Microsoft Security Bulletin (MS00-076)
Patch Available for "Cached Web Credentials" Vulnerability
Originally posted: October 12, 2000
Microsoft has released a patch that eliminates a security
vulnerability in Microsoft(r) Internet Explorer. Under a daunting
set of conditions, the vulnerability could enable a malicious user to
obtain another user's userid and password to a web site.
Frequently asked questions regarding this vulnerability
and the patch can be found at
When a user authenticates to a secured web page via Basic
Authentication, IE caches the userid and password that were used, in
order to minimize the number of times the user must authenticate to
the same site. By design, IE should only send the cached credentials
to secured pages on the site. However, it will actually send them to
non-secure pages on the site as well. If a malicious user had
complete control of another user's network communications, he could
wait until another user logged onto a secured site, then spoof a
request for a non-secured page in order to collect the credentials.
The vulnerability does not provide a means by which the malicious
user could force the other user to log onto a secure page of his
choice, and could only be used to reveal credentials that had been
cached during the current IE session.
Affected Software Versions
- Microsoft Internet Explorer 4.x
- Microsoft Internet Explorer 5.x prior to version 5.5
Note: Internet Explorer 5.5 is not affected by this vulnerability.
Customers using IE 5.5 do not need to take any action.
Note: The patch requires IE 5.01 SP1 to install. Customers who
install this patch on other versions may receive a message reading
"This update does not need to be installed on this system". This
message is incorrect. More information is available in KB article
Note: As discussed in Affected Software Versions, this vulnerability
does not affect IE 5.5.
Note: Per the normal security support policy for IE, security patches
for Internet Explorer version 4.x are no longer being produced.
Microsoft recommends that IE 4.x customers who are concerned about
this issue consider upgrading to either IE 5.01 SP1 or IE 5.5.
Note: The fix for this issue will be included in IE 5.01 SP2.
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 MS00-076,
- Microsoft Knowledge Base article Q273868 discusses this issue
and will be available soon.
- Microsoft TechNet Security web site,
Obtaining Support on this Issue
This is a fully supported patch. Information on contacting Microsoft
Support Services is available at
Microsoft thanks ACROS Security (http://www.acros.si) for reporting
this issue to us and working with us to protect customers.
- October 12, 2000: Bulletin Created.
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Last Updated October 12, 2000