- Internet Explorer had a broken HTTP digest authentication before v7 and

there are servers "out there" that relies on the client doing this broken
  Digest authentication. Apache even comes with an option to work with such
  broken clients.

  The difference is only for URLs that contain a query-part (a '?'-letter and
  text to the right of it).

  libcurl now supports this quirk, and you enable it by setting the
  CURLAUTH_DIGEST_IE bit in the bitmask you pass to the CURLOPT_HTTPAUTH or
  CURLOPT_PROXYAUTH options. They are thus individually controlled to server
  and proxy.

lib/http_digest.c

@@ -356,7 +356,25 @@ CURLcode Curl_output_digest(struct connectdata *conn,
     5.1.1 of RFC 2616)
   */
 
-  md5this = (unsigned char *)aprintf("%s:%s", request, uripath);
+  /* So IE browsers < v7 cut off the URI part at the query part when they
+     evaluate the MD5 and some (IIS?) servers work with them so we may need to
+     do the Digest IE-style. Note that the different ways cause different MD5
+     sums to get sent.
+
+     Apache servers can be set to do the Digest IE-style automatically using
+     the BrowserMatch feature:
+     http://httpd.apache.org/docs/2.2/mod/mod_auth_digest.html#msie
+
+     Further details on Digest implementation differences:
+     http://www.fngtps.com/2006/09/http-authentication
+  */
+  if(authp->iestyle && (tmp = strchr((char *)uripath, '?'))) {
+    md5this = (unsigned char *)aprintf("%s:%.*s", request,
+                                       (int)(tmp - (char *)uripath), uripath);
+  }
+  else
+    md5this = (unsigned char *)aprintf("%s:%s", request, uripath);
+
   if(!md5this) {
     free(ha1);
     return CURLE_OUT_OF_MEMORY;