Microsoft failed to update its servers with a basic patch and was among the casualties in the weekend's SQL Slammer debacle.
The worm only hits SQL servers not fixed with a patch released by Microsoft in July.
According to emails obtained by Associated Press, some units within the software giant failed to install the patch.
A red-faced spokesman declined to say which areas or how many computers at Microsoft were affected.
But he admitted to Associated Press that some servers were left unfixed because administrators "didn't get around to it when they should have".
Other victims of the worm included Bank of America and Continental Airlines, both of which suffered network slowdowns as a result.
In 2000, Microsoft was one of the victims of the 'I Love You' virus which exploited a known flaw in its Outlook email program.
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