Microsoft Security Bulletin MS09-020 - Important Vulnerabilities in Internet Information Services (IIS) Could Allow Elevation of Privilege (970483)

Microsoft Security Bulletin MS09-020 - Important
Vulnerabilities in Internet Information Services (IIS) Could Allow Elevation of Privilege (970483)
Published: June 9, 2009

Version: 1.0
General Information
Executive Summary

This security update resolves one publicly disclosed vulnerability and one privately reported vulnerability in Microsoft Internet Information Services (IIS). The vulnerabilities could allow elevation of privilege if an attacker sent a specially crafted HTTP request to a Web site that requires authentication. These vulnerabilities allow an attacker to bypass the IIS configuration that specifies which type of authentication is allowed, but not the file system-based access control list (ACL) check that verifies whether a file is accessible by a given user. Successful exploitation of these vulnerabilities would still restrict the attacker to the permissions granted to the anonymous user account by the file system ACLs.

This security update is rated Important for Microsoft Internet Information Services on all supported editions of Microsoft Windows 2000, Windows XP, and Windows Server 2003. For more information, see the subsection, Affected and Non-Affected Software, in this section.

The security update addresses the vulnerabilities by modifying the way that the WebDAV extension for IIS handles HTTP requests. For more information about the vulnerabilities, see the Frequently Asked Questions (FAQ) subsection for the specific vulnerability entry under the next section, Vulnerability Information.

This security update addresses the vulnerability described in Microsoft Security Advisory 971492.

Recommendation. The majority of customers have automatic updating enabled and will not need to take any action because this security update will be downloaded and installed automatically. Customers who have not enabled automatic updating need to check for updates and install this update manually. For information about specific configuration options in automatic updating, see Microsoft Knowledge Base Article 294871.

For administrators and enterprise installations, or end users who want to install this security update manually, Microsoft recommends that customers apply the update at the earliest opportunity using update management software, or by checking for updates using the Microsoft Update service.

See also the section, Detection and Deployment Tools and Guidance, later in this bulletin.

Known Issues. None
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Affected and Non-Affected Software

The following software have been tested to determine which versions or editions are affected. Other versions or editions are either past their support life cycle or are not affected. To determine the support life cycle for your software version or edition, visit Microsoft Support Lifecycle.

Affected Software
Operating System Component Maximum Security Impact Aggregate Severity Rating Bulletins Replaced by this Update

Microsoft Windows 2000 Service Pack 4

Microsoft Internet Information Services 5.0

Elevation of Privilege

Important

None

Windows XP Professional Service Pack 2 and Windows XP Professional Service Pack 3

Microsoft Internet Information Services 5.1

Elevation of Privilege

Important

None

Windows XP Professional x64 Edition Service Pack 2

Microsoft Internet Information Services 6.0

Elevation of Privilege

Important

None

Windows Server 2003 Service Pack 2

Microsoft Internet Information Services 6.0

Elevation of Privilege

Important

None

Windows Server 2003 x64 Edition Service Pack 2

Microsoft Internet Information Services 6.0

Elevation of Privilege

Important

None

Windows Server 2003 with SP2 for Itanium-based Systems

Microsoft Internet Information Services 6.0

Elevation of Privilege

Important

None

Non-Affected Software
Operating System Component

Windows Vista, Windows Vista Service Pack 1, and Windows Vista Service Pack 2

Microsoft Internet Information Services 7.0

Windows Vista x64 Edition, Windows Vista x64 Edition Service Pack 1, and Windows Vista x64 Edition Service Pack 2

Microsoft Internet Information Services 7.0

Windows Server 2008 for 32-bit Systems and Windows Server 2008 for 32-bit Systems Service Pack 2

Microsoft Internet Information Services 7.0

Windows Server 2008 for x64-based Systems and Windows Server 2008 for x64-based Systems Service Pack 2

Microsoft Internet Information Services 7.0

Windows Server 2008 for Itanium-based Systems and Windows Server 2008 for Itanium-based Systems Service Pack 2

Microsoft Internet Information Services 7.0
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Frequently Asked Questions (FAQ) Related to This Security Update

Where are the file information details?
Refer to the reference tables in the Security Update Deployment section for the location of the file information details.

Why does this update address several reported security vulnerabilities?
This update contains support for several vulnerabilities because the modifications that are required to address these issues are located in related files. Instead of having to install several updates that are almost the same, customers need to install this update only.

I am using an older release of the software discussed in this security bulletin. What should I do?
The affected software listed in this bulletin have been tested to determine which releases are affected. Other releases are past their support life cycle. To determine the support life cycle for your software release, visit Microsoft Support Lifecycle.

It should be a priority for customers who have older releases of the software to migrate to supported releases to prevent potential exposure to vulnerabilities. For more information about the Windows Product Lifecycle, visit Microsoft Support Lifecycle. For more information about the extended security update support period for these software versions or editions, visit Microsoft Product Support Services.

Customers who require custom support for older releases must contact their Microsoft account team representative, their Technical Account Manager, or the appropriate Microsoft partner representative for custom support options. Customers without an Alliance, Premier, or Authorized Contract can contact their local Microsoft sales office. For contact information, visit Microsoft Worldwide Information, select the country, and then click Go to see a list of telephone numbers. When you call, ask to speak with the local Premier Support sales manager. For more information, see the Windows Operating System Product Support Lifecycle FAQ.
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Vulnerability Information

Severity Ratings and Vulnerability Identifiers

The following severity ratings assume the potential maximum impact of the vulnerability. For information regarding the likelihood, within 30 days of this security bulletin's release, of the exploitability of the vulnerability in relation to its severity rating and security impact, please see the Exploitability Index in the June bulletin summary. For more information, see Microsoft Exploitability Index.
Vulnerability Severity Rating and Maximum Security Impact by Affected Software
Affected Software IIS 5.0 WebDAV Authentication Bypass Vulnerability - CVE-2009-1122 IIS 5.1 and 6.0 WebDAV Authentication Bypass Vulnerability - CVE-2009-1535 Aggregate Severity Rating

Microsoft Internet Information Services (IIS) 5.0 on Microsoft Windows 2000 Service Pack 4

Important
Elevation of Privilege

Not applicable

Important

Microsoft Internet Information Services (IIS) 5.1 on Windows XP Professional Service Pack 2 and Windows XP Professional Service Pack 3

Not applicable

Important
Elevation of Privilege

Important

Microsoft Internet Information Services (IIS) 6.0 on Windows XP Professional x64 Edition Service Pack 2

Not applicable

Important
Elevation of Privilege

Important

Microsoft Internet Information Services (IIS) 6.0 on Windows Server 2003 Service Pack 2

Not applicable

Important
Elevation of Privilege

Important

Microsoft Internet Information Services (IIS) 6.0 on Windows Server 2003 x64 Edition Service Pack 2

Not applicable

Important
Elevation of Privilege

Important

Microsoft Internet Information Services (IIS) 6.0 on Windows Server 2003 with SP2 for Itanium-based Systems

Not applicable

Important
Elevation of Privilege

Important
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IIS 5.0 WebDAV Authentication Bypass Vulnerability - CVE-2009-1122

An elevation of privilege vulnerability exists in the way that the WebDAV extension for IIS handles HTTP requests. An attacker could exploit this vulnerability by creating a specially crafted anonymous HTTP request to gain access to a location that should require authentication.

To view this vulnerability as a standard entry in the Common Vulnerabilities and Exposures list, see CVE-2009-1122.

Mitigating Factors for IIS 5.0 WebDAV Authentication Bypass Vulnerability - CVE-2009-1122

Mitigation refers to a setting, common configuration, or general best-practice, existing in a default state, that could reduce the severity of exploitation of a vulnerability. The following mitigating factors may be helpful in your situation:
•

Microsoft Internet Information Services versions 5.1, 6.0, and 7.0 are not affected by this vulnerability.
•

The anonymous user account is denied write access by default. In order to successfully exploit this vulnerability with write access, the anonymous user account would need to have write access ACLs set within the IIS folder structure. However, by default, the anonymous user account only has read access ACLs set. On IIS 6.0, there is an explicit deny access control entry (ACE) for write permissions for the default anonymous user account. Unless overridden by the administrator, this deny ACE will be inherited by all children under the default Web site root.
•

File system ACLs are enforced. This vulnerability allows for an attacker to bypass the IIS configuration that specifies which type of authentication is allowed, but not the file system-based ACL check that verifies whether a file is accessible by a given user. Successful exploitation of the vulnerability would still restrict the attacker to the permissions granted to the anonymous user account on the file system ACL level. Therefore this vulnerability cannot be used to exceed the level of access granted to the anonymous user account through file system ACLs. The default anonymous user account is configured as the IUSR_<computername> account.
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Workarounds for IIS 5.0 WebDAV Authentication Bypass Vulnerability - CVE-2009-1122

Workaround refers to a setting or configuration change that does not correct the underlying vulnerability but would help block known attack vectors before you apply the update. Microsoft has tested the following workarounds and states in the discussion whether a workaround reduces functionality:
•

Disable WebDAV

If WebDAV functionality is not required on the server, it can be disabled.

For IIS 5.0, WebDAV can be disabled as described in Microsoft Knowledge Base Article 241520.

Impact of workaround. After applying this workaround, WebDAV requests will not be served by IIS.
•

Alternate method to disable WebDAV on IIS 5.0

The IIS Lockdown Tool 2.1 can be used to disable WebDAV as well. See Microsoft Knowledge Base Article 325864.
•

To start the installation immediately, click Open or Run this program from its current location.
•

To copy the download to your computer for installation at a later time, click Save or Save this program to disk.

Impact of workaround. This method achieves its results by installing UrlScan. By default, UrlScan blocks requests to WebDAV by detecting either HTTP verbs or headers that would be mapped to WebDAV. If the IIS Lockdown tool is uninstalled, UrlScan will remain. To uninstall UrlScan, unregister the UrlScan ISAPI filter from the IIS Manager snap-in.
•

Change file system ACLs to deny access to the anonymous user account

If WebDAV functionality is required, NTFS ACLs should be used to control access to resources on the server. If resources should be inaccessible to the anonymous user through WebDAV, appropriate read or write NTFS ACLs should be applied based on desired access to that resource. For more information on setting ACLs for IIS content, see the following articles:
•

Microsoft Knowledge Base Article 271071
•

Microsoft Knowledge Base Article 812614

Impact of workaround. Applying this workaround may have adverse effects on products that require content to be accessed by the IUSR_<computername> account. Examples of these products include System Center Configuration Manager 2007.
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FAQ for IIS 5.0 WebDAV Authentication Bypass Vulnerability - CVE-2009-1122

What is the scope of the vulnerability?
This is an elevation of privilege vulnerability that could allow an attacker to bypass an authentication requirement to access content on an IIS Server.

What causes the vulnerability?
The WebDAV extension does not properly decode a specially crafted requested URL. This causes WebDAV to apply an incorrect configuration when handling the request. If the applied configuration allows anonymous access, a specially crafted request can bypass authentication.

Note that IIS would still process such a request in the security context of the configured anonymous user account. Therefore, this vulnerability cannot be used to bypass NTFS ACLs. The restrictions imposed on the anonymous user account by file system ACLs will still be enforced.

What is the anonymous user account?
When Web sites and applications running on the Web server require anonymous access, IIS must be configured with a user account specifically for anonymous access. This account will then be used to access content on the file system when requested by the anonymous Web user. In a default configuration, IIS uses the anonymous user account IUSR_<computername>, where <computername> is the name of the computer on which IIS is running. However, it is possible to change the anonymous user identity to use a different local user account or a domain user account as the anonymous user account.

What is WebDAV?
Web Distributed Authoring and Versioning (WebDAV) is an extension to the Hypertext Transfer Protocol (HTTP) that defines how basic file functions such as copy, move, delete and create are performed by a computer using HTTP.

What is an access control list (ACL)?
An access control list (ACL) is a list of security protections that applies to an object. (An object can be a file, process, event, or anything else having a security descriptor.) An entry in an ACL is an access control entry (ACE). There are two types of access control list, discretionary and system. Each ACE in an ACL identifies access rights allowed, denied, or audited for that trustee. For information, see the MSDN article, Access Control Lists.

What might an attacker use the vulnerability to do?
An attacker who successfully exploited this vulnerability would be able to bypass the IIS configuration that specifies which type of authentication is allowed, but not the file system-based ACL check that verifies whether a file is accessible by a given user. Successful exploitation of the vulnerability would restrict the attacker to the permissions granted to the anonymous user account on file system ACL level. This could allow an attacker to anonymously access information that should only be accessible to an authenticated user, or write files to locations where the anonymous user account is granted write permission.

How could an attacker exploit the vulnerability?
An attacker could exploit the vulnerability by creating a specially crafted HTTP request to a Web site that requires authentication, and thereby gain unauthorized access to protected resources.

What systems are primarily at risk from the vulnerability?
Microsoft Windows 2000 systems on which IIS version 5.0 is used and where WebDAV is enabled are primarily at risk. Typically these are Web servers or application servers, but they may also be developer or Web designer or other workstation systems on which IIS is used with WebDAV. Systems that deviate from the default configuration in that the anonymous user account has been granted write permission are at increased risk.

What does the update do?
The update addresses the vulnerability by modifying the way that the WebDAV extension for IIS handles HTTP requests.

When this security bulletin was issued, had this vulnerability been publicly disclosed?
No. Microsoft received information about this vulnerability through responsible disclosure. Microsoft had not received any information to indicate that this vulnerability had been publicly disclosed when this security bulletin was originally issued.

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.
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IIS 5.1 and 6.0 WebDAV Authentication Bypass Vulnerability - CVE-2009-1535

An elevation of privilege vulnerability exists in the way that the WebDAV extension for IIS handles HTTP requests. An attacker could exploit this vulnerability by creating a specially crafted anonymous HTTP request to gain access to a location that typically requires authentication.

To view this vulnerability as a standard entry in the Common Vulnerabilities and Exposures list, see CVE-2009-1535.

Mitigating Factors for IIS 5.1 and 6.0 WebDAV Authentication Bypass Vulnerability - CVE-2009-1535

Mitigation refers to a setting, common configuration, or general best-practice, existing in a default state, that could reduce the severity of exploitation of a vulnerability. The following mitigating factors may be helpful in your situation:
•

Microsoft Internet Information Server versions 5.0 and 7.0 are not affected by this vulnerability.
•

WebDAV is not enabled by default on IIS 6.0. On IIS 6.0 running on Windows Server 2003 systems, WebDAV is not enabled in the default configuration. Unless WebDAV has been enabled by an administrator on these systems, the vulnerability is not exposed.
•

The anonymous user account is denied write access by default. In order to successfully exploit this vulnerability with write access, the anonymous user account would need to have write access ACLs set within the IIS folder structure. However, by default, the anonymous user account only has read access ACLs set. On IIS 6.0, there is an explicit deny ACE for the default anonymous user account. Unless overridden by the administrator, this deny ACE will be inherited by all children under the default Web site root.
•

File system ACLs are enforced. This vulnerability allows and attacker to bypass the IIS configuration that specifies which type of authentication is allowed, but not the file system-based ACL check that verifies whether a file is accessible by a given user. Successful exploitation of the vulnerability would still restrict the attacker to the permissions granted to the anonymous user account at the file system ACL level. Therefore this vulnerability cannot be used to exceed the level of access granted to the anonymous user account through file system ACLs. The default anonymous user account is configured as the IUSR_<computername> account.
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Workarounds for IIS 5.1 and 6.0 WebDAV Authentication Bypass Vulnerability - CVE-2009-1535

Workaround refers to a setting or configuration change that does not correct the underlying vulnerability but would help block known attack vectors before you apply the update. Microsoft has tested the following workarounds and states in the discussion whether a workaround reduces functionality:
•

Disable WebDAV

If WebDAV functionality is not required on the server, it can be disabled.

For IIS 5.0 and IIS 5.1, WebDAV can be disabled as described in Microsoft Knowledge Base Article 241520.

For IIS 6.0, WebDAV can be disabled using the following steps:

Launch the IIS Manager MMC snap-in.

In the left-hand pane, expand the local computer icon.

Click on Web Service Extensions beneath this item.

In the right-hand pane, select WebDAV and click the Prohibit button.

Impact of workaround. After applying this workaround, WebDAV requests will not be served by IIS.
•

Alternate method to disable WebDAV on IIS 5.1 and IIS 6.0

Microsoft UrlScan Filter v3.1 can be used to disable WebDAV as well.

For IIS 5.1 and IIS 6.0, download Microsoft UrlScan Filter v3.1 from one of the following:
•

For 32-bit systems: Microsoft Urlscan Filter v3.1 (x86)
•

For x64-based systems: Microsoft Urlscan Filter v3.1 (x64)

Then do one of the following:
•

To start the installation immediately, click Open or Run this program from its current location.
•

To copy the download to your computer for installation at a later time, click Save or Save this program to disk.

Impact of workaround. This method achieves its results by installing UrlScan. By default, UrlScan blocks requests to WebDAV by detecting either HTTP verbs or headers that would be mapped to WebDAV.
•

Change file system ACLs to deny access to the anonymous user account

If WebDAV functionality is required, NTFS ACLs should be used to control access to resources on the server. If resources should be inaccessible to the anonymous user through WebDAV, appropriate read or write NTFS ACLs should be applied based on desired access to that resource. For more information on setting ACLs for IIS content, see the following articles:
•

Microsoft Knowledge Base Article 271071
•

Microsoft Knowledge Base Article 812614

Impact of workaround. Applying this workaround may have adverse effects on products that require content to be accessed by the IUSR_<computername> account. Examples of these products include System Center Configuration Manager 2007.
Top of sectionTop of section

FAQ for IIS 5.1 and 6.0 WebDAV Authentication Bypass Vulnerability - CVE-2009-1535

What is the scope of the vulnerability?
This is an elevation of privilege vulnerability that could allow an attacker to bypass an authentication requirement to access content on an IIS server.

What causes the vulnerability?
The WebDAV extension does not properly decode a specially crafted requested URL. This causes WebDAV to apply an incorrect configuration when handling the request. If the applied configuration allows anonymous access, a specially crafted request can bypass authentication.

Note that IIS would still process such a request in the security context of the configured anonymous user account. Therefore, this vulnerability cannot be used to bypass NTFS ACLs. The restrictions imposed on the anonymous user account by file system ACLs will still be enforced.

What is the anonymous user account?
When Web sites and applications running on the Web server require anonymous access, IIS must be configured with a user account specifically for anonymous access. This account will then be used to access content on the file system when requested by the anonymous Web user. In a default configuration, IIS uses the anonymous user account IUSR_<computername>, where <computername> is the name of the computer on which IIS is running. However, it is possible to change the anonymous user identity to use a different local user account or a domain user account as the anonymous user account.

What is WebDAV?
Web Distributed Authoring and Versioning (WebDAV) is an extension to the Hypertext Transfer Protocol (HTTP) that defines how basic file functions such as copy, move, delete and create are performed by a computer using HTTP.

What might an attacker use the vulnerability to do?
An attacker who successfully exploited this vulnerability would be able to bypass the IIS configuration that specifies which type of authentication is allowed, but not the file system-based ACL check that verifies whether a file is accessible by a given user. Successful exploitation of the vulnerability would restrict the attacker to the permissions granted to the anonymous user account on the file system ACL level. This could allow an attacker to anonymously access information that should only be accessible to an authenticated user, or write files to locations where the anonymous user account is granted write permission.

How could an attacker exploit the vulnerability?
An attacker could exploit the vulnerability by creating a specially crafted HTTP request to a Web site that requires authentication, and thereby gain unauthorized access to protected resources.

What systems are primarily at risk from the vulnerability?
Windows XP and Windows Server 2003 systems on which IIS version 5.1 or 6.0 is used and where WebDAV is enabled are primarily at risk. Typically these systems are Web servers or application servers, but they may also be developer or Web designer or other workstation systems on which IIS is used with WebDAV. Systems that deviate from the default configuration in that the anonymous user account has been granted write permission are at increased risk.

What does the update do?
The update addresses the vulnerability by modifying the way that the WebDAV extension for IIS handles HTTP requests.

When this security bulletin was issued, had this vulnerability been publicly disclosed?
Yes. This vulnerability has been publicly disclosed. This vulnerability was first described in Microsoft Security Advisory 971492 as Common Vulnerability and Exposure number CVE-2009-1535. This vulnerability has also been assigned Common Vulnerability and Exposure number CVE-2009-1676.

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.

Other Information
Acknowledgments

Microsoft thanks the following for working with us to help protect customers:
•

Yamata Li of Palo Alto Networks for reporting the IIS 5.0 WebDAV Authentication Bypass Vulnerability (CVE-2009-1122)
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To improve security protections for customers, Microsoft provides vulnerability information to major security software providers in advance of each monthly security update release. Security software providers can then use this vulnerability information to provide updated protections to customers via their security software or devices, such as antivirus, network-based intrusion detection systems, or host-based intrusion prevention systems. To determine whether active protections are available from security software providers, please visit the active protections Web sites provided by program partners, listed in Microsoft Active Protections Program (MAPP) Partners.

Support
•

Customers in the U.S. and Canada can receive technical support from Security Support or 1-866-PCSAFETY. There is no charge for support calls that are associated with security updates. For more information about available support options, see Microsoft Help and Support.
•

International customers can receive support from their local Microsoft subsidiaries. There is no charge for support that is associated with security updates. For more information about how to contact Microsoft for support issues, visit the International Support Web site.

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Revisions
•

V1.0 (June 9, 2009): Bulletin published.