Network Working Group C. Newman Internet Draft: Using SASL/GSSAPI with Telnet Innosoft Document: draft-newman-telnet-sasl-00.txt January 1998 Using SASL and GSSAPI with Telnet Status of this memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Introduction Telnet has its own custom authentication negotiation framework defined in ''Telnet Authentication Option'' [TELNET-AUTH]. This has primarily been used for Kerberos v4 [TELNET-KRB], but is largely unused otherwise as there is only limited development of telnet products and protocols. It is desirable to have telnet leverage development of new security services for new protocols. Therefore, future use of the Telnet authentication option is deprecated in favor of a new SASL [SASL] authentication option suitable for use with SASL and GSSAPI [GSSAPI] mechanisms. This service can complement use of TLS with Telnet [TELNET-TLS]. [NOTE: This proposal is in response to a request by the TN3270e WG to have SASL or GSSAPI available when TLS is too heavy-weight. I will not request a telnet option number for this until there is rough concensus that it is a good idea. Public discussion of this mechanism may take place on the tn3270e@list.nih.gov mailing list with a subscription address of listserv@list.nih.gov. Private comments may be sent to the author]. Newman [Page 1]
Internet Draft Using SASL/GSSAPI with Telnet January 1998 1. Conventions Used in this Document The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in this document are to be interpreted as described in "Key words for use in RFCs to Indicate Requirement Levels" [KEYWORDS]. Formal syntax is defined using ABNF [ABNF]. In examples, "C:" and "S:" indicate lines sent by the client and server respectively. 2. Mechanism Requirements It is important that all telnet implementations are capable of interoperable authentication without the use of unencrypted plaintext passwords. For the usage scenarios with the TN3270 protocol, it looks like the TLS protocol in combination with traditional embedded plaintext passwords will be preferable because TN3270 is often performed by a proxy with no knowledge of the users. This suggests it is the correct mandatory to implement mechanism for TN3270. Regular telnet servers, on the other hand, have been successfully modified to support mechanisms such as Kerberos, and a full TLS layer might be more expensive than necessary in many cases. This is where a lighter-weight SASL or GSSAPI mechanism may be preferable. Unfortunately, the CRAM-MD5 [CRAM-MD5] SASL mechanism (which is likely to be the mandatory to implement choice for a number of services) is particularly unsuited to the Telnet protocol as a machine which supports both CRAM-MD5 and remote login risks exposing the authentication database and thus risks an attacker gaining the ability to impersonate all users to any CRAM-MD5 authenticated service with the same passphrase. The SCRAM-MD5 [SCRAM] mechanism might suffice, but is probably too new to gain rough concensus. This leaves OTP-SHA1 [OTP-SASL] as the preferred choice for the mandatory to implement lightweight authentication mechanism in combination with this telnet extension. [NOTE: This issue is certainly open for debate, as is the wisdom of replacing the old telnet AUTHENTICATE option]. Newman [Page 2]
Internet Draft Using SASL/GSSAPI with Telnet January 1998 3. Kerberos V4 Compatibility Both SASL and the old Telnet authentication option offer Kerberos V4 mechanisms. It is usually not desirable to deploy two incompatible mechanisms for the same function, however, the KERBEROS_V4 SASL mechanism is more resistant to reply attacks and provides encryption services. Currently deployed Kerberos V4 telnet implementations use encryption support which was documented in an expired internet draft and is susceptible to active attacks. Implementations which offer support for the KERBEROS_V4 SASL mechanism SHOULD also implement the old Telnet authentication option Kerberos v4 mechanism. This will provide better interoperability with deployed implementations. When both options are available, the KERBEROS_V4 SASL mechanism SHOULD be used in preference to the old telnet authentication mechanism. The author is not aware of implementation of Kerberos V5 via the old Telnet authentication option. Therefore the GSSAPI SASL mechanism is the preferred method for Kerberos V5. [NOTE: anyone else know of a Kerb5 telnet?] 4. SASL Telnet Option The SASL telnet option is telnet option number XXX. For historical reasons, the GSSAPI/SASL service name for this SASL profile is "rcmd". #define TELOPT_SASL XXX It has the following subnegotiation options: #define TELSASL_LIST 0 #define TELSASL_START 1 #define TELSASL_STEP 2 #define TELSASL_SUCCESS 3 #define TELSASL_FAIL 4 The SASL telnet option is negotiated only one way. The server offers the SASL option with "WILL SASL" and the client announces support with "DO SASL." Once the option is successfully negotiated, the server sends the LIST subnegotiation containing a space separated list of SASL mechanisms available: S: IAC WILL SASL C: IAC DO SASL S: IAC SB SASL LIST "KERBEROS_V4 GSSAPI CRAM-MD5 OTP-SHA1" IAC SB Newman [Page 3]
Internet Draft Using SASL/GSSAPI with Telnet January 1998 The client then sends the START subnegotiation to begin a SASL exchange with the server. This subnegotiation contains the mechanism name followed by an ASCII NUL character followed by the initial client response, if present. The client is not required to wait for the LIST message from the server prior to sending the START message. C: IAC SB SASL START "CRAM-MD5" NUL IAC SE This is followed by a series of STEP messages containing SASL messages for the client and server respectively: S: IAC SB SASL STEP "<1896.697170952@postoffice.reston.mci.net>" IAC SE C: IAC SB SASL STEP "tim b913a602c7eda7a495b4e6e7334d3890" IAC SE Note that it is important to perform IAC doubling if the octet value hexidecimal FF occurs in any SASL data. The server indicates successful completion of the exchange by sending the "SUCCESS" subnegotiation which MAY contain an optional final mutual authentication step. S: IAC SB SASL SUCCESS IAC SE If a SASL security layer is negotiated, it begins on the server end immediately after the SASL SUCCESS subnegotiation, and begins on the client end immediately after the last client START or STEP subnegotiation once the SUCCESS subnegotiation is received. The server indicates failure by sending the FAIL message with an optional error code: S: IAC SB SASL FAIL BADAUTH IAC SE The following error codes are defined: #define TELSASL_BADAUTH 0 /* authentication failed */ #define TELSASL_BADPROT 1 /* protocol violation */ #define TELSASL_NOTAUTHZ 2 /* authorization failed */ #define TELSASL_EXPIRED 3 /* passphrase/credentials expired */ #define TELSASL_ENCRYPT 4 /* encryption or stronger mech needed */ #define TELSASL_TOOWEAK 5 /* mechanism too weak for user */ #define TELSASL_TRANS 6 /* transition needed to use new mech */ #define TELSASL_DISABLED 7 /* account disabled */ BADAUTH This indicates that the user does not exist or the Newman [Page 4]
Internet Draft Using SASL/GSSAPI with Telnet January 1998 authentication failed for a reason other than those listed below. BADPROT This indicates the client attempted to use a mechanism not supported by the server, or the protocol for the SASL mechanism was not followed. NOTAUTHZ This indicates the client successfully authenticated, but is not authorized to login to the service with the requested SASL authorization identity. EXPIRED This indicates that the client passphrase, one time passphrase or public key certificate has expired and can be updated with an appropriate passphrase/credential change protocol. ENCRYPT This indicates that the requested client mechanism is not permitted without an encryption layer, such as that provided by TLS. The client may activate such encryption, or try a stronger mechanism. TOOWEAK This indicates that security policy does not permit the requested user to use the requested mechanism. For example, an administrative user might be required to use a stronger mechanism. TRANSThis indicates the user has a valid verifier in a server authentication database but the requested mechanism can not be used with that verifier. This also indicates that if the client changes the passphrase or does a one-time authentication with a plaintext passphrase mechanism (preferably encrypted), then the appropriate authentication database for the requested mechanism will be initialized. DISABLED This indicates that the user's account has been disabled. The user must contact a system administrator to get their account re-enabled. 10. Security Considerations This inherits the security considerations of SASL [SASL] and any underlying mechanism used. Newman [Page 5]
Internet Draft Using SASL/GSSAPI with Telnet January 1998 The SASL LIST subnegotiation is not integrity protected and is thus susceptible to tampering by an active attacker. The client can address this issue by having a configurable list of acceptable mechanisms. In addition, if a SASL integrity protection layer is negotiated on, the server SHOULD re-issue the SASL LIST subnegotiation after the integrity layer is active so the client has the option of checking for tampering. A client which supports a weaker integrity protected mechanism and a stronger mechanism SHOULD verify the re-issued SASL LIST subnegotiation is unchanged if the weaker integrity protected mechanism is used. With some SASL mechanisms, the ENCRYPT or TOOWEAK error codes will be generated after sensitive information has been exposed. For this reason, clients SHOULD be configurable to disable weaker mechanisms which might reveal sensitive information and SHOULD do so for user, mechanism and server combinations which result in these error codes. The TRANS error code could be spuriously generated by an active attacker. For this reason, the client SHOULD NOT use a weaker mechanism in response to a TRANS error code without explicit user permission. The TRANS error code can also be used to probe for untransitioned users at a site. For this reason, sites must consider the tradeoffs between a user-friendly transition to a stronger mechanism and the risks entailed by permitting such transitions. 11. References [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd, November 1997. [CRAM-MD5] Klensin, Catoe, Krumviede, "IMAP/POP AUTHorize Extension for Simple Challenge/Response", RFC 2195, MCI, September 1997. [GSSAPI] Linn, "Generic Security Service Application Program Interface, Version 2", RFC 2078, OpenVision Technologies, January 1997. [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, Harvard University, March 1997. [OTP-SASL] Newman, "One Time Password SASL mechanism", work in progress. [SASL] Myers, "Simple Authentication and Security Layer (SASL)", RFC 2222, Netscape Communications, October 1997. Newman [Page 6]
Internet Draft Using SASL/GSSAPI with Telnet January 1998 [SCRAM] Newman, "Salted Challenge Response Authentication Mechanism (SCRAM)", work in progress. [TELNET-AUTH] Borman, "Telnet Authentication Option", RFC 1416, Cray Research, Inc., February 1993. [TELNET-KRB] Borman, "Telnet Authentication: Kerberos Version 4", RFC 1411, Cray Research, Inc., January 1993. 12. Author's Address Chris Newman Innosoft International, Inc. 1050 Lakes Drive West Covina, CA 91790 USA Email: chris.newman@innosoft.com Newman [Page 7]