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I am unable to access the local machine through a CNAME alias and receive an "Access denied" error after upgrading to Service Pack 1 of Windows Server 2003.

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This is caused by the "loopback check" security enhancement introduced in Service Pack 1 for Windows Server 2003.

This security feature prevents access to the local server through its fully qualified domain name or CNAME alias, and is enabled by default.

This feature can be disabled by setting the following registry DWORD to 1:


A server reboot will be required for this change to take effect.

Note that disabling this feature will expose your server to man-in-the-middle NTLM attacks, which this feature is intended to prevent.

More information about this problem is available from MSDN KB926642.

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