IV) If no mechlistMIC token was included but the MIC token
exchange is required, the negotiation SHALL be terminated.
GSS_Accept_sec_context() indicates GSS_S_DEFECTIVE_TOKEN.
c) In the case that the chosen mechanism exchanges an odd number of
mechanism tokens (i.e., the initiator sends the last mechanism
token), the initiator does the following when generating the
negotiation message containing the last mechanism token: if the
negState was request-mic in the first reply from the target, a
mechlistMIC token MUST be included; otherwise, the mechlistMIC
token is OPTIONAL. (Note that the MIC token exchange is required
if a mechanism other than the initiator’s first choice is chosen.)
In the case that the optimistic mechanism token is the only
mechanism token for the initiator’s preferred mechanism, the
mechlistMIC token is OPTIONAL. Whether the mechlistMIC token is
included, GSS_Init_sec_context() indicates GSS_S_CONTINUE_NEEDED.
Initiators that wish to be compatible with legacy Windows SPNEGO
implementations, as described in Appendix C, should not generate a
mechlistMIC token when the MIC token exchange is not required.
The acceptor then processes the last mechanism token and does one
of the following:
I) If a mechlistMIC token was included and is correctly
verified, GSS_Accept_sec_context() indicates GSS_S_COMPLETE.
The output negotiation message contains a mechlistMIC token
and an accept_complete state. The initiator MUST then verify
this mechlistMIC token.
II) If a mechlistMIC token was included but is incorrect, the
negotiation SHALL be terminated. GSS_Accept_sec_context()
indicates GSS_S_DEFECTIVE_TOKEN.
III) If no mechlistMIC token was included and the mechlistMIC
token exchange is not required, GSS_Accept_sec_context()
indicates GSS_S_COMPLETE. The output negotiation message
contains an accept_complete state.
IV) In the case that the optimistic mechanism token is also the
last mechanism token (when the initiator’s preferred
mechanism is accepted by the target) and the target sends a
request-mic state but the initiator did not send a
mechlistMIC token, the target then MUST include a mechlistMIC
token in that first reply. GSS_Accept_sec_context()
indicates GSS_S_CONTINUE_NEEDED. The initiator MUST verify
the received mechlistMIC token and generate a mechlistMIC
token to send back to the target. The target SHALL, in turn,
verify the returned mechlistMIC token and complete the
negotiation.
V) If no mechlistMIC token was included and the acceptor sent a
request-mic state in the first reply message (the exchange of
MIC tokens is required), the negotiation SHALL be terminated.
GSS_Accept_sec_context() indicates GSS_S_DEFECTIVE_TOKEN.
6. Extensibility
Two mechanisms are provided for extensibility. First, the ASN.1
structures in this specification MAY be expanded by IETF standards
action. Implementations receiving unknown fields MUST ignore these
fields.
Secondly, OIDs corresponding to a desired mechanism attribute (i.e.,
mechanism variants) may be included in the set of preferred
mechanisms by an initiator. The acceptor can choose to honor this
request by preferring mechanisms that have the included attributes.
Future work within the Kitten working group is expected to
standardize common attributes that SPNEGO mechanisms may wish to
support. At this time, it is sufficient to say that initiators MAY
include OIDs that do not correspond to mechanisms. Such OIDs MAY
influence the acceptor’s choice of mechanism. As discussed in
Section 5, if there are mechanisms that, if present in the
initiator’s list of mechanisms, might be preferred by the acceptor
instead of the initiator’s preferred mechanism, the acceptor MUST
demand the MIC token exchange. As the consequence, acceptors MUST
demand the MIC token exchange if they support negotiation of
attributes not available in the initiator’s preferred mechanism,
regardless of whether the initiator actually requested these
attributes.
7. Security Considerations
In order to produce the MIC token for the mechanism list, the
mechanism must provide integrity protection. When the selected
mechanism does not support integrity protection, the negotiation is
vulnerable: an active attacker can force it to use a security
mechanism that is not mutually preferred but is acceptable to the
target.
This protocol provides the following guarantees when per-message
integrity services are available on the established mechanism
context, and the mechanism list was altered by an adversary such that
a mechanism that is not mutually preferred could be selected:
a) If the last mechanism token is sent by the initiator, both peers
shall fail;
b) If the last mechanism token is sent by the acceptor, the acceptor
shall not complete and the initiator, at worst, shall complete
with its preferred mechanism being selected.
The negotiation may not be terminated if an alteration was made but
had no material impact.
The protection of the negotiation depends on the strength of the
integrity protection. In particular, the strength of SPNEGO is no
stronger than the integrity protection of the weakest mechanism
acceptable to GSS-API peers.
Note that where there exist multiple mechanisms with similar context
tokens, but different semantics, such that some or all of the
mechanisms’ context tokens can be easily altered so that one
mechanism’s context tokens may pass for another of the similar
mechanism’s context tokens, then there may exist a downgrade or
similar attacks. For example, if a given family of mechanisms uses
the same context token syntax for two or more variants and depends on
the OID in the initial token’s pseudo-ASN.1/DER wrapper, but does not
provide integrity protection for that OID, then there may exist an
attack against those mechanisms. SPNEGO does not generally defeat
such attacks.
In all cases, the communicating peers are exposed to the denial of
service threat.
8. Acknowledgments
The authors wish to thank Sam Hartman, Nicolas Williams, Ken Raeburn,
Martin Rex, Jeff Altman, Tom Yu, Cristian Ilac, Simon Spero, and Bill
Sommerfeld for their comments and suggestions during the development
of this document.
Luke Howard provided a prototype of this protocol in Heimdal and
resolved several issues in the initial version of this document.
Eric Baize and Denis Pinkas wrote the original SPNEGO specification
[RFC2478] of which some of the text has been retained in this
document.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[X690] ASN.1 encoding rules: Specification of Basic Encoding Rules
(BER), Canonical Encoding Rules (CER) and Distinguished
Encoding Rules (DER), ITU-T Recommendation X.690 (1997) |
ISO/IEC International Standard 8825-1:1998.
9.2. Informative References
[RFC2478] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998.
Appendix A. SPNEGO ASN.1 Module
SPNEGOASNOneSpec {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanism(5) snego (2) modules(4) spec2(2)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
MechType ::= OBJECT IDENTIFIER
-- OID represents each security mechanism as suggested by
-- [RFC2743]
MechTypeList ::= SEQUENCE OF MechType
NegotiationToken ::= CHOICE {
negTokenInit [0] NegTokenInit,
negTokenResp [1] NegTokenResp
}
NegTokenInit ::= SEQUENCE {
mechTypes [0] MechTypeList,
reqFlags [1] ContextFlags OPTIONAL,
-- inherited from RFC 2478 for backward compatibility,
-- RECOMMENDED to be left out
mechToken [2] OCTET STRING OPTIONAL,
mechListMIC [3] OCTET STRING OPTIONAL,
...
}
NegTokenResp ::= SEQUENCE {
negState [0] ENUMERATED {
accept-completed (0),
accept-incomplete (1),
reject (2),
request-mic (3)
} OPTIONAL,
-- REQUIRED in the first reply from the target
supportedMech [1] MechType OPTIONAL,
-- present only in the first reply from the target
responseToken [2] OCTET STRING OPTIONAL,
mechListMIC [3] OCTET STRING OPTIONAL,
...
}
ContextFlags ::= BIT STRING {
delegFlag (0),
mutualFlag (1),
replayFlag (2),
sequenceFlag (3),
anonFlag (4),
confFlag (5),
integFlag (6)
} (SIZE (32))
END
Appendix B. GSS-API Negotiation Support API
In order to provide to a GSS-API caller (the initiator or the target
or both) with the ability to choose among the set of supported
mechanisms, a reduced set of mechanisms for negotiation and two
additional APIs are defined:
o GSS_Get_neg_mechs() indicates the set of security mechanisms
available on the local system to the caller for negotiation, for
which appropriate credentials are available.
o GSS_Set_neg_mechs() specifies the set of security mechanisms to be
used on the local system by the caller for negotiation, for the
given credentials.
B.1. GSS_Set_neg_mechs Call
Inputs:
o cred_handle CREDENTIAL HANDLE, -- NULL specifies default
-- credentials
o mech_set SET OF OBJECT IDENTIFIER
Outputs:
o major_status INTEGER,
o minor_status INTEGER
Return major_status codes:
o GSS_S_COMPLETE indicates that the set of security mechanisms
available for negotiation has been set to mech_set.
o GSS_S_FAILURE indicates that the requested operation could not be
performed for reasons unspecified at the GSS-API level.
This allows callers to specify the set of security mechanisms that
may be negotiated with the credential identified by cred_handle.
This call is intended to support specialized callers who need to
restrict the set of negotiable security mechanisms from the set of
all security mechanisms available to the caller (based on available
credentials). Note that if more than one mechanism is specified in
mech_set, the order in which those mechanisms are specified implies a
relative preference.
B.2. GSS_Get_neg_mechs Call
Input:
o cred_handle CREDENTIAL HANDLE -- NULL specifies default --
credentials
Outputs:
o major_status INTEGER,
o minor_status INTEGER,
o mech_set SET OF OBJECT IDENTIFIER
Return major_status codes:
o GSS_S_COMPLETE indicates that the set of security mechanisms
available for negotiation has been returned in mech_set.
o GSS_S_FAILURE indicates that the requested operation could not be
performed for reasons unspecified at the GSS-API level.
This allows callers to determine the set of security mechanisms
available for negotiation with the credential identified by
cred_handle. This call is intended to support specialized callers
who need to reduce the set of negotiable security mechanisms from the
set of supported security mechanisms available to the caller (based
on available credentials).
Note: The GSS_Indicate_mechs() function indicates the full set of
mechanism types available on the local system. Since this call has
no input parameter, the returned set is not necessarily available for
all credentials.
Appendix C. Changes since RFC 2478
SPNEGO implementations in Microsoft Windows 2000/Windows XP/Windows
Server 2003 have the following behavior: no mechlistMIC is produced
and mechlistMIC is not processed if one is provided; if the initiator
sends the last mechanism token, the acceptor will send back a
negotiation token with an accept_complete state and no mechlistMIC
token. In addition, an incorrect OID (1.2.840.48018.1.2.2) can be
used to identify the GSS-API Kerberos Version 5 mechanism.
The following changes have been made to be compatible with these
legacy implementations.
* NegTokenTarg is changed to negTokenResp and is the message format
for all subsequent negotiation tokens.
* NegTokenInit is the message for the initial negotiation message,
and only that message.
* mechTypes in negTokenInit is not optional.
* If the selected mechanism is also the most preferred mechanism for
both peers, it is safe to omit the MIC tokens.
If at least one of the two peers implements the updated pseudo
mechanism in this document, the negotiation is protected.
The following changes are to address problems in RFC 2478.
* reqFlags is not protected, therefore it should not impact the
negotiation.
* DER encoding is required.
* GSS_GetMIC() input is clarified.
* Per-message integrity services are requested for the negotiated
mechanism.
* Two MIC tokens are exchanged, one in each direction.
An implementation that conforms to this specification will not
inter-operate with a strict RFC 2748 implementation. Even if the new
implementation always sends a mechlistMIC token, it will still fail
to inter-operate. If it is a server, it will fail because it
requests a mechlistMIC token using an option that older
implementations do not support. Clients will tend to fail as well.
As an alternative to the approach chosen in this specification, we
could have documented a correct behavior that is fully backward
compatible with RFC 2478 and included an appendix on how to inter-
operate with existing incorrect implementations of RFC 2478.
As a practical matter, the SPNEGO implementers within the IETF have
valued interoperability with the Microsoft implementations. We were
unable to choose to maintain reasonable security guarantees, to
maintain interoperability with the Microsoft implementations, and to
maintain interoperability with correct implementations of RFC 2478.
The working group was not aware of any RFC 2478 implementations
deployed on the Internet. Even if there are such implementations, it
is unlikely that they will inter-operate because of a critical flaw
in the description of the encoding of the mechanism list in RFC 2478.
With the approach taken in this specification, security is ensured
between new implementations all the time while maintaining
interoperability with the implementations deployed within the IETF
community. The working group believes that this justifies breaking
compatibility with a correct implementation of RFC 2478.
Appendix D. mechListMIC Computation Example
The following is an example to illustrate how the mechListMIC field
would be computed.
The initial part of the DER encoding of NegTokenInit is constructed
as follows (the "nn" are length encodings, possibly longer than one
octet):
30 -- identifier octet for constructed SEQUENCE (NegTokenInit)
nn -- length
-- contents octets of the SEQUENCE begin with
-- DER encoding of "[0] MechTypeList":
A0 -- identifier octet for constructed [0]
nn -- length
-- contents of the constructed [0] are DER encoding
-- of MechTypeList (which is a SEQUENCE):
30 -- identifier octet for constructed SEQUENCE
nn -- length
-- contents octets of the SEQUENCE begin with
-- DER encoding of OBJECT IDENTIFIER:
06 -- identifier octet for primitive OBJECT IDENTIFIER
09 -- length
2A 86 48 86 F7 12 01 02 02 -- Kerberos V5
-- {1 2 840 113554 1 2 2}
If a mechlistMIC needs to be generated (according to the rules in
Section 5), it is computed by using the DER encoding of the type
MechTypeList data from the initiator’s NegTokenInit token as input to
the GSS_GetMIC() function. In this case, the MIC would be computed
over the following octets:
DER encoding of MechTypeList:
30 nn 06 09 2A 86 48 86 F7 12 01 02 02 ...
Note that the identifier octet and length octet(s) for constructed
[0] (A0 nn) are not included in the MIC computation.
Authors’ Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: lzhu@microsoft.com
Paul Leach
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: paulle@microsoft.com
Karthik Jaganathan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: karthikj@microsoft.com
Wyllys Ingersoll
Sun Microsystems
1775 Wiehle Avenue, 2nd Floor
Reston, VA 20190
US
EMail: wyllys.ingersoll@sun.com
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