The attack, which resulted in the loss of intellectual property belonging to Google and perhaps to other companies, leveraged an Internet Explorer vulnerability.
Mike Reavy, Microsoft's director of security response, said on Thursday that the Internet Explorer flaw was "one of several attack mechanisms that were used."
The warning comes at a bad time for Microsoft, which has been hoping that Windows 7 adoption will reverse Internet Explorer's ongoing loss of market share. According to NetApplications, Internet Explorer's global market share declined 11 out of 12 months in 2009.
Asked about the French and German recommendations, a Microsoft spokesperson provided the following statement: "In regards to the recent Internet Explorer vulnerability, we have not seen successful attacks on Internet Explorer 8. As such, Microsoft continues to recommend customers upgrade to Internet Explorer 8 to benefit from its improved security protections."
The company also said that it had not seen successful attacks on Internet Explorer 7. But it warned that there have been reports of proof-of-concept code that exploits the vulnerability in Internet Explorer 7 on Windows XP and Vista. Microsoft said it was investigating these claims.
McAfee on Friday said that it had seen exploit code published on mailing lists and at least one Web site.
Websense, a computer security company, on Monday confirmed the Internet Explorer 7 is vulnerable in its default configuration while Internet Explorer 8 is not. Due to the fact that the vulnerability can be used in a drive-by download attack -- an attack triggered by visiting a malicious Web site or opening a specially-crafted e-mail message -- the company predicts that it will be exploited on a large scale.