The company's assertion came after a of more than 500 SSL (secure socket layer) certificates, including several that could be used to impersonate Microsoft's update services, was revealed by Dutch authorities and several other affected developers.

"Attackers are not able to leverage a fraudulent Windows Update certificate to install malware via the Windows Update servers," said Jonathan Ness, an engineer with the Microsoft Security Response Center (MSRC), in a post. "The Windows Update client will only install binary payloads signed by the actual Microsoft root certificate, which is issued and secured by Microsoft."

Seven of the 531 certificates now known to have been fraudulently obtained by hackers in July were for the domains update.microsoft.com and windowsupdate.com, while another six were for *.microsoft.com.

According to Microsoft, the certificates issued for windowsupdate.com couldn't be used by attackers because the company no longer uses that domain. (Windows Update is now at windowsupdate.microsoft.com..) However, those for update.microsoft.com -- the domain for Microsoft Update -- and the wildcard *.microsoft.com could be.

As Ness said, updates delivered via Microsoft's services are signed with a separate certificate that's closely held by the company.