How Kerberos user-to-user authentication works?

The Kerberos user-to-user (U2U) authentication mechanism enables a client to authenticate to a service that is not in possession of the long-term secret key. U2U allows one principal to authenticate using a ticket encrypted with the session key from a TGT issued to another principal. This article discusses the messages involved in the mechanism. It illustrates various scenarios and applicability settings. It finally provides samples of packet exchanges.

Table of Contents

Preamble 2
Glossary 2
Refresher: Basic Kerberos authentication 2

The Authentication Service Exchange 2
The Ticket-Granting Service Exchange 3
The Client / Application Server Exchange 3

User-to-User Authentication Exchange 3

User-to-User Mechanism 3
User-to-User GSS-API messages 4
User-to-User tickets are not cached 5

How is U2U authentication initiated? 5

Deliberate client awareness 5
U2U via KDC policy 5
U2U via server policy 5

U2U illustrations 5

Deliberate Kerberos U2U from client: CredSSP in Remote Desktop Session 5
Kerberos U2U: Enabling SSO with NTLM for smart card sign-in 6
Kerberos U2U via KDC policy: Offering Remote Assistance 6

Conclusion 7

Appendix 1. Related Kerberos Error codes 7
Appendix 2. Steps of standard CredSSP’s Kerberos U2U from client 8
Appendix 3. Steps of Kerberos U2U via KDC policy in Offering Remote Assistance 24
References 48

Preamble

This article is not about Public Key Cryptography Based User-to-User (PKU2U). PKU2U is used in homegroup environments to authenticate peers by leveraging local certificates between peer computers.
This article is not about Service for User and Constrained Delegation, both referred to as S4U [MS-SFU]. S4U enables an application service to obtain a Kerberos service ticket on behalf of a user (S4U2Self, S4U2Proxy).
We assume the reader is familiar with Kerberos authentication [RFC4120] and GSS-API concepts [RFC2743]. We start the article with a terse overview of basic Kerberos authentication before elaborating on U2U.

Glossary

API – Application Program Interface
AP – Application server
AS – Authentication service
GSS – Generic Security Service
KDC - Key distribution center
PAC – Privilege Account Certificate
SPN - Service Principal Name
SPNEGO - Simple and Protected GSSAPI Negotiation Mechanism
TGT - Ticket granting ticket
TGS - Ticket granting service
U2U – User-to-user

Refresher: Basic Kerberos authentication

In its classic form, the Kerberos protocol supports authentication between a client and an application server. It consists of three main message exchanges.

The Authentication Service Exchange

The AS exchange consists of KRB_AS_REQ and KRB_AS_REP. In the KRB_AS_REQ, the client requests a TGT from the KDC’s Authentication Service (AS). In the KRB_AS_REP, the AS returns the TGT, which is the user's initial ticket meant only for use by the TGS. The TGT includes a session key that the client can use to encrypt communication with the TGS.
The TGT is encrypted with a key only known by the KDC (the KDC key is shared by all KDCs in the realm). Thus, the TGT is opaque to all entities except the KDC itself. The TGT has a short validity time (typically 10 hours).
The encrypted part of AS exchange uses the key that the client shares with the KDC.

The Ticket-Granting Service Exchange

The TGS exchange involves KRB_TGS_REQ and KRB_TGS_REP. The client requests the TGS to issue a service ticket for the target service. In the KRB_TGS_REQ, the client presents the TGT, an authenticator, and identifies the target service by its SPN.
The TGS validates the TGT and the authenticator. It then issues a service ticket. It encrypts the ticket with the long-term key that is shared by the KDC and the target service. The ticket includes the client's identity and a copy of the session key. The TGS encrypts the client's copy of the session key with the key that the KDC shares with the client.

The Client / Application Server Exchange

To authenticate to the server, the client composes KRB_AP_REQ and sends the service ticket along with a new authenticator to the target server. The server decrypts the ticket with its long-term key and validates the authenticator. In Windows, the service also creates an access token based on the user’s PAC.
Optionally, the client might have requested mutual authentication with the target server. If so, the target server retrieves the client’s timestamp from the authenticator, encrypts it with the session key, and returns it to the client as part of the KRB_AP_REP.

User-to-User Authentication Exchange

U2U authentication provides a method to perform authentication when the verifier does not have access to the service’s long-term key. The basic form of Kerberos authentication is not suitable for peer-to-peer authentication. The U2U variant of Kerberos enables authentication of such peers. The idea is to obtain a TGS ticket encrypted with the session key from another TGT issued to another user, instead of the long-term server key that would have not be accessible in the acceptor’s context.
The following sections summarize the user-to-user authentication sequence.

User-to-User Mechanism

The OID for user-to-user mechanism is:

1.2.840.113554.1.2.2.3

It is a child OID of GSSAPI Kerberos OID 1.2.840.113554.1.2.2.

The U2U mech-type extends the conventional Kerberos GSS-API protocol and adds a round trip with the server to retrieve the TGT from the service.

User-to-User GSS-API messages

In the GSS-API encapsulated message exchange for U2U Kerberos, the request token has an innerContextToken with a 2-octet TOK_ID field containing 04 00 (KERB-TGT-REQUEST) followed by the message:

KERB-TGT-REQUEST ::= SEQUENCE {
pvno[0]                         INTEGER,
msg-type[1]                     INTEGER,
server-name[2]                  PrincipalName OPTIONAL,
realm[3]                        Realm OPTIONAL

All fields are defined in the Kerberos RFC4120 and informational RFC draft for U2U [U2UDraft].
msg-type is KRB_TGT_REQ (16).

The response message is a KERB_TGT_REPLY token which has an innerContextToken with a 2-octet TOK_ID field containing 04 01 (KERB-TGT-REPLY) followed by the message as follows:

KERB-TGT-REPLY ::= SEQUENCE {
pvno[0]                         INTEGER,
msg-type[1]                     INTEGER,
ticket[2]                       Ticket
}
msg-type is KRB_TGT_REP (17). The ticket contains the TGT for the service.

Upon receipt of the service TGT, the client initiator can now request the KDC to issue a service ticket by using a KERB-TGS-REQ with the KDC-Options ENC-TKT-IN-SKEY and the service TGT in the additional-tickets of the KDC-REQ-BODY.

Because ENC-TKT-IN-SKEY option is present in the KRB_KDC_REQ, the Sname may be absent. However, the Sname for U2U ticket is typically set to the principal name (i.e. user name) whose TGT the server supplied to the client. If Sname is absent, the KDC will use the name from the client in the additional ticket.

The TGS honors the request and issues the ticket (KERB-TGS-REP). It encrypts the ticket with the session key of the TGT in the additional ticket.

The client finally continues with the AP exchange. In the KERB-AP-REQ, the client sets USE-SESSION-KEY flag in the AP-Options to indicate to the server that U2U authentication is in use, and that the service ticket is encrypted in the session key from the server's TGT rather than in the server's long-term key.

User-to-User tickets are not cached

U2U service tickets are not cacheable. A U2U ticket is only usable to one instance of a client application. On the other hand, to cache a ticket, it must be valid to all instances of the service that share that SPN. Multiple authentications that share a cacheable service ticket will likewise share a session key.

How is U2U authentication initiated?

The Kerberos protocol does not specify the client’s trigger to U2U authentication. It is in the application protocol sequence that the client obtains the server’s TGT. Once this pre-requisite is fulfilled, the client can initiate user-to-user authentication as previously described.
U2U is often used in cases where SPNEGO starts Kerberos and then fails either with KDC_ERR_MUST_USE_USER2USER from the KDC or with KRB_AP_ERR_USER_TO_USER_REQUIRED from the server.

Deliberate client awareness

When the client is aware that the application server requires user-to-user authentication, it will explicitly request U2U mechanism in the first call to GSS_Init_Sec_Context.

U2U via KDC policy

The KDC may implement a policy on user accounts to reject conventional service ticket requests. When a KERB_TGS_REQ that does not include a second ticket with an ENC_TKT_IN_SKEY KDC Option, the KDC will respond with KDC_ERR_MUST_USE_USER2USER (0x1B).

U2U via server policy

When a client sends a conventional KRB_AP_REQ and the service requires U2U authentication, the server will respond with KRB_AP_ERR_USER_TO_USER_REQUIRED (0x45). The server may provide its TGT in the data of the error message.

U2U illustrations

Deliberate Kerberos U2U from client: CredSSP in Remote Desktop Session

For Kerberos under CredSSP, the client chooses to do U2U on its own without a prior error about U2U being required. The client initiates U2U because CredSSP calls InitializeSecurityContext with the ISC_REQ_USE_SESSION_KEY flag. That flag requires a new session key, which Kerberos honors by doing U2U.
When one observes network packets of Windows-based CredSPP, the Kerberos exchanges start with a conventional TGS ticket request, which is unexpected. This conventional TGS exchange could be explained by the fact that the code that forces to transition to U2U is emulating the reception of the user-to-user required error, and thus after the point where we get the normal ticket.  That initial TGS preceding the first CredSSP TSRequest is a useless ticket retrieval. Indeed, that ticket is not presented in the TSRequest. Instead the client deliberately decides to send a U2U inside an SPNEGO token of the TSRequest.
CredSSP resorts to U2U mechanism because U2U service tickets are not cacheable. When CredSSP uses U2U, it gets a unique session key to encrypt credentials and this remains private to the process instance. Had that connection used Kerberos without U2U, then the session key used to protect the credentials would have been available to any process running as the user that shares the user’s ticket cache.
Note that the SPNEGO token of the first CSSP TSRequest has UserToUserMechanism in the MechToken = InitialContextToken{ThisMech: UserToUserMechanism (1.2.840.113554.1.2.2.3) . . .}
Even though the MechTypes list contains:
MechTypes: {MsKerberosToken (1.2.840.48018.1.2.2); KerberosToken (1.2.840.113554.1.2.2); Negoex (1.3.6.1.4.1.311.2.2.30); NLMP (1.3.6.1.4.1.311.2.2.10)}
Here, the MechType is not really being negotiated.
In the appendix, we provide illustrative packets of the exchange. The outer TLS session of the CredSSP packets has been decrypted.

Kerberos U2U: Enabling SSO with NTLM for smart card sign-in

Kerberos U2U helps to provide single-sign-on (SSO) with NTLM for a smart card sign-in. When the user logs in a client with a smartcard, Kerberos U2U enables SSO to network services that require NTLM. One such scenario is a remote desktop connection to a non-domain-joined server where the client needs to delegate its credentials to the server which is not capable of Kerberos authentication.
The client uses Kerberos U2U to retrieve the NTOWF hash of the user’s password. It starts with a PKINIT exchange of KERB_AS_REQ / KERB_AS_REP with AS-PK-REQ / AS-PK-REP as specified in [MS-PKCA]. By using the PIN to unlock the smartcard, the client reads the principal name and certificate, then signs the AS-REQ which the KDC will validate before issuing the TGT.
The client then retrieves a U2U TGS with the user name as Sname. In the U2U TGS’s KDC-REQ-BODY, the client uses the ENC-TKT-IN-SKEY option and supplies its TGT in the additional tickets. This allows to decrypt the resulting PAC (with the session key) and get the NTOWF in NTLM_SUPPLEMENTAL_CREDENTIAL.
The encrypted NTOWF is part of the PAC and thus gets propagated to service tickets that the user acquires, but only the system to which the user logs on interactively can decrypt it. This allows a user that logs on with a smartcard to acquire credentials necessary to access resources that require NTLM.
Note that the encrypted NTOWF in the PAC is not due to U2U. The client uses Kerberos U2U because a U2U ticket is un-cacheable. The session key remains privy to that process instance. When another authentication is needed, there is a separate ticket request.

Kerberos U2U via KDC policy: Offering Remote Assistance

This example illustrates U2U authentication with the trigger via KDC policy. It uses the scenario of an Expert providing an unsolicited Remote Assistance (RA) to a Novice. The Expert initiates the RA offer with “msra /offerRA <Novice-FQDN>” command.
To establish the RA session to the host identified by the Novice-FQDN, the Expert needs to obtain a Kerberos ticket to the user account of the Novice.
First, the Expert’s computer tries to obtain a Kerberos ticket for the user account on the Novice's computer. The user account does not have any service principal name (SPN) that is registered. Thus, the KDC service on the DC returns the error KDC_S_PRINCIPAL_UNKNOWN. The extended error information conveys the client should fall back to U2U authentication (the KDC reply indicates KDC_ERR_MUST_USE_USER2USER in its extended error information).
The Expert computer sends the U2U token to the Novice’s computer and obtain the user’s TGT. In this scenario, this U2U token exchange happens as part of the MSRPC Bind because the RA session goes over DCOM (MSRPC).
The Expert computer uses an SPN with "RestrictedKrbHost" service class because it has the server name but not the identity of the service. This SPN is meant to authenticate with the server and not the service.
Afterwards, it proceeds U2U authentication exchange, as previously described.
In the appendix, we show an example of parsed packets of the exchange.

Conclusion

In this article, we discussed how Kerberos user-to-user authentication works. It is often an unspoken extension to Kerberos. We described the message exchange and provided examples of applications. The appendixes provide samples of U2U authentication exchanges.

Appendix 1. Related Kerberos Error codes

Error code, Error macro, Description

0x6, KDC_ERR_C_PRINCIPAL_UNKNOWN, Client not found in Kerberos database
0x7, KDC_ERR_S_PRINCIPAL_UNKNOWN, Server not found in Kerberos database
0x1B, KDC_ERR_MUST_USE_USER2USER, Server principal valid for user2user only
0x45, KRB_AP_ERR_USER_TO_USER_REQUIRED, User-to-user authorization is required
A network packet capture generally shows the error codes in KERB-ERROR responses. To further diagnose, the Kerberos Event log can be useful, notably when there is extended error information.
For diagnostic purpose, you can set the following registry to enable Kerberos events. This should not be left enabled on live production, otherwise it will fill up unnecessary disk space.
https://support.microsoft.com/en-us/help/262177/how-to-enable-kerberos-event-logging
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\Kerberos\Parameters
Registry Value: LogLevel
Value Type: REG_DWORD
Value Data: 0x1

Appendix 2. Steps of standard CredSSP’s Kerberos U2U from client

This client’s TGT will be used later during U2U TGS.

KerberosV5 KerberosV5:AS Request Cname: Administrator Realm: contoso4 Sname: krbtgt/contoso4
KerberosV5 KerberosV5:KRB_ERROR  - KDC_ERR_PREAUTH_REQUIRED (25)
KerberosV5 KerberosV5:AS Request Cname: Administrator Realm: contoso4 Sname: krbtgt/contoso4
KerberosV5 KerberosV5:AS Response Ticket[Realm: CONTOSO4.COM, Sname: krbtgt/CONTOSO4.COM]

The following TGS ticket (to Sname: TERMSRV/sut01tp4.contoso4.com) will not be used, although it is retrieved.

KerberosV5 KerberosV5:TGS Request Realm: CONTOSO4.COM Sname: TERMSRV/sut01tp4.contoso4.com
KerberosV5 KerberosV5:TGS Response Cname: Administrator

The client initiates U2U.

CSSP CredSSP:TSRequest, Version: 3

  TLSSSLData: Transport Layer Security (TLS) Payload Data
+ TLS: TLS Rec Layer-1 SSL Application Data
- Cssp: CredSSP
- TsRequest:
+ SequenceHeader:
+ Tag0:
+ Version: 3
+ Tag1:
- NegoTokens:
+ SequenceOfHeader:
+ SequenceHeader:
+ Tag0:
+ AsnOctetStringHeader:
- NegoToken:
- InitialContextToken:
+ ApplicationHeader:
+ ThisMech: SpnegoToken (1.3.6.1.5.5.2)
- InnerContextToken: 0x1
- SpnegoToken: 0x1
+ ChoiceTag:
- NegTokenInit:
+ SequenceHeader:
+ Tag0:
- MechTypes: Prefer MsKerberosToken (1.2.840.48018.1.2.2)
+ SequenceHeader:
+ MechType: MsKerberosToken (1.2.840.48018.1.2.2)
+ MechType: KerberosToken (1.2.840.113554.1.2.2)
+ MechType: Negoex (1.3.6.1.4.1.311.2.2.30)
+ MechType: NLMP (1.3.6.1.4.1.311.2.2.10)
+ Tag2:
+ OctetStringHeader:
- MechToken: unused (0)
- MsKerberosToken: unused (0)
- KerberosInitToken:
+ ApplicationHeader:
+ ThisMech: UserToUserMechanism (1.2.840.113554.1.2.2.3)
- InnerContextToken: unused (0)
- KerberosToken: unused (0)
TokId: 0x400
- UsertoUserSessionReq: KRB_TGT_REQ (0x10)
+ SequenceHeader:
+ Tag0:
+ pvno: 5
+ Tag1:
+ MessageType: 16
+ Tag2:
+ ServerName: TERMSRV/sut01tp4.contoso4.com

the client receives the TGT form the server.

CSSP CredSSP:TSRequest, Version: 3

  TLSSSLData: Transport Layer Security (TLS) Payload Data
+ TLS: TLS Rec Layer-1 SSL Application Data
- Cssp: CredSSP
- TsRequest:
+ SequenceHeader:
+ Tag0:
+ Version: 3
+ Tag1:
- NegoTokens:
+ SequenceOfHeader:
+ SequenceHeader:
+ Tag0:
+ AsnOctetStringHeader:
- NegoToken:
- NegotiationToken:
+ ChoiceTag:
- NegTokenResp:
+ SequenceHeader:
+ Tag0:
+ NegState: accept-incomplete (1)
+ Tag1:
+ SupportedMech: MsKerberosToken (1.2.840.48018.1.2.2)
+ Tag2:
+ OctetStringHeader:
- ResponseToken: unused (0)
- MsKerberosToken: unused (0)
- KerberosInitToken:
+ ApplicationHeader:
+ ThisMech: UserToUserMechanism (1.2.840.113554.1.2.2.3)
- InnerContextToken: unused (0)
- KerberosToken: unused (0)
TokId: 0x401
- UsertoUserSessionRep: KRB_TGT_REP (0x11)
+ SequenceHeader:
+ Tag0:
+ pvno: 5
+ Tag1:
+ MessageType: 17
+ Tag2:
- Ticket: Realm: CONTOSO4.COM, Sname: krbtgt/CONTOSO4.COM
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ TktVno: 5
+ Tag1:
+ Realm: CONTOSO4.COM
+ Tag2: 0x1
+ Sname: krbtgt/CONTOSO4.COM
+ Tag3: 0x1
- EncPart:
+ SequenceHeader:
+ Tag0:
+ EType: aes256-cts-hmac-sha1-96 (18)
+ Tag1:
+ KvNo: 2
+ Tag2:
+ Cipher: h:Ì°a---------------12Bz"?Uâ*sä‘:ñ‹?ç;†·8µFnDê1@È\wÐV8–ÄÔ÷uÊdÈvqÙÜ0 ---------------/¤<ϼ»¢þkÎgÈmíXeðoôfòWòùÝ?$5.---------------´äŒ^´ÛÝïcJõ@ûdY’.®?`Ä×53+iw˜
¿ÞFv´&q'`™Íü…01W
PÎø.ºOç3|+g`"CRaØ]#\ÊñÞ(xªþ›•â¸ô屡MŠå.05HÓö“¶ZÆQ†·&-------------------------------------------------------------¾}7^a•çúÅ£Ù.´sÕL†ÖãR1---------------œ{íºÄ/?)\" Lò[áqœU

The client requests service ticket for U2U authentication.

KerberosV5 KerberosV5:TGS Request Realm: CONTOSO4.COM Sname: TERMSRV/sut01tp4.contoso4.com

- Kerberos: TGS Request Realm: CONTOSO4.COM Sname: TERMSRV/sut01tp4.contoso4.com
+ Length: Length = 2674
- TgsReq: Kerberos TGS Request
+ ApplicationTag:
- KdcReq: KRB_TGS_REQ (12)
+ SequenceHeader:
+ Tag1:
+ Pvno: 5
+ Tag2:
+ MsgType: KRB_TGS_REQ (12)
+ Tag3:
- PaData:
+ SequenceOfHeader:
- PaData: PA-TGS-REQ (1)
+ SequenceHeader:
+ Tag1:
+ PaDataType: PA-TGS-REQ (1)
+ Tag2:
+ OctetStringHeader:
- KrbApReq: KRB_AP_REQ (14)
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ PvNo: 5
+ Tag1:
+ MsgType: KRB_AP_REQ (14)
+ Tag2: 0x1
+ ApOptions:
+ Tag3:
- Ticket: Realm: CONTOSO4.COM, Sname: krbtgt/CONTOSO4.COM
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ TktVno: 5
+ Tag1:
+ Realm: CONTOSO4.COM
+ Tag2: 0x1
+ Sname: krbtgt/CONTOSO4.COM
+ Tag3: 0x1
- EncPart:
+ SequenceHeader:
+ Tag0:
+ EType: aes256-cts-hmac-sha1-96 (18)
+ Tag1:
+ KvNo: 2
+ Tag2:
+ Cipher: ¯++ä¨OQòåc ¦¹ÿÑãIªã<»Xðë.`¾bÌÎØI?N¦ÛIXzCXaº´Æ}±?­Õ6
›D£ði­ñËÕ?ÉeÕ?J0W¬šÚDÚù ¸aŠNxDÉ»’ÓQŠh ø˘öÕ¡¼wÊ?$挪ya}|^líà@—ÎëÚ÷.\ê¯z™Ÿðîö¢N‡Ù†¥¦2l Q?Ñ=úG²R¨Mj/lÕ¾­éŒL<?—·…¸Ñ=?ãšF;ùÿ\\¸Ìr“ "
ôägùÖ°k…ÐÈÜG.Qƒ¼»IÐG¼9˜‚yDÿx«åÿ[+H=Ç\ýmù›QÐ
+ Tag4:
+ Authenticator:
+ PaData: PA-PAC-OPTIONS (167)
+ Tag4:
- ReqBody:
+ SequenceHeader:
+ Tag0:
- KdcOptions: 0x40810008
+ KerberosFlagsHeader:
+ Padding:
- KrbFlags: 0x40810008
Reserved:              (0...............................)
Forwardable:           (.1..............................) Ticket to be issued is to have its FORWARDABLE flag set
Forwarded:             (..0.............................) This is not a request for forwarding
Proxiable:             (...0............................) Ticket to be issued is not to have its PROXIABLE flag set
Proxy:                 (....0...........................) This is not a request for a proxy
AllowPostDate:         (.....0..........................) Ticket to be issued is not to have its MAY_POSTDATE flag set
PostDated:             (......0.........................) This is not a request for a postdated ticket
Unused7:               (.......0........................)
Renewable:             (........1.......................) Ticket to be issued is to have its RENEWABLE flag set
Unused9:               (.........0......................)
Unused10:              (..........0.....................)
OptHardwareAuth:       (...........0....................)
Unused12:              (............0...................)
Unused13:              (.............0..................)
CnameInAddlTkt:        (..............0.................) This is not a request for S4U2proxy functionality
Canonicalize:          (...............1................)
Unused16:              (................0000000000......)
DisableTransitedCheck: (..........................0.....) Checking of the transited field is enabled
RenewableOk:           (...........................0....) Renewable ticket is not acceptable
EncTktInSkey:          (............................1...) Indicates that the ticket for the end server is to be encrypted in the session key from the additional TGT provided
Unused29:              (.............................0..)
Renew:                 (..............................0.) Present request is not for a renewal
Validate:              (...............................0) Request is not to validate a postdated ticket
+ Tag2: 0x1
+ Realm: CONTOSO4.COM
+ Tag3:
+ Sname: TERMSRV/sut01tp4.contoso4.com
+ Tag5: 0x1
+ Till: 09/13/2037 02:48:05 UTC
+ Tag7:
+ Nonce: 794422766 (0x2F59EDEE)
+ Tag8:
+ Etype:
+ TagA:
+ EncAuthorizationData:
+ TagB:
- AdditionalTickets:
+ SequenceOfHeader:
- Ticket: Realm: CONTOSO4.COM, Sname: krbtgt/CONTOSO4.COM
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ TktVno: 5
+ Tag1:
+ Realm: CONTOSO4.COM
+ Tag2: 0x1
+ Sname: krbtgt/CONTOSO4.COM
+ Tag3: 0x1
- EncPart:
+ SequenceHeader:
+ Tag0:
+ EType: aes256-cts-hmac-sha1-96 (18)
+ Tag1:
+ KvNo: 2
+ Tag2:
+ Cipher: h:Ì°a---------------12Bz"?Uâ*sä‘:ñ‹?ç;†·8µFnDê1@È\wÐV8–ÄÔ÷uÊdÈvqÙÜ0 ---------------/¤<ϼ»¢þkÎgÈmíXeðoôfòWòùÝ?$5.---------------´äŒ^´ÛÝïcJõ@ûdY’.®?`Ä×53+iw˜
¿ÞFv´&q'`™Íü…01W
PÎø.ºOç3|+g`"CRaØ]#\ÊñÞ(xªþ›•â¸ô屡MŠå.05HÓö“¶ZÆQ†·&-------------------------------------------------------------¾}7^a•çúÅ£Ù.´sÕL†ÖãR1---------------œ{íºÄ/?)\" Lò[áqœU

KerberosV5 KerberosV5:TGS Response Cname: Administrator

- Kerberos: TGS Response Cname: Administrator
+ Length: Length = 1638
- TgsRep: Kerberos TGS Response
+ ApplicationTag:
- KdcRep: KRB_TGS_REP (13)
+ SequenceHeader:
+ Tag0:
+ PvNo: 5
+ Tag1:
+ MsgType: KRB_TGS_REP (13)
+ Tag3:
+ Crealm: CONTOSO4.COM
+ Tag4:
+ Cname: Administrator
+ Tag5:
- Ticket: Realm: CONTOSO4.COM, Sname: TERMSRV/sut01tp4.contoso4.com
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ TktVno: 5
+ Tag1:
+ Realm: CONTOSO4.COM
+ Tag2: 0x1
+ Sname: TERMSRV/sut01tp4.contoso4.com
+ Tag3: 0x1
- EncPart:
+ SequenceHeader:
+ Tag0:
+ EType: aes256-cts-hmac-sha1-96 (18)
+ Tag2:
+ Cipher: ­[¼ˆÀfÙ÷&v.'æÿÒ~”´Ñ95D³3›Ø‹éæD@OÆæÆþ@œ?¹oÙÁ}&&MáÛ`? ‘oh¼õ?Îýƒíˆû---------------÷68”öR®˜ÏïV‹?ö5¾_ú>´I4˜
?ØDχɚ&”ù«yaøýª·mv?ïa%ß À
+ Tag6:
+ EncPart:

CSSP CredSSP:TSRequest, Version: 3

  TLSSSLData: Transport Layer Security (TLS) Payload Data
+ TLS: TLS Rec Layer-1 SSL Application Data
- Cssp: CredSSP
- TsRequest:
+ SequenceHeader:
+ Tag0:
+ Version: 3
+ Tag1:
- NegoTokens:
+ SequenceOfHeader:
+ SequenceHeader:
+ Tag0:
+ AsnOctetStringHeader:
- NegoToken:
- NegotiationToken:
+ ChoiceTag:
- NegTokenResp:
+ SequenceHeader:
+ Tag0:
+ NegState: accept-incomplete (1)
+ Tag2:
+ OctetStringHeader:
- ResponseToken: unused (0)
- KerberosToken: unused (0)
- KerberosInitToken:
+ ApplicationHeader:
+ ThisMech: UserToUserMechanism (1.2.840.113554.1.2.2.3)
- InnerContextToken: unused (0)
- KerberosToken: unused (0)
TokId: Krb5ApReq (0x100)
- ApReq: KRB_AP_REQ (14)
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ PvNo: 5
+ Tag1:
+ MsgType: KRB_AP_REQ (14)
+ Tag2: 0x1
- ApOptions:
+ KerberosFlagsHeader:
+ Padding:
- KrbFlags: 0x60000000
Reserved:       (0...............................)
UseSessionKey:  (.1..............................)
MutualRequired: (..1.............................)
Unused:         (...00000000000000000000000000000)
+ Tag3:
- Ticket: Realm: CONTOSO4.COM, Sname: TERMSRV/sut01tp4.contoso4.com
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ TktVno: 5
+ Tag1:
+ Realm: CONTOSO4.COM
+ Tag2: 0x1
+ Sname: TERMSRV/sut01tp4.contoso4.com
+ Tag3: 0x1
- EncPart:
+ SequenceHeader:
+ Tag0:
+ EType: aes256-cts-hmac-sha1-96 (18)
+ Tag2:
+ Cipher: ­[¼ˆÀfÙ÷&v.'æÿÒ~”´Ñ95D³3›Ø‹éæD@OÆæÆþ@œ?¹oÙÁ}&&MáÛ`? ‘oh¼õ?Îýƒíˆû---------------÷68”öR®˜ÏïV‹?ö5¾_ú>´I4˜
?ØDχɚ&”ù«yaøýª·mv?ïa%ß À
+ Tag4:
+ Authenticator:

CSSP CredSSP:TSRequest, Version: 3

  TLSSSLData: Transport Layer Security (TLS) Payload Data
+ TLS: TLS Rec Layer-1 SSL Application Data
- Cssp: CredSSP
- TsRequest:
+ SequenceHeader:
+ Tag0:
+ Version: 3
+ Tag1:
- NegoTokens:
+ SequenceOfHeader:
+ SequenceHeader:
+ Tag0:
+ AsnOctetStringHeader:
- NegoToken:
- NegotiationToken:
+ ChoiceTag:
- NegTokenResp:
+ SequenceHeader:
+ Tag0:
+ NegState: accept-incomplete (1)
+ Tag2:
+ OctetStringHeader:
- ResponseToken: KRB_AP_REP (15)
- KerberosToken: KRB_AP_REP (15)
- KerberosInitToken:
+ ApplicationHeader:
+ ThisMech: UserToUserMechanism (1.2.840.113554.1.2.2.3)
- InnerContextToken: KRB_AP_REP (15)
- KerberosToken: KRB_AP_REP (15)
TokId: Krb5ApRep (0x200)
+ ApRep: KRB_AP_REP (15)
+ Tag3:
+ OctetStringHeader:
+ MechListMic: KRB_AP_REP (15)

CSSP CredSSP:TSRequest, Version: 3

  TLSSSLData: Transport Layer Security (TLS) Payload Data
+ TLS: TLS Rec Layer-1 SSL Application Data
- Cssp: CredSSP
- TsRequest:
+ SequenceHeader:
+ Tag0:
+ Version: 3
+ Tag1:
- NegoTokens:
+ SequenceOfHeader:
+ SequenceHeader:
+ Tag0:
+ AsnOctetStringHeader:
- NegoToken:
- NegotiationToken:
+ ChoiceTag:
- NegTokenResp:
+ SequenceHeader:
+ Tag0:
+ NegState: accept-completed (0)
+ Tag3:
+ OctetStringHeader:
- MechListMic: unused (0)
- KerberosToken: unused (0)
TokId: GSS_GetMIC (0x404)
- Krb5GssV2GetMic:
+ Flags: 4 (0x4)
Filler: Binary Large Object (5 Bytes)
SndSeq: 792999904 (0x2F4437E0)
SgnCksum: Binary Large Object (218 Bytes)

CSSP CredSSP:TSRequest, Version: 3

  TLSSSLData: Transport Layer Security (TLS) Payload Data
+ TLS: TLS Rec Layer-1 SSL Application Data
- Cssp: CredSSP
- TsRequest:
+ SequenceHeader:
+ Tag0:
+ Version: 3
+ Tag3:
+ PubKeyAuth: Encrypted

CSSP CredSSP:TSRequest, Version: 3

  TLSSSLData: Transport Layer Security (TLS) Payload Data
+ TLS: TLS Rec Layer-1 SSL Application Data
- Cssp: CredSSP
- TsRequest:
+ SequenceHeader:
+ Tag0:
+ Version: 3
+ Tag2:
+ AuthInfo: Encrypted

Appendix 3. Steps of Kerberos U2U via KDC policy in Offering Remote Assistance

KerberosV5:AS Request Cname: myuser01 Realm: CONTOSO.COM Sname: krbtgt/CONTOSO.COM
KerberosV5:KRB_ERROR  - KDC_ERR_PREAUTH_REQUIRED (25)
KerberosV5:AS Request Cname: myuser01 Realm: CONTOSO.COM Sname: krbtgt/CONTOSO.COM
KerberosV5:AS Response Ticket[Realm: CONTOSO.COM, Sname: krbtgt/CONTOSO.COM]
KerberosV5:TGS Request Realm: CONTOSO.COM Sname: RestrictedKrbHost/sut2016rs1.contoso.com
KerberosV5:TGS Response Cname: myuser01
. . .
KerberosV5:TGS Request Realm: CONTOSO.COM Sname: krbtgt/CONTOSO
KerberosV5:TGS Response Cname: myuser01

KerberosV5:TGS Request Realm: CONTOSO.COM Sname: Administrator

 - Kerberos: TGS Request Realm: CONTOSO.COM Sname: Administrator
+ Length: Length = 1542
- TgsReq: Kerberos TGS Request
+ ApplicationTag:
- KdcReq: KRB_TGS_REQ (12)
+ SequenceHeader:
+ Tag1:
+ Pvno: 5
+ Tag2:
+ MsgType: KRB_TGS_REQ (12)
+ Tag3:
- PaData:
+ SequenceOfHeader:
- PaData: PA-TGS-REQ (1)
+ SequenceHeader:
+ Tag1:
+ PaDataType: PA-TGS-REQ (1)
+ Tag2:
+ OctetStringHeader:
- KrbApReq: KRB_AP_REQ (14)
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ PvNo: 5
+ Tag1:
+ MsgType: KRB_AP_REQ (14)
+ Tag2: 0x1
+ ApOptions:
+ Tag3:
+ Ticket: Realm: CONTOSO.COM, Sname: krbtgt/CONTOSO.COM
+ Tag4:
+ Authenticator:
+ Tag4:
- ReqBody:
+ SequenceHeader:
+ Tag0:
+ KdcOptions: 0x40810000
+ Tag2: 0x1
+ Realm: CONTOSO.COM
+ Tag3:
+ Sname: Administrator
+ Tag5: 0x1
+ Till: 09/13/2037 02:48:05 UTC
+ Tag7:
+ Nonce: 1167598226 (0x45982292)
+ Tag8:
+ Etype:
+ TagA:
+ EncAuthorizationData:

KerberosV5:KRB_ERROR  - KDC_ERR_S_PRINCIPAL_UNKNOWN (7)

- Kerberos: KRB_ERROR  - KDC_ERR_S_PRINCIPAL_UNKNOWN (7)
+ Length: Length = 105
- KrbError: KRB_ERROR (30)
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ PvNo: 5
+ Tag1:
+ MsgType: KRB_ERROR (30)
+ Tag4:
+ Stime: 04/08/2017 06:06:33 UTC
+ Tag5:
+ SuSec: 429948
+ Tag6:
+ ErrorCode: KDC_ERR_S_PRINCIPAL_UNKNOWN (7)
+ Tag9:
+ Realm: CONTOSO.COM
+ TagA:
+ Sname: Administrator
+ TagC:
- EData:
- EData: 0
0  ---------------?¡------------------------------------------------------------
+ AsnOctetStringHeader:
OctetStream: 04 0D 30 0B 30 09 A0 03 02 01 80 A1 02 04 00

In the Kerberos error message, the extended error data has
Error Code: 0x1b KDC_ERR_MUST_USE_USER2USER

The Kerberos event log entry looks like the following:
- <Event xmlns="https://schemas.microsoft.com/win/2004/08/events/event">
- <System>
<Provider Name="Microsoft-Windows-Security-Kerberos" Guid="{98E6CFCB-EE0A-41E0-A57B-622D4E1B30B1}" EventSourceName="Kerberos" />
<EventID Qualifiers="32768">3</EventID>
<Version>0</Version>
<Level>2</Level>
<Task>0</Task>
<Opcode>0</Opcode>
<Keywords>0x80000000000000</Keywords>
<TimeCreated SystemTime="2017-04-08T06:06:33.640231200Z" />
<EventRecordID>8201</EventRecordID>
<Correlation />
<Execution ProcessID="0" ThreadID="0" />
<Channel>System</Channel>
<Computer>client2016rs1.contoso.com</Computer>
<Security />
</System>
- <EventData>
<Data Name="LogonSession" />
<Data Name="ClientTime" />
<Data Name="ServerTime">6:6:33.0000 4/8/2017 Z</Data>
<Data Name="ErrorCode">0x1b</Data>
<Data Name="ErrorMessage">Unknown Error</Data>
<Data Name="ExtendedError" />
<Data Name="ClientRealm" />
<Data Name="ClientName" />
<Data Name="ServerRealm">CONTOSO.COM</Data>
<Data Name="ServerName">Administrator</Data>
<Data Name="TargetName">Administrator@CONTOSO.COM</Data>
<Data Name="ErrorText" />
<Data Name="File">9</Data>
<Data Name="Line">11b8</Data>
<Binary>300B3009A003020180A1020400</Binary>
</EventData>
</Event>

the client sends UsertoUserSessionReq: KRB_TGT_REQ (0x10) to the server.

- MSRPC: c/o Bind: unknown UUID{00000001-0000-0000-C000-000000000046}  Call=0x2  Assoc Grp=0x0  Xmit=0x16D0  Recv=0x16D0
- Bind: {00000001-0000-0000-C000-000000000046} unknown
RpcVers: 5 (0x5)
RpcVersMinor: 0 (0x0)
PType: 0x0B - Bind
+ PfcFlags: 7 (0x7)
+ PackedDrep: 0x10
FragLength: 304 (0x130)
AuthLength: 136 (0x88)
CallId: 2 (0x2)
MaxXmitFrag: 5840 (0x16D0)
MaxRecvFrag: 5840 (0x16D0)
AssocGroupId: 0 (0x0)
+ PContextElem:
- AuthVerifier: 0x1
AuthType: RPC_C_AUTHN_GSS_NEGOTIATE - The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism selects either NT LAN Manager (NTLM) or Kerberos authentication.
AuthLevel: dce_c_authn_level_pkt_integrity - This level offers the dce_c_authn_level_pkt services plus per-PDU modification and deletion detection.
AuthPadLength: 0 (0x0)
AuthReserved: 0 (0x0)
AuthContextId: 0 (0x0)
- AuthValue:
- GssApi:
- InitialContextToken:
+ ApplicationHeader:
+ ThisMech: SpnegoToken (1.3.6.1.5.5.2)
- InnerContextToken: 0x1
- SpnegoToken: 0x1
+ ChoiceTag:
- NegTokenInit:
+ SequenceHeader:
+ Tag0:
- MechTypes: Prefer MsKerberosToken (1.2.840.48018.1.2.2)
+ SequenceHeader:
+ MechType: MsKerberosToken (1.2.840.48018.1.2.2)
+ MechType: KerberosToken (1.2.840.113554.1.2.2)
+ MechType: Negoex (1.3.6.1.4.1.311.2.2.30)
+ MechType: NLMP (1.3.6.1.4.1.311.2.2.10)
+ Tag2:
+ OctetStringHeader:
- MechToken: unused (0)
- MsKerberosToken: unused (0)
- KerberosInitToken:
+ ApplicationHeader:
+ ThisMech: UserToUserMechanism (1.2.840.113554.1.2.2.3)
- InnerContextToken: unused (0)
- KerberosToken: unused (0)
TokId: 0x400
- UsertoUserSessionReq: KRB_TGT_REQ (0x10)
+ SequenceHeader:
+ Tag0:
+ pvno: 5
+ Tag1:
+ MessageType: 16
+ Tag2:
+ ServerName: Administrator
+ Tag3:
+ Realm: CONTOSO

The server requests TGT for the user from the KDC.

KerberosV5:AS Request Cname: Administrator Realm: CONTOSO.COM Sname: krbtgt/CONTOSO.COM
KerberosV5:KRB_ERROR  - KDC_ERR_PREAUTH_REQUIRED (25)

KerberosV5:AS Request Cname: Administrator Realm: CONTOSO.COM Sname: krbtgt/CONTOSO.COM

- Kerberos: AS Request Cname: Administrator Realm: CONTOSO.COM Sname: krbtgt/CONTOSO.COM
+ Length: Length = 312
- AsReq: Kerberos AS Request
+ ApplicationTag:
- KdcReq: KRB_AS_REQ (10)
+ SequenceHeader:
+ Tag1:
+ Pvno: 5
+ Tag2:
+ MsgType: KRB_AS_REQ (10)
+ Tag3:
+ PaData:
+ Tag4:
- ReqBody:
+ SequenceHeader:
+ Tag0:
+ KdcOptions: 0x40810010
+ Tag1:
+ Cname: Administrator
+ Tag2: 0x1
+ Realm: CONTOSO.COM
+ Tag3:
+ Sname: krbtgt/CONTOSO.COM
+ Tag5: 0x1
+ Till: 09/13/2037 02:48:05 UTC
+ Tag6:
+ Rtime: 09/13/2037 02:48:05 UTC
+ Tag7:
+ Nonce: 1118221639 (0x42A6B547)
+ Tag8:
+ Etype:
+ Tag9:
- Addresses:
+ SequenceOfHeader:
- Address: SUT2016RS1
+ SequenceHeader:
+ Tag0:
+ AddrType: Netbios address (20)
+ Tag1:
+ Address: SUT2016RS1

KerberosV5:AS Response Ticket[Realm: CONTOSO.COM, Sname: krbtgt/CONTOSO.COM]

 - Kerberos: AS Response Ticket[Realm: CONTOSO.COM, Sname: krbtgt/CONTOSO.COM]
+ Length: Length = 1583
- AsRep: Kerberos AS Response
+ ApplicationTag:
- KdcRep: KRB_AS_REP (11)
+ SequenceHeader:
+ Tag0:
+ PvNo: 5
+ Tag1:
+ MsgType: KRB_AS_REP (11)
+ Tag2:
+ Padata:
+ Tag3:
+ Crealm: CONTOSO.COM
+ Tag4:
+ Cname: Administrator
+ Tag5:
+ Ticket: Realm: CONTOSO.COM, Sname: krbtgt/CONTOSO.COM
+ Tag6:
+ EncPart:

the client receives UsertoUserSessionRep: KRB_TGT_REP (0x11) from the server

- MSRPC: c/o Bind Ack: unknown UUID{00000001-0000-0000-C000-000000000046} Call=0x2  Assoc Grp=0x55A4AC30  Xmit=0x16D0  Recv=0x16D0
- BindAck:
RpcVers: 5 (0x5)
RpcVersMinor: 0 (0x0)
PType: 0x0C - Bind Ack
+ PfcFlags: 7 (0x7)
+ PackedDrep: 0x10
FragLength: 1303 (0x517)
AuthLength: 1187 (0x4A3)
CallId: 2 (0x2)
MaxXmitFrag: 5840 (0x16D0)
MaxRecvFrag: 5840 (0x16D0)
AssocGroupId: 1436855344 (0x55A4AC30)
+ SecAddr: 54390
Pad2: 0x1
+ PResultList:
- AuthVerifier:
AuthType: RPC_C_AUTHN_GSS_NEGOTIATE - The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism selects either NT LAN Manager (NTLM) or Kerberos authentication.
AuthLevel: dce_c_authn_level_pkt_integrity - This level offers the dce_c_authn_level_pkt services plus per-PDU modification and deletion detection.
AuthPadLength: 0 (0x0)
AuthReserved: 0 (0x0)
AuthContextId: 0 (0x0)
- AuthValue:
- GssApi:
- NegotiationToken:
+ ChoiceTag:
- NegTokenResp:
+ SequenceHeader:
+ Tag0:
+ NegState: accept-incomplete (1)
+ Tag1:
+ SupportedMech: MsKerberosToken (1.2.840.48018.1.2.2)
+ Tag2:
+ OctetStringHeader:
- ResponseToken: unused (0)
- MsKerberosToken: unused (0)
- KerberosInitToken:
+ ApplicationHeader:
+ ThisMech: UserToUserMechanism (1.2.840.113554.1.2.2.3)
- InnerContextToken: unused (0)
- KerberosToken: unused (0)
TokId: 0x401
- UsertoUserSessionRep: KRB_TGT_REP (0x11)
+ SequenceHeader:
+ Tag0:
+ pvno: 5
+ Tag1:
+ MessageType: 17
+ Tag2:
- Ticket: Realm: CONTOSO.COM, Sname: krbtgt/CONTOSO.COM
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ TktVno: 5
+ Tag1:
+ Realm: CONTOSO.COM
+ Tag2: 0x1
+ Sname: krbtgt/CONTOSO.COM
+ Tag3: 0x1
+ EncPart:

The client request TGS from the KDC for the service's user.

KerberosV5:TGS Request Realm: CONTOSO.COM Sname: Administrator

- Kerberos: TGS Request Realm: CONTOSO.COM Sname: Administrator
+ Length: Length = 2692
- TgsReq: Kerberos TGS Request
+ ApplicationTag:
- KdcReq: KRB_TGS_REQ (12)
+ SequenceHeader:
+ Tag1:
+ Pvno: 5
+ Tag2:
+ MsgType: KRB_TGS_REQ (12)
+ Tag3:
- PaData:
+ SequenceOfHeader:
- PaData: PA-TGS-REQ (1)
+ SequenceHeader:
+ Tag1:
+ PaDataType: PA-TGS-REQ (1)
+ Tag2:
+ OctetStringHeader:
- KrbApReq: KRB_AP_REQ (14)
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ PvNo: 5
+ Tag1:
+ MsgType: KRB_AP_REQ (14)
+ Tag2: 0x1
+ ApOptions:
+ Tag3:
+ Ticket: Realm: CONTOSO.COM, Sname: krbtgt/CONTOSO.COM
+ Tag4:
+ Authenticator:
+ PaData: PA-PAC-OPTIONS (167)
+ Tag4:
- ReqBody:
+ SequenceHeader:
+ Tag0:
- KdcOptions: 0x40810008
+ KerberosFlagsHeader:
+ Padding:
- KrbFlags: 0x40810008
Reserved:              (0...............................)
Forwardable:           (.1..............................) Ticket to be issued is to have its FORWARDABLE flag set
Forwarded:             (..0.............................) This is not a request for forwarding
Proxiable:             (...0............................) Ticket to be issued is not to have its PROXIABLE flag set
Proxy:                 (....0...........................) This is not a request for a proxy
AllowPostDate:         (.....0..........................) Ticket to be issued is not to have its MAY_POSTDATE flag set
PostDated:             (......0.........................) This is not a request for a postdated ticket
Unused7:               (.......0........................)
Renewable:             (........1.......................) Ticket to be issued is to have its RENEWABLE flag set
Unused9:               (.........0......................)
Unused10:              (..........0.....................)
OptHardwareAuth:       (...........0....................)
Unused12:              (............0...................)
Unused13:              (.............0..................)
CnameInAddlTkt:        (..............0.................) This is not a request for S4U2proxy functionality
Canonicalize:          (...............1................)
Unused16:              (................0000000000......)
DisableTransitedCheck: (..........................0.....) Checking of the transited field is enabled
RenewableOk:           (...........................0....) Renewable ticket is not acceptable
EncTktInSkey:          (............................1...) Indicates that the ticket for the end server is to be encrypted in the session key from the additional TGT provided
Unused29:              (.............................0..)
Renew:                 (..............................0.) Present request is not for a renewal
Validate:              (...............................0) Request is not to validate a postdated ticket
+ Tag2: 0x1
+ Realm: CONTOSO.COM
+ Tag3:
+ Sname: Administrator
+ Tag5: 0x1
+ Till: 09/13/2037 02:48:05 UTC
+ Tag7:
+ Nonce: 1181218830 (0x4667F80E)
+ Tag8:
+ Etype:
+ TagA:
+ EncAuthorizationData:
+ TagB:
- AdditionalTickets:
+ SequenceOfHeader:
- Ticket: Realm: CONTOSO.COM, Sname: krbtgt/CONTOSO.COM
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ TktVno: 5
+ Tag1:
+ Realm: CONTOSO.COM
+ Tag2: 0x1
+ Sname: krbtgt/CONTOSO.COM
+ Tag3: 0x1
+ EncPart:

KerberosV5:TGS Response Cname: myuser01

- Kerberos: TGS Response Cname: myuser01
+ Length: Length = 1528
- TgsRep: Kerberos TGS Response
+ ApplicationTag:
- KdcRep: KRB_TGS_REP (13)
+ SequenceHeader:
+ Tag0:
+ PvNo: 5
+ Tag1:
+ MsgType: KRB_TGS_REP (13)
+ Tag3:
+ Crealm: CONTOSO.COM
+ Tag4:
+ Cname: myuser01
+ Tag5:
- Ticket: Realm: CONTOSO.COM, Sname: Administrator
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ TktVno: 5
+ Tag1:
+ Realm: CONTOSO.COM
+ Tag2: 0x1
+ Sname: Administrator
+ Tag3: 0x1
- EncPart:
+ SequenceHeader:
+ Tag0:
+ EType: aes256-cts-hmac-sha1-96 (18)
+ Tag2:
+ Cipher: ©lELY³’Ç·ÝÁò·¸@»^_”@}?WSÀ|hÖ÷FÃrÑ«g†á†ü%º?÷ ¢$ÌÿSÖŸ?×íE?ïB£_†?¤tólëÄU?
+ Tag6:
- EncPart:
+ SequenceHeader:
+ Tag0:
+ EType: aes256-cts-hmac-sha1-96 (18)
+ Tag2:
+ Cipher: À{ÙcBØP}½§WcæT%3ׄ·îç”mù×v-=Y¹A%1šãˆr_·âh?

the client does the AP exchange of Kerberos token with the server, and indicates in the APOptions USE-SESSION-KEY flag that the ticket is encrypted with the session key from the server's TGT.

- MSRPC: c/o Alter Cont: unknown  UUID{00000001-0000-0000-C000-000000000046}  Call=0x2
- AlterContext:
RpcVers: 5 (0x5)
RpcVersMinor: 0 (0x0)
PType: 0x0E - Alter Context
+ PfcFlags: 7 (0x7)
+ PackedDrep: 0x10
FragLength: 1659 (0x67B)
AuthLength: 1579 (0x62B)
CallId: 2 (0x2)
MaxXmitFrag: ignored
MaxRecvFrag: ignored
AssocGroupId: ignored
+ PContextElem:
- AuthVerifier: 0x1
AuthType: RPC_C_AUTHN_GSS_NEGOTIATE - The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism selects either NT LAN Manager (NTLM) or Kerberos authentication.
AuthLevel: dce_c_authn_level_pkt_integrity - This level offers the dce_c_authn_level_pkt services plus per-PDU modification and deletion detection.
AuthPadLength: 0 (0x0)
AuthReserved: 0 (0x0)
AuthContextId: 0 (0x0)
- AuthValue:
- GssApi:
- NegotiationToken:
+ ChoiceTag:
- NegTokenResp:
+ SequenceHeader:
+ Tag0:
+ NegState: accept-incomplete (1)
+ Tag2:
+ OctetStringHeader:
- ResponseToken: unused (0)
- KerberosToken: unused (0)
- Kerberos: AP Request Ticket[Realm: CONTOSO.COM, Sname: Administrator]
- ApReq: KRB_AP_REQ (14)
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ PvNo: 5
+ Tag1:
+ MsgType: KRB_AP_REQ (14)
+ Tag2: 0x1
- ApOptions:
+ KerberosFlagsHeader:
+ Padding:
- KrbFlags: 0x60000000
Reserved:       (0...............................)
UseSessionKey:  (.1..............................)
MutualRequired: (..1.............................)
Unused:         (...00000000000000000000000000000)
+ Tag3:
- Ticket: Realm: CONTOSO.COM, Sname: Administrator
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ TktVno: 5
+ Tag1:
+ Realm: CONTOSO.COM
+ Tag2: 0x1
+ Sname: Administrator
+ Tag3: 0x1
- EncPart:
+ SequenceHeader:
+ Tag0:
+ EType: aes256-cts-hmac-sha1-96 (18)
+ Tag2:
+ Cipher: ©lELY³’Ç·ÝÁò·¸@»^_”@}?WSÀ|hÖ÷FÃrÑ«g†á†ü%º?÷ ¢$ÌÿSÖŸ?×íE?ïB£_†?¤tólëÄU?
+ Tag4:
+ Authenticator:

Because MUTUAL-REQUIRED flag was set in AP-Options, the server includes KRB-AP-REP in its response.

- MSRPC: c/o Alter Cont Resp: unknown UUID{00000001-0000-0000-C000-000000000046} Call=0x2  Assoc Grp=0x55A4AC30  Xmit=0x16D0  Recv=0x16D0
- AlterContextResponse:
RpcVers: 5 (0x5)
RpcVersMinor: 0 (0x0)
PType: 0x0F - Alter Context Resp
+ PfcFlags: 7 (0x7)
+ PackedDrep: 0x10
FragLength: 216 (0xD8)
AuthLength: 152 (0x98)
CallId: 2 (0x2)
MaxXmitFrag: ignored
MaxRecvFrag: ignored
AssocGroupId: ignored
+ SecAddr:
+ Pad2: 0x1
+ PResultList:
- AuthVerifier:
AuthType: RPC_C_AUTHN_GSS_NEGOTIATE - The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism selects either NT LAN Manager (NTLM) or Kerberos authentication.
AuthLevel: dce_c_authn_level_pkt_integrity - This level offers the dce_c_authn_level_pkt services plus per-PDU modification and deletion detection.
AuthPadLength: 0 (0x0)
AuthReserved: 0 (0x0)
AuthContextId: 0 (0x0)
- AuthValue:
- GssApi:
- NegotiationToken:
+ ChoiceTag:
- NegTokenResp:
+ SequenceHeader:
+ Tag0:
+ NegState: accept-incomplete (1)
+ Tag2:
+ OctetStringHeader:
- ResponseToken: KRB_AP_REP (15)
- KerberosToken: KRB_AP_REP (15)
- Kerberos: AP Response
- ApRep: KRB_AP_REP (15)
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ PvNo: 5
+ Tag1:
+ MsgType: KRB_AP_REP (15)
+ Tag2: 0x1
- AuthorizationData:
+ SequenceHeader:
+ Tag0:
+ EType: rc4-hmac (23)
+ Tag2:
+ Cipher: Uàs5˜3(,S
¥Ç×Ñ•Kìd/aò---------------Ã/}¦Œ«—ƒm–„Fzº4@¬q1[«u,OCÿ¦ÇŠ}µ˜?D¸j¹šÚÜ|!»[ç¤|/öon‡ˆ=i+ï†(šPpãúN….¼wȆ‚™vºÉWõ

- MSRPC: c/o Alter Cont: unknown  UUID{00000001-0000-0000-C000-000000000046}  Call=0x2
- AlterContext:
RpcVers: 5 (0x5)
RpcVersMinor: 0 (0x0)
PType: 0x0E - Alter Context
+ PfcFlags: 7 (0x7)
+ PackedDrep: 0x10
FragLength: 216 (0xD8)
AuthLength: 136 (0x88)
CallId: 2 (0x2)
MaxXmitFrag: ignored
MaxRecvFrag: ignored
AssocGroupId: ignored
+ PContextElem:
- AuthVerifier: 0x1
AuthType: RPC_C_AUTHN_GSS_NEGOTIATE - The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism selects either NT LAN Manager (NTLM) or Kerberos authentication.
AuthLevel: dce_c_authn_level_pkt_integrity - This level offers the dce_c_authn_level_pkt services plus per-PDU modification and deletion detection.
AuthPadLength: 0 (0x0)
AuthReserved: 0 (0x0)
AuthContextId: 0 (0x0)
- AuthValue:
- GssApi:
- NegotiationToken:
+ ChoiceTag:
- NegTokenResp:
+ SequenceHeader:
+ Tag0:
+ NegState: accept-incomplete (1)
+ Tag2:
+ OctetStringHeader:
- ResponseToken: KRB_AP_REP (15)
- KerberosToken: KRB_AP_REP (15)
- Kerberos: AP Response
- ApRep: KRB_AP_REP (15)
+ ApplicationTag:
+ SequenceHeader:
+ Tag0:
+ PvNo: 5
+ Tag1:
+ MsgType: KRB_AP_REP (15)
+ Tag2: 0x1
- AuthorizationData:
+ SequenceHeader:
+ Tag0:
+ EType: rc4-hmac (23)
+ Tag2:
+ Cipher: ówûdf__ÇßMk†¦;N”ß³“ºãu:ïýVwhû™F0ÔuDš>©×¤‡Ÿ7@Pü¤º8MÚ­Ü 4H
+ Tag3:
+ OctetStringHeader:
- MechListMic: KRB_AP_REP (15)
- KerberosToken: KRB_AP_REP (15)
TokId: GSS_GetMIC (0x404)
- Krb5GssV2GetMic:
+ Flags: 4 (0x4)
Filler: Binary Large Object (5 Bytes)
SndSeq: 1181218827 (0x4667F80B)
SgnCksum: Binary Large Object (12 Bytes)

- MSRPC: c/o Alter Cont Resp: unknown UUID{00000001-0000-0000-C000-000000000046} Call=0x2  Assoc Grp=0x55A4AC30  Xmit=0x16D0  Recv=0x16D0
- AlterContextResponse:
RpcVers: 5 (0x5)
RpcVersMinor: 0 (0x0)
PType: 0x0F - Alter Context Resp
+ PfcFlags: 7 (0x7)
+ PackedDrep: 0x10
FragLength: 105 (0x69)
AuthLength: 41 (0x29)
CallId: 2 (0x2)
MaxXmitFrag: ignored
MaxRecvFrag: ignored
AssocGroupId: ignored
+ SecAddr:
+ Pad2: 0x1
+ PResultList:
- AuthVerifier:
AuthType: RPC_C_AUTHN_GSS_NEGOTIATE - The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism selects either NT LAN Manager (NTLM) or Kerberos authentication.
AuthLevel: dce_c_authn_level_pkt_integrity - This level offers the dce_c_authn_level_pkt services plus per-PDU modification and deletion detection.
AuthPadLength: 0 (0x0)
AuthReserved: 0 (0x0)
AuthContextId: 0 (0x0)
- AuthValue:
- GssApi:
- NegotiationToken:
+ ChoiceTag:
- NegTokenResp:
+ SequenceHeader:
+ Tag0:
+ NegState: accept-completed (0)
+ Tag3:
+ OctetStringHeader:
- MechListMic: unused (0)
- KerberosToken: unused (0)
TokId: GSS_GetMIC (0x404)
- Krb5GssV2GetMic:
+ Flags: 5 (0x5)
Filler: Binary Large Object (5 Bytes)
SndSeq: 1096155632 (0x415601F0)
SgnCksum: Binary Large Object (12 Bytes)

References

[RFC4120] The Kerberos Network Authentication Service (V5)
https://tools.ietf.org/rfc/rfc4120.txt
[U2UDraft] User to User Kerberos Authentication using GSS-API
https://tools.ietf.org/html/draft-swift-win2k-krb-user2user-03
[RFC2743] "Generic Security Service Application Program Interface Version 2, Update 1"
https://tools.ietf.org/rfc/rfc2743.txt
[MS-CSSP]: Credential Security Support Provider (CredSSP) Protocol
https://msdn.microsoft.com/en-us/library/cc226764.aspx
[MS-SPNG]: Simple and Protected GSS-API Negotiation Mechanism (SPNEGO) Extension
https://msdn.microsoft.com/en-us/library/cc247021.aspx
[MS-RA]: Remote Assistance Protocol
https://msdn.microsoft.com/en-us/library/cc240013.aspx
[MS-PKCA]: Public Key Cryptography for Initial Authentication (PKINIT) in Kerberos Protocol
https://msdn.microsoft.com/en-us/library/cc238455.aspx
[PKU2U] Introducing PKU2U in Windows
https://technet.microsoft.com/en-us/library/dd560634(v=ws.10).aspx
[MS-SFU]: Kerberos Protocol Extensions: Service for User and Constrained Delegation Protocol
https://msdn.microsoft.com/en-us/library/cc246071.aspx

Comments

• Anonymous
  July 14, 2017
  Good job!
• Anonymous
  August 30, 2017
  Mr. Olougouna your incredibly rich and in depth article has provided me with much to dissect and absorb. Thank you.