Kerberos in an ISP environment UNIX/Win2K/Cisco

Transcription

Kerberos in an ISP environment
UNIX/Win2K/Cisco
> Nicolas FISCHBACH
[email protected] - http://www.securite.org/nico/
> Sébastien LACOSTE-SERIS
[email protected] - http://www.securite.org/kaneda/
version 1.13
Agenda
> Kerberos
> Introduction : why did we choose Kerberos ?
> Protocol and Exchanges
> MIT Kerberos and Applications
> 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 and shared keys
> Trusted third party
What is Kerberos not ?
> Kerberos does not provide authorization only
authentication
> Kerberos does not provide data encryption
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)
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 : login/staff@REALM
> service : service/host.fqdn@REALM
> primary : username or service name
> instance : “qualifies” the primary (role)
> realm : authentication domain
Kerberos vocabulary (2)
> keytab : file containing one or more keys (for hosts or
services). Also known as SRVTAB.
> 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.
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)
Kerberos Protocol (1)
> Kerberos Ticket
Domain
Principal Name
Ticket Flags
Encryption Key
Domain
Principal Name
Start Time
End Time
Host Address
Authorization Data
Encrypted
> Kerberos Ticket Exchanges
Key Distribution Center
> Ports :
kinit:
kpasswd (Unix):
kpasswd (Win):
88/udp
749/tdp
464/{tcp,udp}
Authentication
Service
Ticket Granting
Service
et
k
t ic
User
Network Service
> 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)
> 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
Client
ST Request (3)
ST and SK (4)
ST (5)
Server
> Kerberos delegation
KDC
Server
ST Request
Client
TGT + ST
ST and SK
ST
Server
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
> Authentication across domains
KDC
Client
TGT Request
KDC
TGT
ST Request
ST and SK
ST and SK
Server
MIT distribution
> Version used : 5.1
> Provides client and server
> Supported platforms :
UNIXes (xBSD, Linux, Solaris, AIX, HP-UX, OSF/1, ...)
MacOS 10
> DNS can be used for lookups
Kerberized applications
> telnet (with DES encryption) and r-commands
> CVS and ksu, klogin, k*
> SSH 1.2 supports Kerberos V (run at least
version 1.2.30)
> SSL v3.0
> Cygnus Kerbnet (NT, MAC, Unix)
> samba doesn’t (related to MS extensions)
How to Kerberize an application
> All applications can be adapted
> Use of the GSS API
> Transport the ticket within an application
NAT issues
> Host address is included in the tickets
> Need to add NATed IP address in the ticket
> Patch for MIT Kerberos 5.1
Attacks against Kerberos (1)
> Vulnerability in Kerberos password authentication
via KDC AS spoofing : keytab file and register principals
for the service (http://www.monkey.org/~dugsong/kdcspoof.tar.gz)
> Replay attacks : detected (C+S are time synchronized)
> Exposed keys : keys have a limited lifetime
but are multi-session keys
> Temporary file vulnerability :
run krb5-1.2.1+
Attacks against Kerberos (2)
> Passwords guessing : use a good passphrase
> Trojaned clients : OTP
> Implicit trust between realms
> Ticket forwarding
> Others : KDC, shared workstations, ...
*NIX clients
> RedHat (6.2 and 7) provides Kerberos V support
> Install patch RHSA-2001:025-14
> Solaris/OpenBSD only provide Kerberos IV
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
Kerberos V on *NIX clients (2)
> Kerberized Telnet : available
> Kerberized SSH :
> SSH.Com’s SSH 1.2.x and 2.x support Kerberos V
> OpenSSH (as of 2.5.1) doesn’t yet support Kerberos V :
http://www.sxw.org.uk/computing/patches/
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)
> 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 :
Hardware IOS RAM / Flash
26xx
12.0
32 / 8
12.1
48 / 16
72xx
12.0
64 / 16
12.1
64 / 16
Hint: always check with the Cisco IOS Feature Navigator
> Router Configuration :
aaa authentication login default krb5-telnet local
aaa authorization exec default krb5-instance
kerberos local-realm COLT.CH
kerberos srvtab entry host/[email protected] ...
kerberos server COLT.CH 192.168.0.14
kerberos instance map engineering 15
kerberos instance map support 3
kerberos credentials forward
line vty 0 4
ntp server 192.168.0.126
> 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 :
Hardware CatOs
4000
5.2+
6.1
6000
5.4+
6.1
Memory
64
64
64
64 (SE1)
Hint: always check the Release Notes
> Switch Configuration :
#kerberos
set kerberos local-realm COLT.CH
set kerberos clients mandatory
set kerberos credentials forward
set kerberos server COLT.CH 192.168.0.82 88
set kerberos srvtab entry host/[email protected] ...
#authentication
set authentication login kerberos enable telnet primary
set authentication enable kerberos enable telnet primary
#ntp
set ntp client enable
set ntp server 192.168.0.11
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
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 [email protected] localuser
ksetup /mapuser * *
> Windows Time Server (+ registry)
> No kerberized SSH, only a few (broken)
telnet clients
That’s all folks :-)
> Latest version, goodies and additional information
< http://www.securite.org/presentations/krb5/ >
> Q&A
Picture: http://www.inforamp.net/~dredge/funkycomputercrowd.html

Kerberos in an ISP environment UNIX/Win2K/Cisco

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