[NT] Web Client Service Remote Code Execution (MS06-008)

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Web Client Service Remote Code Execution (MS06-008)
------------------------------------------------------------------------


SUMMARY

A remote code execution vulnerability exists in the way that Windows
processes Web Client requests. This could allow an attacker to take
complete control of the affected system.

DETAILS

Vulnerable Systems:
* Microsoft Windows XP Service Pack 1 and Microsoft Windows XP Service
Pack 2
<http://www.microsoft.com/downloads/details.aspx?FamilyId=62535040-5204-4469-B0BF-EAE14567C2D5> Download the update
* Microsoft Windows XP Professional x64 Edition
<http://www.microsoft.com/downloads/details.aspx?FamilyId=9734F634-6869-434F-AAF0-47B70F84D178> Download the update
* Microsoft Windows Server 2003 and Microsoft Windows Server 2003 Service
Pack 1
<http://www.microsoft.com/downloads/details.aspx?FamilyId=FA073183-0C83-4F1C-BE46-A2EE8A1A1440> Download the update
* Microsoft Windows Server 2003 for Itanium-based Systems and Microsoft
Windows Server 2003 with SP1 for Itanium-based Systems
<http://www.microsoft.com/downloads/details.aspx?FamilyId=E186E149-208A-4035-A0FC-E1CBDE4E6FEF> Download the update
* Microsoft Windows Server 2003 x64 Edition
<http://www.microsoft.com/downloads/details.aspx?FamilyId=E2F5413A-0B77-4C18-9BAB-E2470D3D3F4E> Download the update
* Note The security updates for Microsoft Windows Server 2003, Microsoft
Windows Server 2003 Service Pack 1, and Microsoft Windows Server 2003 x64
Edition also apply to Microsoft Windows Server 2003 R2.

Immune Systems:
* Microsoft Windows 2000 Service Pack 4
* Microsoft Windows 98, Microsoft Windows 98 Second Edition (SE), and
Microsoft Windows Millennium Edition (ME)

Mitigating Factors for Web Client Vulnerability -
<http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-0013>
CVE-2006-0013:
* An attacker must have valid logon credentials to exploit this
vulnerability. The vulnerability could not be exploited remotely by
anonymous users. However, the affected component is available remotely to
users who have standard user accounts. In certain configurations,
anonymous users could authenticate as the Guest account. For more
information, see
<http://www.microsoft.com/technet/security/advisory/906574.mspx> Microsoft
Security Advisory 906574.

* By default, the Web Client service is disabled in Windows Server 2003
and Windows Server 2003 Service Pack 1. An administrator would have to
manually enable this service for the system to become vulnerable to this
issue.

* Firewall best practices and standard default firewall configurations
can help protect networks from attacks that originate outside the
enterprise perimeter. Best practices recommend that systems that are
connected to the Internet have a minimal number of ports exposed. By
default, the Internet Connection Firewall that is provided as part of
Windows XP Service Pack 1 and Windows Server 2003 blocks the affected
ports from responding to network-based attempts to exploit this
vulnerability.

Workarounds for Web Client Vulnerability -
<http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-0013>
CVE-2006-0013:
Microsoft has tested the following workarounds. While these workarounds
will not correct the underlying vulnerability, they help block known
attack vectors. When a workaround reduces functionality, it is identified
in the following section.

Disable the Web Client service

Disabling the Web Client service will help protect the affected system
from attempts to exploit this vulnerability. To disable the Web Client
service, follow these steps:

1. Click Start, and then click Control Panel. Alternatively, point to
Settings, and then click Control Panel.
2. Double-click Administrative Tools.
3. Double-click Services.
4. Double-click WebClient.
5. In the Startup type list, click Disabled.
6. Click Stop, and then click OK.

You can also stop and disable the Web Client service by using the
following command at the command prompt:

sc stop WebClient & sc config WebClient start= disabled

Impact of Workaround: If the Web Client service is disabled, Web
Distributed Authoring and Versioning (WebDAV) requests are not
transmitted. If the Web Client service is disabled, any services that
explicitly depend on the Web Client service will not start, and an error
message will be logged in the System log. Windows Server 2003 users will
not be able to use the "Open as Web Folder" functionality.

Use the Group Policy settings to disable the WebClient service on all
affected systems that do not require this feature.
Because the Web Client service is a possible attack vector, disable the
service by using the Group Policy settings. You can disable the startup of
this service at either the local, site, domain, or organizational-unit
level by using Group Policy object functionality in Windows 2000 domain
environments or in Windows Server 2003 domain environments.

Note You may also review the
<http://www.microsoft.com/technet/security/prodtech/windowsserver2003/w2003hg/sgch00.mspx> Windows Server 2003 Security Guide. This guide includes information about how to disable services.

For more information about Group Policy, visit the following Microsoft Web
sites:

*
<http://www.microsoft.com/technet/prodtechnol/windowsserver2003/technologies/directory/activedirectory/stepbystep/gpfeat.mspx> Step-by-Step Guide to Understanding the Group Policy Feature Set
*
<http://www.microsoft.com/windows2000/techinfo/howitworks/management/grouppolwp.asp> Windows 2000 Group Policy
*
<http://www.microsoft.com/technet/prodtechnol/windowsserver2003/technologies/management/gp/default.mspx> Group Policy in Windows Server 2003

Impact of Workaround: If the Web Client service is disabled, Web
Distributed Authoring and Versioning (WebDAV) requests are not
transmitted. If the Web Client service is disabled, any services that
explicitly depend on the Web Client service will not start, and an error
message will be logged in the System log. Windows Server 2003 users will
not be able to use the "Open as Web Folder" functionality.

Block TCP ports 139 and 445 at the firewall:

Although WebDAV uses TCP port 80 for outbound communication, TCP ports 139
and 445 can be used inbound to attempt to connect to this service and try
to exploit this vulnerability. Blocking them at the firewall can help
prevent systems that are behind that firewall from attempts to exploit
this vulnerability. We recommend that you block all unsolicited inbound
communication from the Internet to help prevent attacks that may use other
ports. For more information about ports, visit the following Web site.

To help protect from network-based attempts to exploit this vulnerability,
use a personal firewall, such as the
<http://go.microsoft.com/fwlink/?LinkId=33335> Internet Connection
Firewall, which is included with Windows XP and with Windows Server 2003.

By default, the Internet Connection Firewall feature in Windows XP and in
Windows Server 2003 helps protect your Internet connection by blocking
unsolicited incoming traffic. We recommend that you block all unsolicited
incoming communication from the Internet. In Windows XP Service Pack 2
this features is called the Windows Firewall.

To enable the Internet Connection Firewall feature by using the Network
Setup Wizard, follow these steps:

1. Click Start, and then click Control Panel.
2. In the default Category View, click Network and Internet Connections,
and then click Setup or change your home or small office network. The
Internet Connection Firewall feature is enabled when you select a
configuration in the Network Setup Wizard that indicates that your system
is connected directly to the Internet.

To configure Internet Connection Firewall manually for a connection,
follow these steps:

1. Click Start, and then click Control Panel.
2. In the default Category View, click Networking and Internet
Connections, and then click Network Connections.
3. Right-click the connection on which you want to enable Internet
Connection Firewall, and then click Properties.
4. Click the Advanced tab.
5. Click to select the Protect my computer or network by limiting or
preventing access to this computer from the Internet check box, and then
click OK.

Note If you want to enable certain programs and services to communicate
through the firewall, click Settings on the Advanced tab, and then select
the programs, the protocols, and the services that are required.

To help protect from network-based attempts to exploit this vulnerability,
enable advanced TCP/IP filtering on systems that support this feature.

You can enable advanced TCP/IP filtering to block all unsolicited inbound
traffic. For more information about how to configure TCP/IP filtering, see
<http://support.microsoft.com/kb/309798> Microsoft Knowledge Base Article
309798.

To help protect from network-based attempts to exploit this vulnerability,
block the affected ports by using IPSec on the affected systems.

Use Internet Protocol security (IPSec) to help protect network
communications. Detailed information about IPSec and about how to apply
filters is available in <http://support.microsoft.com/kb/313190>
Microsoft Knowledge Base Article 313190 and
<http://support.microsoft.com/kb/813878> Microsoft Knowledge Base Article
813878.

FAQ for Web Client Vulnerability -
<http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-0013>
CVE-2006-0013:
What is the scope of the vulnerability?
This is a remote code execution vulnerability. An attacker who
successfully exploited this vulnerability could take complete control of
an affected system. An attacker could then install programs; view, change,
or delete data; or create new accounts with full user rights. To attempt
to exploit the vulnerability, an attacker must have valid logon
credentials. The vulnerability could not be exploited by anonymous users.
An attacker could also use this vulnerability to perform a local elevation
of privilege attack.

What causes the vulnerability?
An unchecked buffer in the WebClient service.

What is the Web Client service?
The Web Client service allows applications to access documents on the
Internet. Web Client extends the networking capability of Windows by
allowing standard Win32 applications to create, read, and write files on
Internet file servers by using the WebDAV protocol. The WebDAV protocol is
a file-access protocol that is described in XML and that travels over the
Hypertext Transfer Protocol (HTTP). By using standard HTTP, WebDAV runs
over the existing Internet infrastructure. For example, WebDAV runs over
firewalls and routers.

If the Web Client service is stopped, you will be prevented from using the
Web Publishing Wizard to publish data to the Internet for locations that
use the WebDAV protocol. If this service is disabled, any services that
explicitly depend on this service will not start. For more information on
WebDAV, see the following
<http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/844f5e01-4b9e-4dac-897e-2a0bb33f28af.mspx> product documentation.

What might an attacker use the vulnerability to do?
An attacker who successfully exploited this vulnerability could take
complete control of an affected system. An attacker could then install
programs; view, change, or delete data; or create new accounts with full
user rights.

Who could exploit the vulnerability?
To attempt to exploit the vulnerability, an attacker must have valid logon
credentials. The vulnerability could not be exploited by anonymous users.
Even though the Web Client service is used to support the WebDAV protocol
over the Internet, an authenticated attacker must perform the steps that
are required to attempt to exploit this issue. If the Guest account has
been enabled on an affected system, then this attack could be performed by
any user. For more information, see
<http://www.microsoft.com/technet/security/advisory/906574.mspx> Microsoft
Security Advisory 906574.

How could an attacker exploit the vulnerability?
An attacker would first have to authenticate to the system. An attacker
could then try to exploit the vulnerability directly over a network by
creating a series of specially crafted messages and sending them to an
affected system. The messages could then cause the affected system to
execute code.

What systems are primarily at risk from the vulnerability?
All affected operating systems are at risk from this vulnerability. The
Internet Connection Firewall that is provided as part of Windows XP
Service Pack 1 and Windows Server 2003 blocks the affected ports from
responding to network-based attempts to exploit this vulnerability. The
Internet Connection Firewall is not enabled by default on Windows XP
Service Pack 1. By default, the Web Client service is disabled in Windows
Server 2003.

Note By default, exceptions will be automatically created after enabling
File & Printer Sharing which allow access from the network. This access is
limited to the local subnet by default.

Could the vulnerability be exploited over the Internet?
Yes. An attacker could try to exploit this vulnerability over the
Internet. Firewall best practices and standard default firewall
configurations can help protect against attacks that originate from the
Internet. Microsoft has provided information about how you can help
protect your PC. End users can visit the
<http://go.microsoft.com/fwlink/?LinkId=21169> Protect Your PC Web site.
IT professionals can visit the
<http://go.microsoft.com/fwlink/?LinkId=21171> Security Guidance Center
Web site.

What does the update do?
The update removes the vulnerability by modifying the way that the Web
Client service validates the length of a message before it passes the
message to the allocated buffer.

When this security bulletin was issued, had this vulnerability been
publicly disclosed?
No. Microsoft received information about this vulnerability through
responsible disclosure

When this security bulletin was issued, had Microsoft received any reports
that this vulnerability was being exploited?
No. Microsoft had not received any information to indicate that this
vulnerability had been publicly used to attack customers and had not seen
any examples of proof of concept code published when this security
bulletin was originally issued.

How does this vulnerability relate to the Web Client Vulnerability that is
corrected by MS05-028?
Both vulnerabilities were in Web Client service. However, this update
addresses a new vulnerability that was not addressed as part of MS05-028.
MS05-028 helps protect against the vulnerability that is discussed in that
bulletin, but does not address this new vulnerability. This update
replaces MS05-028. You must install this update to help protect your
system against both vulnerabilities.


ADDITIONAL INFORMATION

The information has been provided by Microsoft Security.
The original article can be found at:
<http://www.microsoft.com/technet/security/Bulletin/MS06-008.mspx>
http://www.microsoft.com/technet/security/Bulletin/MS06-008.mspx



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