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

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

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

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

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

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

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

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

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

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

Kerberos Protocol (2) Kerberos Ticket Exchanges Ports : kinit: 88/udp kpasswd (Unix):749/tdp kpasswd (Win): 464/{tcp,udp} © 2001 Sécurité.Org Key Distribution Center Authentication Service Ticket Granting Service User Network Service

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

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 © 2001 Sécurité.Org Client KDC Server ST Request (3) ST and SK (4) ST (5)

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 © 2001 Sécurité.Org

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

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

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

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

Kerberos V on *NIX clients (2) Kerberized Telnet : available Kerberized SSH : > SSH.Coms 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

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

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

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

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

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

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

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

Thats all folks :-) Latest version Q&A © 2001 Sécurité.Org Picture: