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Re: NTLM HTTP Authentication is insecure by design - a new writeup by Amit Klein
From: "Amit Klein (AKsecurity)" <aksecurity () hotpop com>
Date: Tue, 19 Jul 2005 20:22:59 +0200

Dear 3APA3A,

On 19 Jul 2005 at 13:35, 3APA3A wrote:

Dear Amit Klein (AKsecurity),

--Monday, July 18, 2005, 9:40:32 PM, you wrote to bugtraq () securityfocus com:

AKA>                      NTLM HTTP Authentication
AKA>               (and possibly other connection-oriented
AKA>           HTTP authentication and authorization protocols)
AKA>                       is insecure by design

 NTLM  authentication is insecure by design for external authentication,
 because  of  "single  signon" ideology. NTLM authentication can be used
 for  NTLM  proxy  attack. For example, attacker who can hijack or spoof
 server  connection  with NTLM authentication can use this connection to
 access  different  Web server, mail server or file server with client's
 credentials.  This  is  known for many years and was discussed for many

While not downplaying this remark, I'd say that:

1. By this argument, any non-cryptographically strong protocol should be considered 
insecure by design. I don't think that's what people have in mind. I think that per each 
protocol, there's a set of expectations. For example, no-one expects NTLM auth to protect 
data in transit. But people do expect NTLM not to by bypassed by silly methods.

2. I see a big difference between spoofing and hijacking attacks, which require non trivial 
skill set, and the attack I described, which, given the right preconditions, is so easy to 
mount that it can be done so accidentally.

Few  years  ago  Internet  Explorer  was  patched  to  use NTLM
 authentication  only  for  local  network zone. Local network are hosts
 with  NetBIOS name (for example WEBSRV, excluded by default from proxy)
 and list of proxy exclusions.

Uh, I don't think so. From my experiments with IE 6.0, it happily engages in NTLM 
authentication on non local network sites. In fact, there are many sites on the Internet 
which require NTLM authentication. For example, OWA 2000/2003...

 So,  under default configuration, NTLM will not be used through proxy
 server, at least in Internet Explorer.

As a result of my former comment, I have to disagree. There ARE websites that require NTLM 
authentication, IE DOES perform it, so there's no theoretic reason why there shouldn't be 
proxy servers in between.


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