ISSalert: Internet Security Systems Security Advisory: Microsoft IIS Remote Denial of Service Attack

Internet Security Systems Security Advisory
May 11, 2000

Microsoft IIS Remote Denial of Service Attack

Synopsis:
Internet Security Systems (ISS) X-Force has determined that Microsoft
Internet Information Server (IIS) is vulnerable to a remote Denial of
Service (DoS) attack. IIS is a popular web server application for Windows
NT, and comprises the majority of Windows NT based web servers. This
vulnerability may allow a remote attacker to effectively disable vulnerable
versions of IIS.

Impact:
This vulnerability causes a Windows NT system to consume 100% CPU usage. The
inetinfo.exe process cannot be stopped, requiring a full reboot of the
server.

Affected Versions:
Microsoft IIS version 4.0 is affected. IIS version 5.0 is affected, however
the impact is limited.

Description:
Microsoft Internet Information Server is a popular web server that runs
exclusively on Windows NT. The vulnerability exists primarily in IIS 4.0 and
to a limited extent in 5.0. IIS uses IISADMPWD virtual directory to give
users the ability to change passwords. When IIS is installed, it creates
the directory %system32%\inetsrv\iisadmpwd that contains .htr files used
for web-based password administration. Only when the virtual directory
IISADMPWD is created does the ability to change passwords become enabled.
On vulnerable systems, an attacker can send a malformed request to force
inetinfo.exe to utilize 100% of the CPU and adversely affect the ability
of IIS to field requests. After the vulnerability has been exploited, the
inetinfo.exe process cannot be stopped, requiring a full reboot of the
server to regain functionality. The effect on IIS 5.0 is not as severe.
If the vulnerability is exploited against this version of IIS, access to
any .htr file on the server fails. CPU utilization does not increase to
100% as it does in version 4.0.

Recommendations:

Microsoft has made patches available for IIS versions 4 and 5. ISS X-Force
recommends that these patches be installed immediately.

Internet Information Server 4.0:
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=20905

Internet Information Server 5.0:
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=20903

The ISS X-Force recommends deleting the IISADMPWD virtual directory as
follows:

IIS 4.0
Start the Microsoft Management Console for IIS.
Click the Windows Start Menu.
Select Programs.
Select Windows NT 4.0 Option Pack.
Select Microsoft Internet Information Server.
Select Internet Service Manager.

In the left-hand pane, follow the path below and drill down the tree to the
IISADMPWD virtual directory:
Console Root\Internet Information Server\<Computer Name>\Default Web Site\IISADMPWD

Right-click the IISADMPWD virtual directory and select Delete from the
pop-up menu item.

IIS 5.0
Start the Microsoft Management Console for IIS.
Click the Windows Start Menu.
Select Programs.
Select Administrative Tools.
Select Internet Service Manager.

In the left-hand pane, follow the path below and drill down the tree to the
IISADMPWD virtual directory:
Internet Information Server\<Computer Name>\Default Web Site\IISADMPWD

Right-click the IISADMPWD virtual directory and select Delete from the
pop-up menu item.

The ISS X-Force has updated the ISS SAFEsuite security assessment software,
Internet Scanner, to detect this vulnerability in X-Press Update 3.6.

Additional Information:

The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CAN-2000-0304 to this issue. This is a candidate for inclusion in the CVE
list (http://cve.mitre.org), which standardizes names for security problems.

Credits:
This vulnerability was discovered by Steven Maks of ISS and researched by
Dan Ingevaldson of ISS X-Force. Internet Security Systems would like to
thank Microsoft for their response and handling of this vulnerability.

About Internet Security Systems (ISS)

Internet Security Systems (ISS) is a leading global provider of security
management solutions for the Internet. By providing industry-leading
SAFEsuite security software, remote managed security services, and strategic
consulting and education offerings, ISS is a trusted security provider to
its customers, protecting digital assets and ensuring safe and uninterrupted
e-business. ISS' security management solutions protect more than 5,500
customers worldwide including 21 of the 25 largest U.S. commercial banks, 10
of the largest telecommunications companies and over 35 government agencies.
Founded in 1994, ISS is headquartered in Atlanta, GA, with additional
offices throughout North America and international operations in Asia,
Australia, Europe, Latin America and the Middle East. For more information,
visit the Internet Security Systems web site at www.iss.net or call
888-901-7477.

Copyright (c) 2000 Internet Security Systems, Inc.

Permission is hereby granted for the redistribution of this Alert
electronically. It is not to be edited in any way without express consent of
the X-Force. If you wish to reprint the whole or any part of this Alert in
any other medium excluding electronic medium, please e-mail [email protected]
<mailto:[email protected]> for permission.

Disclaimer
The information within this paper may change without notice. Use of this
information constitutes acceptance for use in an AS IS condition. There are
NO warranties with regard to this information. In no event shall the author
be liable for any damages whatsoever arising out of or in connection with
the use or spread of this information. Any use of this information is at the
user's own risk.

X-Force PGP Key available at: <http://xforce.iss.net/sensitive.php3&gt; as well
as on MIT's PGP key server and PGP.com's key server.

Please send suggestions, updates, and comments to: X-Force [email protected] of
Internet Security Systems, Inc.