Microsoft Security Advisory (911052)

Microsoft is aware of public reports of proof-of-concept code that seeks to exploit a possible vulnerability in Microsoft Windows 2000 Service Pack 4 and in Microsoft Windows XP Service Pack 1. This vulnerability could allow an attacker to levy a denial of service attack of limited duration.

On Windows XP Service Pack 1, an attacker must have valid logon credentials to try 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. Customers who have installed Windows XP Service Pack 2 are not affected by this vulnerability. Additionally, customers running Windows Server 2003 and Windows Server 2003 Service Pack 1 are not affected by this vulnerability.

Microsoft is not aware of active attacks that use this vulnerability or of customer impact at this time. However, Microsoft is actively monitoring this situation to keep customers informed and to provide customer guidance as necessary.

Microsoft is concerned that this new report of a vulnerability in Windows 2000 Service Pack 4 and Windows XP Service Pack 1 was not disclosed responsibly, potentially putting computer users at risk. We continue to encourage responsible disclosure of vulnerabilities. We believe the commonly accepted practice of reporting vulnerabilities directly to a vendor serves everyone's best interests. This practice helps to ensure that customers receive comprehensive, high-quality updates for security vulnerabilities without exposure to malicious attackers while the update is being developed.

Mitigating Factors:

• On Windows XP Service Pack 1 an attacker must have valid logon credentials to try 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 Microsoft Security Advisory 906574.

• Customers who are running Windows XP Service Pack, Windows Server 2003 and Windows Server 2003 Service Pack 1 are not affected by this vulnerability.

• 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.

Workarounds

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.

• To help protect against anonymous network-based connection attempts to exploit this vulnerability, configure the RestrictAnonymous registry setting to a more restrictive setting:

By default on Windows 2000, the RestrictAnonymous entry is set to a value of 0, which does not restrict Anonymous users. By setting the registry entry to a value of 2, Anonymous users will have no access without explicit anonymous permissions. For more information about how to use the RestrictAnonymous registry entry in Windows 2000, see Microsoft Knowledge Base Article 246261.

Block the following at the firewall:

• UDP ports 135, 137, 138, and 445, and TCP ports 135, 139, 445, and 593
• All unsolicited inbound traffic on ports greater than 1024
• Any other specifically configured RPC port
• If installed, COM Internet Services (CIS) or RPC over HTTP, which listen on ports 80 and 443

To help protect from network-based attempts to exploit this vulnerability, use a personal firewall, such as the Internet Connection Firewall, which is included with Windows XP Service Pack 1.

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

To help protect from network-based attempts to exploit this vulnerability, block the affected ports by using IPsec on the affected systems.
http://www.microsoft.com/technet/security/advisory/911052.mspx