains valid on another party. This specification does not provide detailed guidance on how EAP methods derive the MSK and EMSK, how the AAA-Key is derived from the MSK and/or EMSK, or how the TSKs are derived from the AAA-Key. The development and validation of key derivation algorithms is difficult, and as a result, EAP methods SHOULD re-use well established and analyzed mechanisms for key derivation (such as those specified in IKE [RFC2409] or TLS [RFC2246]), rather than inventing new ones. EAP methods SHOULD also utilize well established and analyzed mechanisms for MSK and EMSK derivation. Further details on EAP Key Derivation are provided within [KEYFRAME].
7.11. Weak Ciphersuites
If after the initial EAP authentication, data packets are sent without per-packet authentication, integrity, and replay protection, an attacker with access to the media can inject packets, "flip bits" within existing packets, replay packets, or even hijack the session completely. Without per-packet confidentiality, it is possible to snoop data packets. To protect against data modification, spoofing, or snooping, it is recommended that EAP methods supporting mutual authentication and key derivation (as defined by Section 7.2.1) be used, along with lower layers providing per-packet confidentiality, authentication, integrity, and replay protection. Additionally, if the lower layer performs ciphersuite negotiation, it should be understood that EAP does not provide by itself integrity protection of that negotiation. Therefore, in order to avoid downgrading attacks which would lead to weaker ciphersuites being used, clients implementing lower layer ciphersuite negotiation SHOULD protect against negotiation downgrading. This can be done by enabling users to configure which ciphersuites are acceptable as a matter of security policy, or the ciphersuite negotiation MAY be authenticated using keying material derived from the EAP authentication and a MIC algorithm agreed upon in advance by lower-layer peers.7.12. Link Layer
There are reliability and security issues with link layer indications in PPP, IEEE 802 LANs, and IEEE 802.11 wireless LANs: [a] PPP. In PPP, link layer indications such as LCP-Terminate (a link failure indication) and NCP (a link success indication) are not authenticated or integrity protected. They can therefore be spoofed by an attacker with access to the link. [b] IEEE 802. IEEE 802.1X EAPOL-Start and EAPOL-Logoff frames are not authenticated or integrity protected. They can therefore be spoofed by an attacker with access to the link. [c] IEEE 802.11. In IEEE 802.11, link layer indications include Disassociate and Deauthenticate frames (link failure indications), and the first message of the 4-way handshake (link success indication). These messages are not authenticated or integrity protected, and although they are not forwardable, they are spoofable by an attacker within range.
In IEEE 802.11, IEEE 802.1X data frames may be sent as Class 3 unicast data frames, and are therefore forwardable. This implies that while EAPOL-Start and EAPOL-Logoff messages may be authenticated and integrity protected, they can be spoofed by an authenticated attacker far from the target when "pre-authentication" is enabled. In IEEE 802.11, a "link down" indication is an unreliable indication of link failure, since wireless signal strength can come and go and may be influenced by radio frequency interference generated by an attacker. To avoid unnecessary resets, it is advisable to damp these indications, rather than passing them directly to the EAP. Since EAP supports retransmission, it is robust against transient connectivity losses.7.13. Separation of Authenticator and Backend Authentication Server
It is possible for the EAP peer and EAP server to mutually authenticate and derive a AAA-Key for a ciphersuite used to protect subsequent data traffic. This does not present an issue on the peer, since the peer and EAP client reside on the same machine; all that is required is for the client to derive the AAA-Key from the MSK and EMSK exported by the EAP method, and to subsequently pass a Transient Session Key (TSK) to the ciphersuite module. However, in the case where the authenticator and authentication server reside on different machines, there are several implications for security. [a] Authentication will occur between the peer and the authentication server, not between the peer and the authenticator. This means that it is not possible for the peer to validate the identity of the authenticator that it is speaking to, using EAP alone. [b] As discussed in [RFC3579], the authenticator is dependent on the AAA protocol in order to know the outcome of an authentication conversation, and does not look at the encapsulated EAP packet (if one is present) to determine the outcome. In practice, this implies that the AAA protocol spoken between the authenticator and authentication server MUST support per-packet authentication, integrity, and replay protection. [c] After completion of the EAP conversation, where lower layer security services such as per-packet confidentiality, authentication, integrity, and replay protection will be enabled, a secure association protocol SHOULD be run between the peer and authenticator in order to provide mutual authentication between
the peer and authenticator, guarantee liveness of transient
session keys, provide protected ciphersuite and capabilities
negotiation for subsequent data, and synchronize key usage.
[d] A AAA-Key derived from the MSK and/or EMSK negotiated between the
peer and authentication server MAY be transmitted to the
authenticator. Therefore, a mechanism needs to be provided to
transmit the AAA-Key from the authentication server to the
authenticator that needs it. The specification of the AAA-key
derivation, transport, and wrapping mechanisms is outside the
scope of this document. Further details on AAA-Key Derivation
are provided within [KEYFRAME].
7.14. Cleartext Passwords
This specification does not define a mechanism for cleartext password
authentication. The omission is intentional. Use of cleartext
passwords would allow the password to be captured by an attacker with
access to a link over which EAP packets are transmitted.
Since protocols encapsulating EAP, such as RADIUS [RFC3579], may not
provide confidentiality, EAP packets may be subsequently encapsulated
for transport over the Internet where they may be captured by an
attacker.
As a result, cleartext passwords cannot be securely used within EAP,
except where encapsulated within a protected tunnel with server
authentication. Some of the same risks apply to EAP methods without
dictionary attack resistance, as defined in Section 7.2.1. For
details, see Section 7.6.
7.15. 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 may enable an authenticator to impersonate
another authenticator or communicate incorrect information via out-
of-band mechanisms (such as via a AAA or lower layer protocol).
Where EAP is used in pass-through mode, the EAP peer typically does
not verify the identity of the pass-through authenticator, it only
verifies that the pass-through authenticator is trusted by the EAP
server. This creates a potential security vulnerability.
Section 4.3.7 of [RFC3579] describes how an EAP pass-through
authenticator acting as a AAA client can be detected if it attempts
to impersonate another authenticator (such by sending incorrect NAS-
Identifier [RFC2865], NAS-IP-Address [RFC2865] or NAS-IPv6-Address
[RFC3162] attributes via the AAA protocol). However, it is possible for a pass-through authenticator acting as a AAA client to provide correct information to the AAA server while communicating misleading information to the EAP peer via a lower layer protocol. For example, it is possible for a compromised authenticator to utilize another authenticator's Called-Station-Id or NAS-Identifier in communicating with the EAP peer via a lower layer protocol, or for a pass-through authenticator acting as a AAA client to provide an incorrect peer Calling-Station-Id [RFC2865][RFC3580] to the AAA server via the AAA protocol. In order to address this vulnerability, EAP methods may 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. Where discrepancies are found, these SHOULD be logged; additional actions MAY also be taken, such as denying access.7.16. Protected Result Indications
Within EAP, Success and Failure packets are neither acknowledged nor integrity protected. Result indications improve resilience to loss of Success and Failure packets when EAP is run over lower layers which do not support retransmission or synchronization of the authentication state. In media such as IEEE 802.11, which provides for retransmission, as well as synchronization of authentication state via the 4-way handshake defined in [IEEE-802.11i], additional resilience is typically of marginal benefit. Depending on the method and circumstances, result indications can be spoofable by an attacker. A method is said to provide protected result indications if it supports result indications, as well as the "integrity protection" and "replay protection" claims. A method supporting protected result indications MUST indicate which result indications are protected, and which are not. Protected result indications are not required to protect against rogue authenticators. Within a mutually authenticating method, requiring that the server authenticate to the peer before the peer will accept a Success packet prevents an attacker from acting as a rogue authenticator.
However, it is possible for an attacker to forge a Success packet after the server has authenticated to the peer, but before the peer has authenticated to the server. If the peer were to accept the forged Success packet and attempt to access the network when it had not yet successfully authenticated to the server, a denial of service attack could be mounted against the peer. After such an attack, if the lower layer supports failure indications, the authenticator can synchronize state with the peer by providing a lower layer failure indication. See Section 7.12 for details. If a server were to authenticate the peer and send a Success packet prior to determining whether the peer has authenticated the authenticator, an idle timeout can occur if the authenticator is not authenticated by the peer. Where supported by the lower layer, an authenticator sensing the absence of the peer can free resources. In a method supporting result indications, a peer that has authenticated the server does not consider the authentication successful until it receives an indication that the server successfully authenticated it. Similarly, a server that has successfully authenticated the peer does not consider the authentication successful until it receives an indication that the peer has authenticated the server. In order to avoid synchronization problems, prior to sending a success result indication, it is desirable for the sender to verify that sufficient authorization exists for granting access, though, as discussed below, this is not always possible. While result indications may enable synchronization of the authentication result between the peer and server, this does not guarantee that the peer and authenticator will be synchronized in terms of their authorization or that timeouts will not occur. For example, the EAP server may not be aware of an authorization decision made by a AAA proxy; the AAA server may check authorization only after authentication has completed successfully, to discover that authorization cannot be granted, or the AAA server may grant access but the authenticator may be unable to provide it due to a temporary lack of resources. In these situations, synchronization may only be achieved via lower layer result indications. Success indications may be explicit or implicit. For example, where a method supports error messages, an implicit success indication may be defined as the reception of a specific message without a preceding error message. Failures are typically indicated explicitly. As described in Section 4.2, a peer silently discards a Failure packet received at a point where the method does not explicitly permit this
to be sent. For example, a method providing its own error messages might require the peer to receive an error message prior to accepting a Failure packet. Per-packet authentication, integrity, and replay protection of result indications protects against spoofing. Since protected result indications require use of a key for per-packet authentication and integrity protection, methods supporting protected result indications MUST also support the "key derivation", "mutual authentication", "integrity protection", and "replay protection" claims. Protected result indications address some denial-of-service vulnerabilities due to spoofing of Success and Failure packets, though not all. EAP methods can typically provide protected result indications only in some circumstances. For example, errors can occur prior to key derivation, and so it may not be possible to protect all failure indications. It is also possible that result indications may not be supported in both directions or that synchronization may not be achieved in all modes of operation. For example, within EAP-TLS [RFC2716], in the client authentication handshake, the server authenticates the peer, but does not receive a protected indication of whether the peer has authenticated it. In contrast, the peer authenticates the server and is aware of whether the server has authenticated it. In the session resumption handshake, the peer authenticates the server, but does not receive a protected indication of whether the server has authenticated it. In this mode, the server authenticates the peer and is aware of whether the peer has authenticated it.8. Acknowledgements
This protocol derives much of its inspiration from Dave Carrel's AHA document, as well as the PPP CHAP protocol [RFC1994]. Valuable feedback was provided by Yoshihiro Ohba of Toshiba America Research, Jari Arkko of Ericsson, Sachin Seth of Microsoft, Glen Zorn of Cisco Systems, Jesse Walker of Intel, Bill Arbaugh, Nick Petroni and Bryan Payne of the University of Maryland, Steve Bellovin of AT&T Research, Paul Funk of Funk Software, Pasi Eronen of Nokia, Joseph Salowey of Cisco, Paul Congdon of HP, and members of the EAP working group. The use of Security Claims sections for EAP methods, as required by Section 7.2 and specified for each EAP method described in this document, was inspired by Glen Zorn through [EAP-EVAL].
9. References
9.1. Normative References
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1994] Simpson, W., "PPP Challenge Handshake Authentication Protocol (CHAP)", RFC 1994, August 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2243] Metz, C., "OTP Extended Responses", RFC 2243, November 1997. [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. [RFC2289] Haller, N., Metz, C., Nesser, P. and M. Straw, "A One-Time Password System", RFC 2289, February 1998. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, November 2000. [IEEE-802] Institute of Electrical and Electronics Engineers, "Local and Metropolitan Area Networks: Overview and Architecture", IEEE Standard 802, 1990. [IEEE-802.1X] Institute of Electrical and Electronics Engineers, "Local and Metropolitan Area Networks: Port-Based Network Access Control", IEEE Standard 802.1X, September 2001.
9.2. Informative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network Authentication Service (V5)", RFC 1510, September 1993. [RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness Recommendations for Security", RFC 1750, December 1994. [RFC2246] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication Protocol (EAP)", RFC 2284, March 1998. [RFC2486] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC 2486, January 1999. [RFC2408] Maughan, D., Schneider, M. and M. Schertler, "Internet Security Association and Key Management Protocol (ISAKMP)", RFC 2408, November 1998. [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions", RFC 2433, October 1998. [RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy Implementation in Roaming", RFC 2607, June 1999. [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661, August 1999. [RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication Protocol", RFC 2716, October 1999. [RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson, "Stream Control Transmission Protocol", RFC 2960, October 2000. [RFC3162] Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IPv6", RFC 3162, August 2001. [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002. [RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial In User Service) Support For Extensible Authentication Protocol (EAP)", RFC 3579, September 2003. [RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G. and J. Roese, "IEEE 802.1X Remote Authentication Dial In User Service (RADIUS) Usage Guidelines", RFC 3580, September 2003. [RFC3692] Narten, T., "Assigning Experimental and Testing Numbers Considered Useful", BCP 82, RFC 3692, January 2004. [DECEPTION] Slatalla, M. and J. Quittner, "Masters of Deception", Harper-Collins, New York, 1995. [KRBATTACK] Wu, T., "A Real-World Analysis of Kerberos Password Security", Proceedings of the 1999 ISOC Network and Distributed System Security Symposium, http://www.isoc.org/isoc/conferences/ndss/99/ proceedings/papers/wu.pdf. [KRBLIM] Bellovin, S. and M. Merrit, "Limitations of the Kerberos authentication system", Proceedings of the 1991 Winter USENIX Conference, pp. 253-267, 1991. [KERB4WEAK] Dole, B., Lodin, S. and E. Spafford, "Misplaced trust: Kerberos 4 session keys", Proceedings of the Internet Society Network and Distributed System Security Symposium, pp. 60-70, March 1997.
[PIC] Aboba, B., Krawczyk, H. and Y. Sheffer, "PIC, A Pre-IKE Credential Provisioning Protocol", Work in Progress, October 2002. [IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", Work in Progress, January 2004. [PPTPv1] Schneier, B. and Mudge, "Cryptanalysis of Microsoft's Point-to- Point Tunneling Protocol", Proceedings of the 5th ACM Conference on Communications and Computer Security, ACM Press, November 1998. [IEEE-802.11] Institute of Electrical and Electronics Engineers, "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", IEEE Standard 802.11, 1999. [SILVERMAN] Silverman, Robert D., "A Cost-Based Security Analysis of Symmetric and Asymmetric Key Lengths", RSA Laboratories Bulletin 13, April 2000 (Revised November 2001), http://www.rsasecurity.com/rsalabs/bulletins/ bulletin13.html. [KEYFRAME] Aboba, B., "EAP Key Management Framework", Work in Progress, October 2003. [SASLPREP] Zeilenga, K., "SASLprep: Stringprep profile for user names and passwords", Work in Progress, March 2004. [IEEE-802.11i] Institute of Electrical and Electronics Engineers, "Unapproved Draft Supplement to Standard for Telecommunications and Information Exchange Between Systems - LAN/MAN Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Specification for Enhanced Security", IEEE Draft 802.11i (work in progress), 2003. [DIAM-EAP] Eronen, P., Hiller, T. and G. Zorn, "Diameter Extensible Authentication Protocol (EAP) Application", Work in Progress, February 2004. [EAP-EVAL] Zorn, G., "Specifying Security Claims for EAP Authentication Types", Work in Progress, October 2002.
[BINDING] Puthenkulam, J., "The Compound Authentication Binding Problem", Work in Progress, October 2003. [MITM] Asokan, N., Niemi, V. and K. Nyberg, "Man-in-the- Middle in Tunneled Authentication Protocols", IACR ePrint Archive Report 2002/163, October 2002, <http://eprint.iacr.org/2002/163>. [IEEE-802.11i-req] Stanley, D., "EAP Method Requirements for Wireless LANs", Work in Progress, February 2004. [PPTPv2] Schneier, B. and Mudge, "Cryptanalysis of Microsoft's PPTP Authentication Extensions (MS- CHAPv2)", CQRE 99, Springer-Verlag, 1999, pp. 192-203.
Appendix A. Changes from RFC 2284
This section lists the major changes between [RFC2284] and this document. Minor changes, including style, grammar, spelling, and editorial changes are not mentioned here. o The Terminology section (Section 1.2) has been expanded, defining more concepts and giving more exact definitions. o The concepts of Mutual Authentication, Key Derivation, and Result Indications are introduced and discussed throughout the document where appropriate. o In Section 2, it is explicitly specified that more than one exchange of Request and Response packets may occur as part of the EAP authentication exchange. How this may be used and how it may not be used is specified in detail in Section 2.1. o Also in Section 2, some requirements have been made explicit for the authenticator when acting in pass-through mode. o An EAP multiplexing model (Section 2.2) has been added to illustrate a typical implementation of EAP. There is no requirement that an implementation conform to this model, as long as the on-the-wire behavior is consistent with it. o As EAP is now in use with a variety of lower layers, not just PPP for which it was first designed, Section 3 on lower layer behavior has been added. o In the description of the EAP Request and Response interaction (Section 4.1), both the behavior on receiving duplicate requests, and when packets should be silently discarded has been more exactly specified. The implementation notes in this section have been substantially expanded. o In Section 4.2, it has been clarified that Success and Failure packets must not contain additional data, and the implementation note has been expanded. A subsection giving requirements on processing of success and failure packets has been added. o Section 5 on EAP Request/Response Types lists two new Type values: the Expanded Type (Section 5.7), which is used to expand the Type value number space, and the Experimental Type. In the Expanded Type number space, the new Expanded Nak (Section 5.3.2) Type has been added. Clarifications have been made in the description of most of the existing Types. Security claims summaries have been added for authentication methods.
o In Sections 5, 5.1, and 5.2, a requirement has been added such that fields with displayable messages should contain UTF-8 encoded ISO 10646 characters. o It is now required in Section 5.1 that if the Type-Data field of an Identity Request contains a NUL-character, only the part before the null is displayed. RFC 2284 prohibits the null termination of the Type-Data field of Identity messages. This rule has been relaxed for Identity Request messages and the Identity Request Type-Data field may now be null terminated. o In Section 5.5, support for OTP Extended Responses [RFC2243] has been added to EAP OTP. o An IANA Considerations section (Section 6) has been added, giving registration policies for the numbering spaces defined for EAP. o The Security Considerations (Section 7) have been greatly expanded, giving a much more comprehensive coverage of possible threats and other security considerations. o In Section 7.5, text has been added on method-specific behavior, providing guidance on how EAP method-specific integrity checks should be processed. Where possible, it is desirable for a method-specific MIC to be computed over the entire EAP packet, including the EAP layer header (Code, Identifier, Length) and EAP method layer header (Type, Type-Data). o In Section 7.14 the security risks involved in use of cleartext passwords with EAP are described. o In Section 7.15 text has been added relating to detection of rogue NAS behavior.
Authors' Addresses
Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA Phone: +1 425 706 6605 Fax: +1 425 936 6605 EMail: bernarda@microsoft.com Larry J. Blunk Merit Network, Inc 4251 Plymouth Rd., Suite 2000 Ann Arbor, MI 48105-2785 USA Phone: +1 734-647-9563 Fax: +1 734-647-3185 EMail: ljb@merit.edu John R. Vollbrecht Vollbrecht Consulting LLC 9682 Alice Hill Drive Dexter, MI 48130 USA EMail: jrv@umich.edu James Carlson Sun Microsystems, Inc 1 Network Drive Burlington, MA 01803-2757 USA Phone: +1 781 442 2084 Fax: +1 781 442 1677 EMail: james.d.carlson@sun.com Henrik Levkowetz ipUnplugged AB Arenavagen 33 Stockholm S-121 28 SWEDEN Phone: +46 708 32 16 08 EMail: henrik@levkowetz.com
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