Word document streamed to HTTP response causes 401 authentication demand - c++

Question

Good morning all,
I have a web app using Classic ASP, hosted on IIS 7.5. I need to create a word document and stream back to the client for display, however when the document is streamed into the response something (an IIS setting?) is causing a 401 error - and i cannot track it down.
A colleague very kindly furnished me with some C++ code which deals with the document building & streaming and an ASP page which builds the COM objects and makes the calls etc.
The document starts life as a template in an IIS virtual directory outside my ASP application. We make a copy, do some find and replace actions and then stream it into the response with the correct MIME type for the browser to handle it however it sees fit.
It seems as soon as the streaming takes place (via a call to ASPTypeLibrary::IResponsePtr piResponse->BinaryWrite()) the 401 response is sent back, until that moment the response looks fine.
The site uses Forms authentication, and i have by this point signed in, the request i see in fiddler definitely has valid session data and the rest of the site is happy with my authentication.
Any ideas? (he asks with a note of desperation)
p.s. I realise i haven't listed all the code, i can if it helps though...
The plot thickens...
I have had success using the ASP page to stream the file into the output using and ADODB.Stream object.
When this is successful Fiddler picks up two HTTP request/responses; the first request gets a 401 back, then the browser sends another request with different cookie data which returns a successful result.
When my COM object is used two requests occur, but the second request also receives a 401...
Points to some security setting to do with COM object? Something i am not adding to the response with the COM object?
As per my response to Eric, my colleague worked a bit of magic and got the thing working, i am still a little confused about why it was caused though...
The line which Magic Al changed was this one:
piResponse->AddHeader( _T("Content-Length"), (LPCTSTR)Length );
Which is called while the response is being built up and what he did was comment it out.
Apparently he noticed that the length written by the BinaryWrite was coming out 13 bytes larger than the length of the file. He tells me that this may be because it is writing out the reserved WORD blocks from the Variant it is given.
So the response header is a bit mangled and the result is an HTTP violation error in fiddler which i had overlooked and somewhere in between client and COM object the mangled 200 response is replaced with a 401.
I guess the moral of this story is that you should always pay attention to fiddler errors and to ensure your Content-Length is correct.
My new question is why 401? why not a 500? And what is likely to be throwing this out? is it coming from IIS?
This is the HTTP text fiddler registers for the exchange:
GET GET [The page address - its on localhost and is an ASP page] HTTP/1.1
Accept: text/html, application/xhtml+xml, */*
Accept-Language: en-GB
User-Agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)
Accept-Encoding: gzip, deflate
Connection: Keep-Alive
Host: localhost
Cookie: SessionUID={E41F5378-FBE2-475E-8F9A-6416AFE2BAA0}; DisplayMethod=0; ShowDataTips=1; LOGONUSER={UserName Info}ASPSESSIONIDQARAQRBD={Session ID} Authorization: Negotiate YH8GBisGAQUFAqB1MHOgMDAuBgorBgEEAYI3AgIKBgkqhkiC9xIBAgIGCSqGSIb3EgECAgYKKwYBBAGCNwICHqI/BD1OVExNU1NQAAEAAACXsgjiCwALADIAAAAKAAoAKAAAAAYBsR0AAAAPRE9DREVWLUpDV0RFVkVMT1BNRU5U
HTTP/1.1 401 Unauthorized
Content-Type: text/html; charset=us-ascii
Server: Microsoft-HTTPAPI/2.0
WWW-Authenticate: Negotiate oYIBYTCCAV2gAwoBAaEMBgorBgEEAYI3AgIKooIBRgSCAUJOVExNU1NQAAIAAAAWABYAOAAAABXCieJQEtMwe36vajD3zwEAAAAA9AD0AE4AAAAGAbEdAAAAD0QARQBWAEUATABPAFAATQBFAE4AVAACABYARABFAFYARQBMAE8AUABNAEUATgBUAAEAFABEAE8AQwBEAEUAVgAtAEoAQwBXAAQAOABEAGUAdgBlAGwAbwBwAG0AZQBuAHQALgBEAG8AYwB1AG0AYQB0AGkAbwBuAC4AYwBvAC4AdQBrAAMATgBEAE8AQwBEAEUAVgAtAEoAQwBXAC4ARABlAHYAZQBsAG8AcABtAGUAbgB0AC4ARABvAGMAdQBtAGEAdABpAG8AbgAuAGMAbwAuAHUAawAFACAARABvAGMAdQBtAGEAdABpAG8AbgAuAGMAbwAuAHUAawAHAAgAKRk7jhfZzQEAAAAA
Date: Thu, 13 Dec 2012 09:52:25 GMT
Content-Length: 341
Proxy-Support: Session-Based-Authentication
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN""http://www.w3.org/TR/html4/strict.dtd">
<HTML><HEAD><TITLE>Not Authorized</TITLE>
<META HTTP-EQUIV="Content-Type" Content="text/html; charset=us-ascii"></HEAD>
<BODY><h2>Not Authorized</h2>
<hr><p>HTTP Error 401. The requested resource requires user authentication.</p>
</BODY></HTML>
------------------------------------------------------------------
GET [The page address - its on localhost and is an ASP page] HTTP/1.1
Accept: text/html, application/xhtml+xml, */*
Accept-Language: en-GB
User-Agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)
Accept-Encoding: gzip, deflate
Connection: Keep-Alive
Cookie: SessionUID={E41F5378-FBE2-475E-8F9A-6416AFE2BAA0}; DisplayMethod=0; ShowDataTips=1; LOGONUSER={UserName Info}ASPSESSIONIDQARAQRBD={Session ID}
Authorization: Negotiate oXcwdaADCgEBoloEWE5UTE1TU1AAAwAAAAAAAABYAAAAAAAAAFgAAAAAAAAAWAAAAAAAAABYAAAAAAAAAFgAAAAAAAAAWAAAABXCiOIGAbEdAAAAD4GPyFfTAkcs1KpJqG4eT0ujEgQQAQAAAPUXp1AtIpqEAAAAAA==
Host: localhost
HTTP/1.1 401 Unauthorized
Cache-Control: private
Content-Type: text/html; charset=utf-8
Server: Microsoft-IIS/7.5
WWW-Authenticate: Negotiate
WWW-Authenticate: NTLM
X-Powered-By: ASP.NET
Date: Thu, 13 Dec 2012 09:52:27 GMT
Content-Length: 6630
Proxy-Support: Session-Based-Authentication
<HTML Page telling you about the error which gets displayed when you cancel the authentication dialog.>