|Internet-Draft||Deprecating RADIUS||October 2022|
|DeKok||Expires 7 April 2023||[Page]|
- RADEXT Working Group
- Intended Status:
- Standards Track
Deprecating RADIUS/UDP and RADIUS/TCP
RADIUS crypto-agility was first mandated as future work by RFC 6421. The outcome of that work was the publication of RADIUS over TLS (RFC 6614) and RADIUS over DTLS (RFC 7360) as experimental documents. Those transport protocols have been in wide-spread use for many years in a wide range of networks. They have proven their utility as replacements for the previous UDP (RFC 2865) and TCP (RFC 6613) transports. With that knowledge, the continued use of insecure transports for RADIUS has serious and negative implications for privacy and security.¶
This document formally deprecates the use of the User Datagram Protocol (UDP) and of the Transport Congestion Protocol (TCP) as transport protocols for RADIUS. These transports are permitted inside of secure networks, but their use even in that environment is strongly discouraged. For all other environments, the use of TLS-based transports is mandated.¶
This note is to be removed before publishing as an RFC.¶
Status information for this document may be found at https://datatracker.ietf.org/doc/draft-dekok-radext-deprecating-radius/.¶
Source for this draft and an issue tracker can be found at https://github.com/freeradius/deprecating-radius.git.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 7 April 2023.¶
Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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The RADIUS protocol [RFC2865] was first standardized in 1997, though its roots go back much earlier to 1993. The protocol uses MD5 [RFC1321] to sign some packets types, and to obfsucate certain attributes such as User-Password. As originally designed, Access-Request packets were entirely unauthenticated, and could be trivially spoofed as discussed in [RFC3579] Section 4.3.2. In order to prevent such spoofing, that specification defined the Message-Authenticator attribute ([RFC3579] Section 3.2) which allowed for packets to carry a signature based on HMAC-MD5.¶
The state of MD5 security was discussed in [RFC6151], which led to the state of RADIUS security being reviewed in [RFC6421] Section 3. The outcome of that review was the remainder of [RFC6421], which created crypto-agility requirements for RADIUS.¶
RADIUS/TLS [RFC6614] and RADIUS/DTLS [RFC7360] were then defined in order to meet the crypto-agility requirements of [RFC6421]. RADIUS/TLS has been in wide-spread use for about a decade, including Eduroam, and more recently OpenRoaming. RADIUS/DTLS has seen less use across the public Internet, but it nonetheless has multiple implementations.¶
As of the writing of this specification, RADIUS/UDP is still widely used, even though it depends on MD5 and "ad hoc" constructions for security. While MD5 has been broken, it is a testament to the design of RADIUS that there have been (as yet) no attacks on RADIUS Authenticator signatures which are stronger than brute-force.¶
However, the probmens with MD5 means that if a someone can view unencrypted RADIUS traffic, even a hobbyist attacker can crack all possible RADIUS shared secrets of eight characters or less. Such attacks can also result in compromise of all passwords carried in the User-Password attribute.¶
Even if a stronger packet signature method was used as in [RFC6218], it would not fully address the issues with RADIUS. Most information in RADIUS is sent in cleartext, and only a few attributes are hidden via obfuscation methods which rely on more "ad hoc" MD5 constructions. The privacy implications of this openness are severe.¶
Any observer of non-TLS RADIUS traffic is able to tell who is logging in to the network, what devices they are using, where they are logging in from, and their approximate location (usually city). With location-based attributes as defined in [RFC5580], a users location may be determined to within 15 or so meters. An observer can use RADIUS accounting packets to determine how long a user is online, and to track a summary of their total traffic (upload and download totals).¶
When RADIUS/UDP is used across the public Internet, the location of corporate executives can potentially be tracked in real-time (usually 10 minute intervals), to within 15 meters. Their devices can be identified, and tracked. Any passwords they send via the User-Password attribute can be be compromised. The negative implications for security and individual safety are obvious.¶
These issues are only partly mitigated when the attributes RADIUS is carrying define their own increased security and privacy. For example, some authentication methods such EAP-TLS, EAP-TTLS, etc. allow for User-Name privacy and for more secure transport of passwords. The use of MAC address randomization can limit device informationidentification to a particular manufacterer, instead of to a unique device.¶
However, these authentication methods are not always used or are not always available. Even if these methods were used ubiquitously, they do not protect all of the information which is publicly available when RADIUS/UDP or RADIUS/TCP is used.¶
It is no longer acceptable for RADIUS to rely on MD5 for security. It is no longer acceptable to send device or location information in clear text. This document therefore deprecates insecure uses of RADIUS, and mandates the use of secure TLS-based transport layers.¶
The use of TLS-based transport ensures complete privacy and security for all RADIUS traffic. An observer is limited to knowing rough activity levels of a client or server. That is, an observer can tell if there are a few users on a NAS, or many users on a NAS. All other information is hidden from all observers.¶
The rest of this document begins a summary of issues with RADIUS, and shows just how trivial it is to crack RADIUS/UDP security. We then mandate the use of TLS transport, and describe what that means. We give recommendations on how current systems can be migrated to using TLS. We conclude with privacy and security considerations.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
RADIUS over the User Datagram Protocol as define above.¶
the Transport Layer Security protocol. Generally when we refer to TLS in this document, we are referring to RADIUS/TLS and/or RADIUS/DTLS.¶
Network Access Server, which is a RADIUS client.¶
There are a number of issues with RADIUS. For one, RADIUS sends most information "in the clear", with obvious privacy implications.¶
Further, MD5 has been broken for over a decade, as summarized in [RFC6151]. No protocol should be using MD5 for anything. Even if MD5 was not broken, computers have gotten substantially faster in the past thirty years. This speed increase makes it possible for the average hobbyist to perform brute-force attacks to crack even seemingly complex shared secrets.¶
We address each of these issues in detail below.¶
Other than a few attributes such as User-Password, all RADIUS traffic is sent "in the clear". The resulting traffic has a large number of privacy issues. We refer to [RFC6973], and specifically to Section 5 of that document for detailed discussion. RADIUS is vulnerable to all of the issues raised by [RFC6973].¶
There are clear privacy and security information with sending user identifiers, and user locations [RFC5580] in clear-text across the Internet. As such, the use of clear-text protocols across insecure networks is no longer acceptable.¶
Attacks on MD5 are summarized in part in [RFC6151]. While there have not been many new attacks in the decade since [RFC6151] was published, that does not mean that further attacks do not exist. It is more likely that no one is looking for new attacks.¶
It is reasonable to expect that new research can further break MD5, but also that such research may not be publicly available.¶
There are similar security issues for the Tunnel-Password attribute, at least in CoA-Request and Disconnect-Request packets.¶
Request Authenticator In Request packets, the Authenticator value is a 16-octet MD5 [RFC1321] checksum, called the Request Authenticator. The Request Authenticator is calculated the same way as for an Accounting-Request, specified in [RFC2866].¶
The NAS and RADIUS accounting server share a secret. The Request Authenticator field in Accounting-Request packets contains a one- way MD5 hash calculated over a stream of octets consisting of the Code + Identifier + Length + 16 zero octets + request attributes + shared secret (where + indicates concatenation). The 16 octet MD5 hash value is stored in the Authenticator field of the Accounting-Request packet.¶
Taken together, these defintions means that for CoA-Request packets, all attribute obfuscation is calculated with the Reply Authenticator being all zeroes.¶
... Change-of-Authorization Messages Request ACK NAK # Attribute ... 0+ 0 0 69 Tunnel-Password (Note 5) ... (Note 5) When included within a CoA-Request, these attributes represent an authorization change request. Where tunnel attributes are included within a successful CoA-Request, all existing tunnel attributes are removed and replaced by the new attribute(s).¶
Call the shared secret S, the pseudo-random 128-bit Request Authenticator (from the corresponding Access-Request packet) R,¶
The assumption that the Request Authenticator is random data is true for Access-Request packets. It is not true for CoA-Request packets¶
Salt The Salt field is two octets in length and is used to ensure the uniqueness of the encryption key used to encrypt each instance of the Tunnel-Password attribute occurring in a given Access-Accept packet. The most significant bit (leftmost) of the Salt field MUST be set (1). The contents of each Salt field in a given Access-Accept packet MUST be unique.¶
Which means that there is only 15 bits of entropy in the Tunnel-Password obfuscation (plus the secret). It is not known if this limitation makes it easier to determine the contents of the Tunnel-Password. However, it cannot be a good thing, and it is one more reason to deprecate RADIUS/UDP.¶
The solution to an insecure protocol using thirty year-old cryptography is to deprecate the insecure cryptography, and to mandate modern cryptographic transport.¶
RADIUS/UDP and RADIUS/TCP MUST NOT be used outside of secure networks. A secure network is one which is known to be safe from eavesdroppers, attackers, etc.¶
For example, if IPSec is used between two systems, then those systems may use RADIUS/UDP or RADIUS/TCP over the IPSec connection.¶
Similarly, RADIUS/UDP and RADIUS/TCP may be used in secure management networks. However, administrators should not assume that such uses are secure.¶
Using RADIUS/UDP and RADIUS/TCP in any environment is still NOT RECOMMENDED. A network misconfiguration could result in the RADIUS traffic being sent over an insecure network. Neither the RADIUS client nor the RADIUS server would be aware of this misconfiguration.¶
In contrast, when TLS is used, the RADIUS endpoints are aware of all security issues, and can enforce security for themselves.¶
The crypto-agility requirements of [RFC6421] are addressed in [RFC6614] Appendix C, and in Section 10.1 of [RFC7360]. For clarity, we repeat the text of [RFC7360] here, with some minor modifications to update references, but not content.¶
Section 4.5 of [RFC6421] requires that the new security methods apply to all packet types. This requirement is satisfied by allowing TLS and DTLS to be used for all RADIUS traffic. In addition, [RFC7360] Section 3, addresses concerns about documenting the transition from legacy RADIUS to crypto-agile RADIUS.¶
We can now finalize the work began in [RFC6421]. We now state that new RADIUS specifications MUST NOT create any new cryptographic primitives to sign individual packets, or to obfscate the contents of any attributes. All security and privacy MUST instead be provided by a secure transport layer such as TLS. Simply using IPSec is not enough, as the use (or not) of IPSec is unknown to the RADIUS application.¶
It is NOT RECOMMENDED to define new attributes which use the content obfuscation methods defined for User-Password or Tunnel-Password. We would like to forbid such constructs entirely. We recognize that RADIUS/UDP will still be in use for many years, and that new standards may require some modicrum of privacy. As a result, it is difficult to forbid the use of these constructs.¶
hat being said, there has been no need since [RFC2868] in 2000 for new attribute which use these obfuscation methods. We believe therefore that there will be no demand for this kind of new attribute.¶
We recognize that it is difficult to upgrade legacy devices with new cryptographic protocols and user interfaces. The problem is made worse because the volume of RADIUS devices which are in use. The exact number is unknown, and can only be approximated. Our best guesses would be in the order of hundreds of thousands, if not millions of RADIUS/UDP devices are in daily use.¶
We therefore need to define a migration path to using secure transports. We give a few migration steps by making stronger recommendations for shared secrets. Where [RFC6614] Section 2.3 makes support for TLS-PSK optional, we now require support for TLS-PSK.¶
TLS uses certificates in most common uses. However, we recognize that it may be difficult to fully upgrade client implementations to allow for certificates to be used with RADIUS/TLS and RADIUS/DTLS. It is therefore RECOMMENDED that client implementations allow the use of a pre-shared key (TLS-PSK). The client implementation can then expose a flag "TLS yes / no", and then a shared secret (now PSK) entry field.¶
Any shared secret used for RADIUS/UDP MUST NOT be used for TLS-PSK.¶
Implementations MUST support PSKs of at least 32 octets, and SHOULD support PSKs of 64 octets. Implementations MUST require that PSKs be at least 16 octets in length. That is, short PSKs MUST NOT be permitted to be used.¶
Administrators SHOULD use PSKs of at least 24 octets, generated using a source of secure random numbers. The script given above can again be used.¶
The need to manage identities associated with PSK is a new requirement for NAS management interfaces, and is a new requirement for RADIUS servers. As such, implementors and operators require guidance.¶
A RADIUS client implementing TLS-PSK MUST update their management interfaces and application programming interfaces (APIs) to label the PSK field as "PSK" or "TLS-PKS, and MUST NOT label the PSK field as "shared secret".¶
When a RADIUS server implements TLS-PSK, the PSK identity replaces the IP address as the logical identifier for a RADIUS client. RADIUS servers MUST be able to look up PSK identity in a subsystem which then returns the actual PSK.¶
We note that due to NAT, etc. there may be multiple RADIUS clients using one public IP address. RADIUS servers MUST support such a configuration, and MUST support unique PSK identities for each client which is deployed in such a scenario.¶
RADIUS servers SHOULD tie PSK identities to a particular permitted IP address or permitted network, as doing so will lower the risk if a PSK is leaked. RADIUS servers MUST permit multiple clients to share one permitted IP address or network.¶
A RADIUS server which accepts TLS-PSK MUST support a unique PSK identifier per RADIUS client. There is no reason to use the same identifier for multiple clients. A RADIUS server which accepts TLS-PSK MUST have a unique PSK per RADIUS client.¶
It is RECOMMENDED that RADIUS clients and server track all used shared secrets and PSKs, and then verify that the following requirements all hold true:¶
The primary focus of this document is addressing privacy considerations for RADIUS.¶
Deprecating insecure transport for RADIUS, and requiring secure transport means that personally identifying information is no longer sent "in the clear". As noted earlier in this document, such information can include MAC addresses, user identifiers, and user locations.¶
The primary focus of this document is addressing security considerations for RADIUS.¶
Deprecating insecure transport for RADIUS, and requiring secure transport means that historical and/or future security issues with the RADIUS protocol no longer apply.¶
Experience has shown that it can be difficult to configure and update certificates in a RADIUS environment. Public Certification Authorities (CAs) will not issue certificates specifically for use by RADIUS servers. As for updates, certificates may be valid for many years. By the time a certificate is up for renewal, the people and processes responsible for it may have changed. Which means that updating certificates can be a complex and error-prone task.¶
Therefore while [RFC6614] suggested that TLS-PSK was optional, we mandate support for TLS-PSK here. We also correct the omission of any discussion related to TLS-PSK identies in that specification, and explain how those identities are used on clients and servers.¶
There are no IANA considerations in this document.¶
RFC Editor: This section may be removed before final publication.¶
- Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
- Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
- Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, DOI 10.17487/RFC2865, , <https://www.rfc-editor.org/info/rfc2865>.
- Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)", RFC 4279, DOI 10.17487/RFC4279, , <https://www.rfc-editor.org/info/rfc4279>.
- Nelson, D., Ed., "Crypto-Agility Requirements for Remote Authentication Dial-In User Service (RADIUS)", RFC 6421, DOI 10.17487/RFC6421, , <https://www.rfc-editor.org/info/rfc6421>.
- Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
- Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, DOI 10.17487/RFC1321, , <https://www.rfc-editor.org/info/rfc1321>.
- Rigney, C., "RADIUS Accounting", RFC 2866, DOI 10.17487/RFC2866, , <https://www.rfc-editor.org/info/rfc2866>.
- Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M., and I. Goyret, "RADIUS Attributes for Tunnel Protocol Support", RFC 2868, DOI 10.17487/RFC2868, , <https://www.rfc-editor.org/info/rfc2868>.
- Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial In User Service) Support For Extensible Authentication Protocol (EAP)", RFC 3579, DOI 10.17487/RFC3579, , <https://www.rfc-editor.org/info/rfc3579>.
- Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B. Aboba, "Dynamic Authorization Extensions to Remote Authentication Dial In User Service (RADIUS)", RFC 5176, DOI 10.17487/RFC5176, , <https://www.rfc-editor.org/info/rfc5176>.
- Tschofenig, H., Ed., Adrangi, F., Jones, M., Lior, A., and B. Aboba, "Carrying Location Objects in RADIUS and Diameter", RFC 5580, DOI 10.17487/RFC5580, , <https://www.rfc-editor.org/info/rfc5580>.
- Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, DOI 10.17487/RFC6151, , <https://www.rfc-editor.org/info/rfc6151>.
- Zorn, G., Zhang, T., Walker, J., and J. Salowey, "Cisco Vendor-Specific RADIUS Attributes for the Delivery of Keying Material", RFC 6218, DOI 10.17487/RFC6218, , <https://www.rfc-editor.org/info/rfc6218>.
- DeKok, A., "RADIUS over TCP", RFC 6613, DOI 10.17487/RFC6613, , <https://www.rfc-editor.org/info/rfc6613>.
- Winter, S., McCauley, M., Venaas, S., and K. Wierenga, "Transport Layer Security (TLS) Encryption for RADIUS", RFC 6614, DOI 10.17487/RFC6614, , <https://www.rfc-editor.org/info/rfc6614>.
- Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., Morris, J., Hansen, M., and R. Smith, "Privacy Considerations for Internet Protocols", RFC 6973, DOI 10.17487/RFC6973, , <https://www.rfc-editor.org/info/rfc6973>.
- DeKok, A., "Datagram Transport Layer Security (DTLS) as a Transport Layer for RADIUS", RFC 7360, DOI 10.17487/RFC7360, , <https://www.rfc-editor.org/info/rfc7360>.
- Winter, S. and M. McCauley, "Dynamic Peer Discovery for RADIUS/TLS and RADIUS/DTLS Based on the Network Access Identifier (NAI)", RFC 7585, DOI 10.17487/RFC7585, , <https://www.rfc-editor.org/info/rfc7585>.