1. Kerberos in an ISP environment
UNIX/Win2K/Cisco
> Nicolas FISCHBACH
Senior IP&Security Engineer - Professional Services Team -
> Sébastien LACOSTE-SERIS
Security Officer - IP Research & Development Manager -
version 1.2

2. Agenda  Kerberos > Introduction : why did we choose Kerberos ?
> Protocol and Exchanges
> Attacks
 Deployment
> UNIX
> Cisco Routers and Switches
> Win2K
 Q&A
© 2001 Sécurité.Org

3. What is Kerberos ?
 Kerberos is a network authentication protocol/system
 Uses time synchronization to :
> limit the use of the keys
> help in detecting replay attacks
 Mutual authentication
 Uses DES, 3DES and shared keys
 Trusted third party
© 2001 Sécurité.Org

4. What is Kerberos not ?  Kerberos does not provide authorization only
authentication
 Kerberos does not provide data encryption
© 2001 Sécurité.Org

5. Why use Kerberos ?  Secure authentication (cryptography)
 No password transmission
 Single Sign On
> SSO is bad for security (Bruce Schneier)
 Centralized authentication management
 IETF Standard (RFC 1510)
© 2001 Sécurité.Org

6. Kerberos vocabulary (1)
 KDC : Key Distribution Center. Holds a database of clients
and servers (called principals) and their private keys
 principal : three-tuple <primary name, instance, realm>
> user :
> service :
 primary : username or service name
 instance : “qualifies” the primary (role)
 realm : authentication domain
© 2001 Sécurité.Org

7. Kerberos vocabulary (2)
 keytab : file containing one or more keys (for hosts or
services). Also known as SRVTAB (Cisco).
 client : an entity that can obtain a ticket (user or host)
 service : host, ftp, krbtgt, pop, etc.
 ticket : credentials (identity of a client for
a particular service)
 TGT : ticket issued by the AS. Allows the
client to obtain additional tickets for the
same realm.
© 2001 Sécurité.Org

8. Key Distribution Center
 Responsible for maintaining master keys for all principles
and issuing Kerberos tickets
 Authentication Service (AS) gives the client a session key
and a Ticket Granting Ticket (TGT)
 Distributes service session keys and ticket
for the service via a Ticket Granting Service
(TGS)
© 2001 Sécurité.Org

9. Realms  A Realm is an authentication domain
> one Kerberos database and a set of KDCs
 Hierarchical organization (new in v5)
 One or two way authentication
 Cross-realm authentication
> transitive cross-realm
> direct between realms
© 2001 Sécurité.Org

10. Kerberos Protocol (1)  Kerberos Ticket Encrypted Domain
Principal Name
Ticket Flags
Encryption Key
Start Time
End Time
Host Address
Authorization Data
Encrypted
© 2001 Sécurité.Org

11. Kerberos Protocol (2)  Kerberos Ticket Exchanges  Ports :
kinit: /udp
kpasswd (Unix): 749/tdp
kpasswd (Win): 464/{tcp,udp}
Key Distribution Center
Authentication
Service
Ticket Granting
Service
User
Network Service
© 2001 Sécurité.Org

12. Kerberos Protocol (3)  Getting a Ticket Granting Ticket (1+2)
> (1) TGT Request
> (2) TGT (to be decrypted with the user’s password hash)
Client
KDC
TGT Request (1)
TGT (2)
© 2001 Sécurité.Org

13. Kerberos Protocol (4)  Getting and using a Service Ticket (3+4+5)
> (3) ST Request (with a TGT)
> (4) ST and session key
> (5) ST for authentication
KDC
ST Request (3)
Client
ST and SK (4)
ST (5)
Server
© 2001 Sécurité.Org

14. NAT issues  Host address is included in the tickets (recommended)
 Need to add NATed IP address in the ticket
 Patch for MIT Kerberos 5.1
 Create TGT without address (not recommended) :
kinit -A <principal>
© 2001 Sécurité.Org

15. Attacks against Kerberos (1)
 Vulnerability in Kerberos password authentication
via KDC AS spoofing : keytab file and register principals
for the service (
 Replay attacks : detected (C+S are time synchronized)
 Exposed keys : keys have a limited lifetime
but are multi-session keys
 Temporary file vulnerability :
run krb
© 2001 Sécurité.Org

16. Attacks against Kerberos (2)
 Passwords guessing : use a good passphrase
 Trojaned clients : OTP
 Implicit trust between realms
 Ticket forwarding
 Others : KDC, shared workstations, ...
© 2001 Sécurité.Org

17. *NIX clients  RedHat (6.2 and 7) provides Kerberos V support
> Install patch RHSA-2001:025-14
 OpenBSD and Solaris 2.8 now support Kerberos V
 Solaris < 2.8 only provides Kerberos IV
© 2001 Sécurité.Org

18. Kerberos V on *NIX clients (1)
 Authentication managed by Kerberos API
 Authorizations defined in user files :
~/.k5login - defines the principal(s) who can login
into account that account
~/.k5users - defines commands that can be launched
via ksu (sudo like)
 PAM alternatives
© 2001 Sécurité.Org

19. Kerberos V on *NIX clients (2)
 Kerberized Telnet : available
 Kerberized SSH :
> SSH.Com’s SSH 1.2.x and 2.x support Kerberos V
> Kerberos V Patches available for OpenSSH :
> OpenSSH w/ Kerberos V on RedHat 7.1
© 2001 Sécurité.Org

20. Kerberos V on Cisco equipment (1)
 Cisco Routers
> Kerberized Telnet
> Password authentication using Kerberos (telnet, SSH
and console)
> Can map instance to Cisco privilege (locally defined)
 Cisco Switches
> Telnet only (SSH available as of 6.1 but
w/o Kerberos support)
© 2001 Sécurité.Org

21. Kerberos V on Cisco equipment (2)
 IOS & memory issues on routers :
> Feature name : Kerberos V client support
> Needed Feature set : at least Enterprise
> Not supported on all hardware, for example :
- Cisco 16xx router
- Cisco GSR (12xxx - Gigabit Switch Router)
> Memory requirements :
Hint: always check with the Cisco IOS Feature Navigator
© 2001 Sécurité.Org

22. Kerberos V on Cisco equipment (3)
 Router Configuration :
aaa authentication login default krb5-telnet local
aaa authorization exec default krb5-instance
kerberos local-realm COLT.CH
kerberos srvtab entry ...
kerberos server COLT.CH
kerberos instance map engineering 15
kerberos instance map support 3
kerberos credentials forward
line vty 0 4
ntp server
© 2001 Sécurité.Org

23. Kerberos V on Cisco equipment (4)
 CatOS & memory issues on switches :
> At least Supervisor Engine Software Release 5.x
> Only supported on Catalyst 4000, 5000 and 6000/6500
> Only supported on SE I (not SE II) on Cat6K
> Memory requirements :
Hint: always check the Release Notes
© 2001 Sécurité.Org

24. Kerberos V on Cisco equipment (5)
 Switch Configuration :
#kerberos
set kerberos local-realm COLT.CH
set kerberos clients mandatory
set kerberos credentials forward
set kerberos server COLT.CH
set kerberos srvtab entry ...
#authentication
set authentication login kerberos enable telnet primary
set authentication enable kerberos enable telnet primary
#ntp
set ntp client enable
set ntp server
© 2001 Sécurité.Org

25. Kerberos V on Win2K stations (1)
 Provides Kerberos authentication for interactive logons
 The protocol is a Security Provider under the SPPI
(Security Support Provider Interface) and is linked to
the LSA (Local Security Authority)
 Ticket cache is provided by the LSA
 Telnetd supports Kerberos
© 2001 Sécurité.Org

26. Kerberos V on Win2K stations (2)
 Support Tools
 Win2K station configuration :
ksetup /setdomain COLT.CH
ksetup /addkdc COLT.CH kdc.colt.ch
ksetup /setmachpassword password
ksetup /mapuser localuser
ksetup /mapuser * *
 Windows Time Server (+ registry)
 No kerberized SSH, only a few (broken)
telnet clients
© 2001 Sécurité.Org

27. That’s all folks :-)  Latest version
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© 2001 Sécurité.Org