Microsoft has re-released an update for Windows XP to correct a snafu that left users vulnerable to potential "man-in-the-middle" attacks for most of last week.
Monday's update addressed a gaffe introduced last week when Microsoft blocked six additional root certificates issued by DigiNotar that were cross-signed by a pair of other certificate authorities (CAs) - having already leaked the patch information four days previously.
Servers run by Dutch CA DigiNotar were hacked starting in June, and attackers stole over 500 SSL (secure socket layer) certificates, including many used by the Dutch government.
SSL certificates are used by websites and browsers to identify a site as legitimate - that gmail.com or hotmail.com are actually what they claim - and illegally-obtained certificates can be abused to disguise unauthorised domains using "man-in-the-middle" attacks to snoop on digital communications and harvest account credentials.
One certificate stolen from DigiNotar was used to spy on 300,000 Iranians for about a month this summer.
Microsoft has now admitted that the update it shipped to Windows XP and Server 2003 users last Tuesday was flawed.
"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 revised support document . "These versions of the update did not contain the digital certificates that were included in earlier updates."
The earlier update, delivered by Microsoft on September 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.
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