"The versions...for Windows XP and for Windows Server 2003 contained only the latest six digital certificates cross-signed by GTE and Entrust," said Microsoft in a . "These versions of the update did not contain the digital certificates that were included in [earlier updates]."

The earlier update, delivered by Microsoft on Sept. 6, blocked five DigiNotar root certificates.

"If you installed update 2616676 and had not already installed update 2607712 or update 2524375, your system would not have been protected from the use of fraudulent digital certificates," Microsoft admitted.

The re-released update for XP and Server 2003 has been added to Windows Update, Microsoft said. Customers who do not have Automatic Updates enabled should manually download and install the new version of the DigiNotar blocker.

Windows Vista, Windows 7, Server 2008 and Server 2008 R2 were not affected by the update goof, said Microsoft.