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||Patch Available for "Session ID Cookie Marking" Vulnerability
||20th November 2000
The following is a Security Bulletin from the Microsoft Product Security
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Microsoft Security Bulletin (MS00-080)
Patch Available for "Session ID Cookie Marking" Vulnerability
Originally posted: October 23, 2000
Updated: November 20, 2000
On October 23, 2000, Microsoft released the original version of this
bulletin, to discuss the availability of a patch that eliminates a
security vulnerability in Microsoft(r) Internet Information Server.
The vulnerability could allow a malicious user to "hijack" another
user's secure web session, under a very restricted set of
On November 20, 2000, we re-released the bulletin to advise customers
using IIS 4.0 on Alpha platforms, or IIS 5.0 on x86 platforms, that
new versions of these patch are available, to correct an error in the
original version of the patch. The x86 IIS 4.0 patch was not affected
by the error, and customers using these systems do not need to take
Frequently asked questions regarding this vulnerability and
the patch can be found at
IIS supports the use of a Session ID cookie to track the current
session identifier for a web session. However, .ASP in IIS does not
support the creation of secure Session ID cookies as defined in RFC
2109. As a result, secure and non-secure pages on the same web site
use the same Session ID. If a user initiated a session with a secure
web page, a Session ID cookie would be generated and sent to the
user, protected by SSL. But if the user subsequently visited a
non-secure page on the same site, the same Session ID cookie would be
exchanged, this time in plaintext. If a malicious user had complete
control over the communications channel, he could read the plaintext
Session ID cookie and use it to connect to the user's session with
the secure page. At that point, he could take any action on the
secure page that the user could take.
The conditions under which this vulnerability could be exploited are
rather daunting. The malicious user would need to have complete
control over the other user's communications with the web site. Even
then, the malicious user could not make the initial connection to the
secure page - only the legitimate user could do that. The patch
eliminates the vulnerability by adding support for secure Session ID
cookies in .ASP pages. (Secure cookies already are supported for all
other types of cookies, under all other technologies in IIS).
The original version of patches for IIS 4.0 Alpha and the IIS 5.0
systems did not install correctly. The IIS 4.0 x86 version of the
patch does install correctly.
Affected Software Versions
- Microsoft Internet Information Server 4.0
- Microsoft Internet Information Services 5.0
- IIS 4.0:
Available from Microsoft Product Support Services
- IIS 5.0:
Note: The patch installs support for secure Session ID cookies, but
does not enable it for reasons of application compatibility. As
discussed in the Knowledge Base article, it can be enabled or
disabled on a site-by-site basis.
- The IIS 4.0 version of this patch can be installed on Windows
NT 4.0 systems running Service Pack 6a, and will be included
in Service Pack 7.
- The IIS 5.0 version of this patch can be installed on Windows
2000 systems with or without Service Pack 1, and will be
included in Service Pack 2.
Note: Additional security patches are available at the Microsoft
Please see the following references for more information related to
- Frequently Asked Questions: Microsoft Security Bulletin MS00-080,
- Microsoft Knowledge Base article Q274149 discusses this issue
and will be available soon.
- RFC 2109, HTTP State Management,
- Microsoft TechNet Security web site,
Obtaining Support on this Issue
This is a fully supported patch. Information on contacting Microsoft
Product Support Services is available at
Microsoft thanks ACROS Security (http://www.acros.si/) and Ron Sires
and C. Conrad Cady of Healinx (http://www.healinx.com/) for reporting
this issue to us and working with us to protect customers.
- October 23, 2000: Bulletin Created.
- November 20, 2000: Bulletin updated to indicate availability of
updated patches for IIS 4.0 on Alpha platforms, and IIS 5.0
on x86 platforms.
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DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
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