Subject: RE: PPTP AUTHENTICATION

From: Gurdeep Singh Pall <[email-masked]>
Date: Wed, 16 Jun 1999 11:42:56 -0700

James, how do you do change password (a feature in ms-chap-v-anything) with
a "standard" protocol?

-----Original Message-----
From: James Carlson [mailto:[email-masked]]
Sent: Wednesday, June 16, 1999 10:11 AM
To: [email-masked]
Subject: Re: PPTP AUTHENTICATION

There's nothing in the standard protocols that would prevent them from
doing that and eliminating MS-CHAP-v-anything.

---

From: James Carlson <[email-masked]>
Date: Wed, 16 Jun 1999 15:20:14 -0400 (EDT)

> James, how do you do change password (a feature in ms-chap-v-anything) with

You do it via a separate mechanism; any that is validated by the
administrator of the authenticating system will do.  Some security
folks may want physical identifiers to do this; some may do it with a
script.

I view the ability to set the password as a bug, not a feature, in
MS-CHAP.  The bug here is that I can use this security hole to break
into an NT system.  To break in, I first steal a copy of the MD4-
hashed registry.  Then, I use this with a modified implementation of
MS-CHAP (note that the Unix version of this is available as free
source code) that doesn't bother doing the meaningless MD4-hash step
-- it just authenticates using the stolen hash value.  Next, I use
MS-CHAP to change the compromised user's *actual* password.  Now I can
both connect to the network and authenticate as that user for access
to systems on the network as well, since NT is reusing this
authentication mechanism to validate user access.

If this hole weren't designed into MS-CHAP, as it isn't in standards-
compliant CHAP, then stealing the registry wouldn't help much.  I
would be able to connect to the network and pass packets, but I
wouldn't have access to any of the nodes on the network, since they
would each want to challenge me for my knowledge of the true password
(which I don't have), or, if the administrator had any smarts at all,
for a *different* password.

-- 
James Carlson, Software Architect                   <[email-masked]>
IronBridge Networks / 55 Hayden Avenue   71.246W   Vox:  +1 781 372 8132
Lexington MA  02421-7996 / USA           42.423N   Fax:  +1 781 372 8090
"PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp

---

From: David Woolley <[email-masked]>
Date: Thu, 17 Jun 1999 22:21:40 +0100 (BST)

> MS-CHAP to change the compromised user's *actual* password.  Now I can

I was under the impression that the same MD4 hashed value was used for
SMB authentication; if so, there is no distinct *actual* password, and
read access compromises the network without the need to change the
password (you can make the same conceptual change to SAMBA as you can
make to pppd).