RE: issue: draft-ietf-radext-digest-auth-06.txt Digest MD5-sess

[Henrik Nordstrom <henrik@henriknordstrom.net>]

On Thu, 5 Jan 2006, Avi Lior wrote:

> So one example is in Mobile IP.  Once the HA has validated the
> Registration Request or Binding Update with RADIUS.  It can continue to
> authentication subsequent bind request or Registration Request received
> from that user. This is only limited by a lifetime received from the AAA
> server.

Which is very similar to what is provided by Digest MD5-sess. The 
credentials given to the RADIUS client is only good for validating within 
a good credibility that next messages is in the same user session. It can 
not be reused in authentication to any other service, or even to start a 
new session or renew the existing session to the same service.

Giving avay the non-session Digest MD5 HA1 is a completely different 
think. It's in the Digest world almost equivalent to giving away the users 
plaintext password.

The only security issue with the MD5-sess HA1 in session mode that I can 
see is it SHOULD be transmitted securely IF MD5-sess is used in session 
mode or if message integrity protection is used as knowledge of the 
MD5-sess HA1 hash allows for limited session theft, limited by session 
nonce count and time limits enforced by the service. But as soon as the 
server-side nonce expires the hash is completely useless for any 
meaningful purposes.



My notes on security issues of the Digest-HA1 attribute in general and 
it's use in Digest session authentication in particular:


MD5, any mode of operation:

Equivalent to plaintext for any service using Digest authentication. 
Knowledge of the MD5 HA1 allow for full theft of the account on the 
service/realm until the password or realm is changed, and consequently 
also full control of integrity protection of both requests and responses.

MD5-sess, one request per nonce:

No session theft is possible as the sessions only last for a single 
message.

But knowing the HA1 allows breaking any response integrity protection. 
Request integrity is protected however as the HA1 is only known after the 
request has been received and processed by the service.

MD5-sess, session based:

In addition to the above it allows for session theft for as long as that 
session is considered valid by the service, including request/response 
integrity protection (except for the first message). Session length is 
usually limited by both nonce count (number of requests) and time. The 
RADIUS server may hint both session limits to the client, but can not 
enforce it.


Senario 1 mode of operation:

If Senario 1 mode of operation is allowed then it mau be considerably 
easier for an attacker to gain access to the HA1. If Scenario 1 mode of 
operation is allowed then he only needs to be able to see the application 
protocol traffic and have access to any RADIUS client allowed to query the 
RADIUS server in this mode to gain access to the Digest-HA1 by replaying 
the seen Digest message details to the RADIUS server. Transport protection 
of the Digest-HA1 attribute does not help against this.



Regards
Henrik

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