How to find if NTLM or Kerberos is used from WWW-Authenticate: Negotiate header

Question

I am programming a client application in .Net that communicates with server via HTTP.

I need to set different request buffering options in case of NTLM and Kerberos authorization.

How to find out if NTLM or Kerberos is used? Is it possible to somehow decode 'WWW-Authenticate: Negotiate' header?

Sheep

Answer 1

You will find answer here.

Short answer is:

1.Capture some successfully authorized request using Fiddler tool.
2.Choose "Inspectors" -> "Headers" tab.
3.Pay attention at "Cookies / Login" section, "Authorization" header.

If the Authorization token begins with "YII" then Kerberos is used, but if it begins with "TlR" then Kerberos is not used.

For example Kerberos:

Authorization: Negotiate YIIVDAYGKwYBE...

Not Kerberos:

Authorization: Negotiate TlRMTVNTUA...

Seasick

Excellent! Very simple, and it works in .Net/C#. Thank you Taras! – Sheep

Also on the Inspectors/Auth tab it will say something like 'Authorization Header (Negotiate) appears to contain a Kerberos ticket' – Sauger

What if begins with an "oS"? – Hanafee

Fiddler sets itself up as a proxy and can cause kerberos to fail in some situations, which would lead to an NTLM fall-back for most Negotiate situations. Microsoft advised me of this in a support ticket in the past, when I THOUGHT I had proven that kerberos wasn't working for a particular application. It is best to use less intrusive logging methods to inspect the authorization header, for instance "netsh trace": blogs.msdn.com/b/canberrapfe/archive/2012/03/31/… – Hrutkay

+1 for "YII", I'm surprised searching for the first few bytes in hex didn't immediately flag up "this is Kerberos" when I was trying to identify the blob". – Bahr

Worth noting: Base64Decode("TlRMTVNT") = "NTLMSS" – Meyerhof

@Hanafee that probably means you are looking at a continued negotiation token, I think the "YII" and "TlR" rule only applies to the first token sent from the browser not subsequent tokens. – Lay

Answer 2

Parsing a Negotiate header is sort of a tedious exercise as it's built using ASN.1 DER.

That said, you may not necessarily need to decode this however to make a good assumption about the payload. While there is a mechanism in GSSAPI for NTLM (more on that below), in my experience clients do not actually use it, they simply send NTLM headers. In my (admittedly strictly controlled) environment, if I see Authorization: NTLM ... then this is guaranteed to be NTLM. If I see Authorization: Negotiate ... then this is guaranteed to be Kerberos.

Strictly speaking, you should look at the mechanism list in the header to determine whether the mechanism was NTLM or Kerberos. I would recommend either using an off-the-shelf ASN.1 decoder, or looking at Microsoft's decoding example. You're going to want to look for the SPNEGO OID (1.3.6.1.5.5.2), then look for the mechanism type sequence within that. The first mechanism in the sequence corresponds to the response token payload, so you can look at that OID to determine the mechanism. Some known OIDs for Kerberos are:

1.2.840.113554.1.2.2 (Kerberos 5)
1.2.840.48018.1.2.2 (Microsoft Kerberos 5)
1.3.5.1.5.2 (Kerberos 5 OID 2)

To my knowledge, the only OID for NTLM is (referenced from this blog):

1.3.6.1.4.1.311.2.2.10 (NLMP NTLM)

Juliettajuliette

It's absolutely legal and possible to use NTLM inside Negotiate, and in my experience, it's pretty common. If you simply base64 decode the header, it should be obvious which SSP is in use. E.g. take a peek inside Fiddler's AUTH tab. – Apanage

@EricLaw: that's why I suggested parsing the mechanism list. – Juliettajuliette

@EricLaw: and if there's some more trivial way to determine the mechanism than by parsing it properly, then that would be helpful knowledge to have. Is that not what fiddler is doing? – Juliettajuliette

There is also an OID for NegoEx: 1.3.6.1.4.1.311.2.2.30. This is used to extend "authorization: negotiate" headers to support new protocols. – Emergency

false="If I see Authorization: Negotiate ... then this is guaranteed to be Kerberos" – Uncircumcision

@Uncircumcision I though that I was pretty clear with the "in my environment" disclaimer, but thanks anyway for the downvote! – Juliettajuliette

Answer 3

If the server advertises to user Negotiate you are free to use Kerberos, NTLM oder something is supported by SPNEGO. Though, there is no guarantee that the server supports every wrapped auth method sent by the client.

Copeck

I am programming it with .Net and I can only specify Negotiate on a client side. So finally I do not know if NTLM or Kerberos was chosen. Any idea how to detect which was selected? – Sheep

Answer 4

Yes; just Base64 decode it and you will see "NTLM" or "HTTP".

C#

v = BitConverter.ToString(Convert.FromBase64String(v.Replace("Negotiate: ","")));
if (v.indexOf("NTLM") > -1) {
    //...
}

Uncircumcision