e-matters GmbH
                          www.e-matters.de

                      -= Security  Advisory =-



     Advisory: Interner Explorer HTTPS certificate attack
 Release Date: 2001/12/22 
       Author: Stefan Esser [s.esser@e-matters.de]

  Application: Microsoft Internet Explorer 5.0/5.5/6.0
     Severity: Vulnerability in IE's SSL Certificate handling allows
               undetected SSL Man-In-The-Middle attacks
         Risk: Very High
Vendor Status: Notified
    Reference: https://security.e-matters.de/advisories/012001.html



Overview:
  
   A flaw in Microsoft Internet Explorer allows an attacker to perform
   a SSL Man-In-The-Middle attack without the majority of users recognising
   it. In fact the only way to detect the attack is to manually compare the
   server name with the name stored in the certificate.
  
   For a basic introduction into SSL MIM attacks I recommend reading:
  
   Phrack #57 - Hang on, Snoopy (by stealth)
   https://www.phrack.org/show.php?p=57&a=13
  
  
Details:

   There is a flaw in the way IE checks HTTPS objects that are embedded into 
   normal HTTP pages. According to my tests IE does only check if the certi-
   ficate of the HTTPS server is properly signed by a trusted CA but totally
   ignores if the cert was issued onto the correct name or has already ex-
   pired. This is in fact not dangerous because the user considers HTTPS 
   objects embedded in a HTTP page not secure. The problem is that IE flags
   the certificate as trusted and caches this certification trust until your
   browser session ends. That means once you visited a normal http page that
   included an image from the MIMed SSL Server, IE will not warn you about 
   an illegal site certificate as long the certificate was signed by f.e. 
   Verisign.

   A possible scenario would be:
  
   Hacker runs a MIM attacking tool for HTTP/HTTPS in the subnet of your 
   site. The HTTP part of the tool auto appends
    
   <img src="https://www.yoursite.com/nonexistent.gif" width=1 height=1>
    
   to any html page that is returned to your customer's browser and the 
   HTTPS part presents his browser a valid but stolen certificate for 
   www.shop.com. IE will only check if the cert was signed by a trusted CA
   when trying to display the image and won't compare the name inside the 
   cert or check the expiration date. If your customer now tries to login
   to your site via HTTPS IE will consider the cert trustworthy without 
   checking it again. Your customer will only be able to determine that he
   was just tricked by manually checking the servername in the cert. But
   you can be sure that only paranoid people would check. The majority of
   people don't even know how they can do so. Imagine the hacker stole the
   cert from "yoursite.de"... How many users of "yoursite.com" would not
   trust a cert that was issued on "yoursite.de". The average user does 
   not know anything about SSL than it's making his payment "secure".


Proof of Concept:

   A proof of concept webpage was put up at https://suspekt.org. Clicking 
   onto the "To the secure page..." link will send your browser to 
   https://suspekt.org without IE warning you that the certificate was not
   issued onto that server.
  
   This is not a MIM but it has the same effect: IE will tell you a page is
   secure although the certificate is illegal and its possible for a third
   party (anyone who owns the given certificate) to decrypt your traffic in
   realtime.


Vendor Response:

   26 November 2001 -   Microsoft was informed about this vulnerability
   27 November 2001 -   Proof of concept page got visited by lots of MS IPs
   01 December 2001 -   Microsoft informed us with a standard reply that
                        they have received the advisory
   12 December 2001 -   Microsoft was informed that were going to release 
                        the advisory within the next 3 days
   13 December 2001 -   Microsoft asked us to wait because the issue is 
                        complex due to the fact a lot of cryptography 
                        is involved
   21 December 2001 -   Microsoft sent an update: no patches yet, 
                        still a complex issue
    

Conclusion:

   Until today Microsoft did not release a patch, they had nearly a month 
   time to fix the bug. Instead they call it a very complex issue. Because
   I don't know the source code of the Internet Explorer I cannot check the
   validity of these claims, but from my point of view fixing this missing
   check is just a matter of copy and pasting a few lines. Unfortunately it
   is christmas time and especially during the last month millions of cus-
   tomers where buying christmas presents on the internet all around the
   world. That means millions of customers were shopping with insuffient 
   protection of their private data. Because there are no patches out yet,
   I strongly recommend that you use Mozilla, Opera or another non MS brow-
   ser to do your internet banking or shopping these days. If you think 
   (for whatever strange reason) that you need the Internet Explorer, 
   ensure that the certificate is the correct by comparing the servername
   in the certificate with the one in your browser...  
 

GPG-Key:

   https://security.e-matters.de/gpg_key.asc
    
   pub  1024D/D19C5835 2001-11-26 e-matters GmbH - Securityteam 
   Key fingerprint = DD27 8C4B CEDE 41A9 5766  39BA AF65 B19C D19C 5835


Copyright 2001 Stefan Esser. All rights reserved.


--- Packet Storm ---


UPDATE: IE https certificate attack
        
Date: 2001/12/25
        
This morning i was googling through the web and found out that
the issue is not that new for Microsoft.
If you compare
https://www.acros.si/aspr/ASPR-1999-12-15-1-PUB.txt
with my advisory at
https://security.e-matters.de/advisories/012001.html
you can see that the same bug was reported 2(!) years ago to
microsoft. At that time (or better half a year later) Microsoft
released the patches for that vulnerability that fixed the
bug within IE 4.0 and the early versions of IE 5.0.
The Microsoft Security Bulletin (MS00-039) clearly states that
IE 5.01 SP1 and IE 5.5 are not vulnerable.
That means, that one of the "security patches" that Microsoft
released since that date reimplemented the bug and made all
IEs vulnerable again.

Stefan Esser