The Wikimedia Foundation is warning its millions of visitors that if they're seeing ads appearing on any of the Foundation's Web sites, then their computer is probably infected with malware.

The Foundation issued a statement on Monday clarifying that it never runs ads on the Web site for Wikipedia, the massive, crowd-sourced encyclopedia. Visitors who are seeing ads for for-profit firms have likely been the victim of a Web based attack, including malicious browser plug-ins, the statement reads.

A variety of malicious browser extensions for Chrome, Firefox and Internet Explorer are responsible for the ads, Wikimedia Foundation Director of Community Advocacy Philippe Beaudette explained. Alternatively, users may be accessing the Web by way of a free Internet connection that injects the ads into the Web pages the viewer loads. "But rest assured: you won't be seeing legitimate advertisements on Wikipedia."

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