Network Security: Kerberos

Tuomas AuraT-110.5240 Network security

Aalto University, Nov-Dec 2012

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OutlineKerberos authenticationKerberos in Windows domains

Kerberos authentication

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4

KerberosShared-key protocol for user login authentication

Uses passwords as shared keysSolves security and scalability problems in password-based authentication in large domainsBased loosely on the Needham-Schroeder secret-key protocol

Kerberos v4 1988- at MITKerberos v5 1993- [RFC 4120]

Updated protocol and algorithmsASN.1 BER message encodingImplemented in Windows 2000 and later

Used in intranets: e.g. university Unix systems, corporate Windows domains

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Kerberos architecture

1.–2. Authentication3.–4. Ticket for a specific service5.–6. Authentication to the service

KDC

TGSAS

Application server B

Client A

1. K

RB

_AS

_RE

Q

2. K

RB

_AS

_RE

P

3. K

RB

_TG

S_R

EQ

4. K

RB

_TG

S_R

EP

5. KRB_AP_REQ

6. KRB_AP_REPap_client.exe ap

_ser

ver.e

xe

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Kerberos architecture (some details)KDC

TGSAS

Application server B

Client A

1. K

RB

_AS

_RE

Q

2. K

RB

_AS

_RE

P

3. K

RB

_TG

S_R

EQ

4. K

RB

_TG

S_R

EP

5. KRB_AP_REQ

6. KRB_AP_REP

TGT

TGT,

KA

T

Ser

vice

tick

et, K

AB

Service ticket

ap_client.exe ap_s

erve

r.exe

krbtgt@RealmY

A@RealmY B@

Rea

lmY

1.–2. Authentication with password → client gets TGT and KAT

3.–4. Authentication with TGT and KAT → client gets

service ticket and KAB

5.–6. Authentication with service ticket and KAB

→ client gets service access

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Message type, version

Kerberos ticket

Same format for both TGT and service ticketCredentials = ticket + keyASN.1 encoding in Kerberos v5“Encryption” also protects integrity, actually encryption and a MAC

Flags: FORWARDABLE, FORWARDED, PROXIABLE, PROXY, MAY-POST-DATE, POSTDATED, INVALID, RENEWABLE, INTINIAL, PRE-AUTHENT, HW-AUTHENTINITIAL flag indicates TGT

REALM, SNAMEServer name and realm

FLAGS

KEY

CNAME, CREALM Client name and realm

TRANSITEDtransit realms

AUTH-TIME, END-TIME

CADDRClient IP address (optional)

AUTORIZATION-DATAApp-specific access constraints E

ncry

pted

with

ser

ver’s

mas

ter k

ey

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Kerberos protocol (more details)Initial login of user A:

1. A → AS: Preauthentication, A, TGS, NA1, AddrA

2. AS → A: A, TGT, EKA (KA-TGS, NA1, TGS, AddrA)

Ticket request:3. A → TGS: TGT, AuthenticatorA-TGS, B, NA2, AddrA

4. TGS → A: A, Ticket, EKA-TGS (KAB, NA2, B, AddrA)

Authentication to server B:5. A → B: Ticket, AuthenticatorAB

6. B → A: AP_REP

KA , KTGS, KB = master keys of A, TGS and BKA-TGS = shared key for A and TGS KAB = shared key for A and B TGT = B, EKTGS (INITIAL, KA-TGS, A, Tauth, Texpiry1, AddrA))

Ticket = B, EKB(KAB, A, Tauth, Texpiry2, AddrA))

Preauthentication = EKA (1 TA)

AuthenticatorA-TGS = EKA-TGS (2 TA)

AuthenticatorAB = EKAB (3 TA)

AP_REP = EKAB(4 TA)

A, B = principal namesTx = timestampAddrA = A’s IP addresses

Notes:

1234) ASN.1 encoding adds type tags to all messages

Encryption mode also protects message integrity

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Kerberos realms

Users and services registered to one KDC form a realmname@realm, e.g. A@X, alice@asia.sales.contoso.com

Cross-realm trust: Two KDCs X and Y share a key (krbtgt@Y is registered in KDC X and krbtgt@X in KDC Y)KDCs believe each other to be honest and competent to name users in their own realm

Cross-realm authentication: Client A@X requests from TGS at realm X a ticket for TGS at realm YThe ticket is encrypted for krbtgt@Y (i.e. TGS at realm Y)Client A@X requests from TGS at realm Y a ticket for server B@Y

Access control at several steps:Local policy at each KDC about when to honor tickets from other realmsLocal policy at B@Y about whether to allow access to users from other realmsACLs at B@Y determine whether the users is allowed to access the particular resources

Possible to transit multiple realms → TRANSITED field in the ticket lists intermediate realms

Local policy at each server about which transit realms are allowed

Server BUser A

Realm X Realm YCross-realm trustUser registration

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Realm hierarchy

Large organizations can have a realm hierarchy Hierarchy follows internet names → easy to find a path between realms→ can filter cross-realm requests based on the namesCan add shortcut links or create even a fully connected graph between KDCsE.g. Windows domain hierarchy

Compare with X.509 certification hierarchy: similarities, differences?

contoso.com

sales.contoso.com dev.contoso.com

euro.sales.contoso.com asia.sales.contoso.com

Bob David Alice

Charlie

Cross-realm trust

User registration

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Password guessing attacksKerberos is vulnerable to password guessing:

Sniffed KRB_AS_REQ or KRB_AS_REP can be used to test candidate passwords → offline brute-force password guessingIn Kerberos v4, anyone could request a password-encrypted TGT from AS → easy to obtain material for password crackingPreauthentication in Kerberos v5 prevents active attacks to obtain material for password cracking → must sniff it

Note: active vs. passive attacksMisleading thinking: active attacks (e.g. MitM) are more difficult to implement than passive attacks (sniffing)Reality: Active attacks can often be initiated by the attacker while passive attacks require attacker to wait for something to sniff → vulnerability to such active attacks is serious

!

!

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PKINITGoal: take advantage of an existing PKI to bootstrap authentication in KerberosReplaces the KRB_AS_REQ / KRB_AS_REP exchange with a public-key protocol

Public-key authentication and encryption to obtain TGTContinue with standard Kerberos → transparent to TGS and application servers

No password, so not vulnerable to password guessingUses DSS signatures and ephemeral DHWindows 2000 and later, now standard [RFC 4556]

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DelegationServer may need to perform tasks on the client’s behalf

Recursive RPC; agents operating when the user is no longer logged in; batch processing at night

Alice can give her TGT or service ticket and key to DavidControlling the use of delegated rights in applications:

Ticket may specify many allowed client IP addressesAuthorization-data field in ticket may contain app-specific restrictionsIt is safer to delegate a service ticket than TGT

Ticket flags related to delegation:FORWARDABLE flag in TGT: the TGT can be used to obtain a new TGT with different IP addressesPROXIABLE flag in TGT: the TGT can be used to obtain service tickets with a different IP address

Kerberos delegation is identity delegationWhen B has A’s ticket and key, B can act as A and nobody can tell the difference → difficult to audit access; similar to sharing passwordsOther protocols delegate only access rights, and the delegate can be identified

Kerberos in Windows domainsThanks to Dieter Gollmann

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Windows access control summaryThe O/S stores security attributes for each processes (subject) in an access tokenToken contains a list of privileges and SIDs (i.e. user and group identifiers)

Permissions are decided by comparing the list of SIDs against a DACLs on an object

The access token is local to the machine, created at login time, and never sent over the network How to authorize access to resources managed by a Windows service (=daemon) on a remote server, e.g. over remote procedure call (RPC)?

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Network credentialsAlice’s user name, SID and network credentials are cached on the user workstation

username and password, or TGT and KA-TGS

Alice’s processes can use her network credentials for remote login

Two authentication protocols: NTLM and Kerberos V5 (RFC 1510)These authentication protocols do not reveal the password to the server

Applications can also ask the user for a different user name and credentials and store them separately

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Tokens and remote accessAccess tokens are meaningful only to the local machine and cannot be sent over network

The server does not trust the client machine to tell who Alice is and which groups she belongs to

Instead, the client authenticates Alice to the server using her network credentials. The server creates a new login session and a new token (on the server) for AliceThe service may now assign the token to a process or thread (=impersonation)The authentication protocols also

provide the server with Alice’s user and group SIDsproduce a session key for protecting data between the client and server

Encryption and authentication of session data is controlled by applications

Different secure session protocol exist for network logon, RPC, COM

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Message type, version

Kerberos ticket in Windows

REALM, SNAMEServer name and realm

FLAGS

KEY

CNAME, CREALM Client name and realm

TRANSITEDtransit realms

AUTH-TIME, END-TIME

CADDRClient IP address (optional)

AUTORIZATION-DATAApp-specific access constrains E

ncry

pted

with

ser

ver’s

mas

ter k

ey

Username, domain

User and group SIDs

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Related readingWilliam Stallings. Network security essentials: applications and standards, 3rd ed. chapter 4.1; 4th ed. chapter 4.1–4.2 (Kerberos v5 only)William Stallings. Cryptography and Network Security, 4th ed.: chapters 14.1 (Kerberos v5)Dieter Gollmann. Computer Security, 2nd ed.: chapter 12.4; 3rd ed. chapter 15.4Kaufmann, Perlman, Speciner. Network security, 2nd ed.: chapter 14

Online: How the Kerberos Version 5 Authentication Protocol Works, http://technet.microsoft.com/en-us/library/cc772815(v=ws.10).aspx

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ExercisesHow does Kerberos fix the flaw in Needham-Schroeder secret-key protocol?Find source code for a Kerberized client/server application (e.g. telnet) and see how it accesses Kerberos servicesWhy is Kerberos used on the intranets and TLS/SSL on the Internet? Could it be the other way?Learn about Encrypted Key Exchange (EKE) and other similar password-based authentication protocols. Which problem do they solve?Should standard protocols include data fields or messages for proprietary extensions? What are the arguments for and against?