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Microsoft Security Advisory (906574)

Clarification of Simple File Sharing and ForceGuest


Microsoft has issued this Security Advisory to clarify information of the issue addressed in Security Bulletin MS05-039 for non-default configurations of Windows XP Service Pack 1. This feature is known as “Simple File Sharing and ForceGuest.” If you are using Windows XP Service Pack 2, enabling Simple File Sharing and ForceGuest does not increase your level of exposure to the MS05-039 security vulnerability. Also, customers that have applied the security update included with MS05-039 are not impacted by this issue. We recommend that customers continue to follow our Protect Your PC guidance of enabling a firewall, getting software updates and installing anti-virus software. Customers can learn more about these steps by visiting the Protect Your PC Web site.

If Simple File Sharing is enabled on a Microsoft Windows XP system that is not joined to a domain, then all users who access this system through the network are forced to use the Guest account. This is the “Network access: Sharing and security model for local accounts security policy setting, and is also known as ForceGuest.

Windows XP mitigates several security vulnerabilities by preventing users who do not have a valid logon credential from accessing the system remotely. An example of this is the vulnerability that is addressed in Microsoft Security Bulletin MS05-039. However, when you enable Simple File Sharing, the Guest account is also enabled and given permission to access the system through the network. Because the Guest account is a valid account when it is enabled, and is given permission to access the system through the network, an attacker could use the Guest account as if they had a valid user account.

There is no known attack that is seeking to exploit this scenario. The Advisory is being issued as a special precaution. There is no change to the update in Security Bulletin MS05-039. Customers who have applied this update are protected in this scenario.

Mitigating Factors:

  • Windows XP Service Pack 2 is not vulnerable remotely to the issue addressed by MS05-039 even when Simple File Sharing enables the Guest account. On Windows XP Service Pack 2, the impact of this vulnerability is only Local Privilege Elevation, and only exploitable if a user has the ability to logon locally to the system.
  • Simple File Sharing is not available on Windows XP systems that are joined to a domain. Domain-joined systems use standard file sharing which does not enable the Guest account or give it permissions to access the system through the network. Windows XP Service Pack 2 is not vulnerable remotely in domain-joined systems or in workgroup-joined systems.
  • Enabling Simple File Sharing does not expose customers who have applied the security updates provided by Microsoft Security Bulletin MS05-039 to the vulnerability that is addressed by that security bulletin.

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The information provided in this advisory is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.


  • August 23, 2005: Advisory published