Visitors to Wikipedia who see advertisements on the site have most likely fallen victim to a browser-based malware infection, Wikimedia Foundation, the organization operating the website, said on Monday.

"We never run ads on Wikipedia," said Philippe Beaudette, director of community advocacy for the Wikimedia Foundation, in a blog post. "If you're seeing advertisements for a for-profit industry ... or anything but our fundraiser, then your web browser has likely been infected with malware."

One example of such malware is a rogue Google Chrome extension called "I want this," Beaudette said. However, similar malicious add-ons might also exist for Mozilla Firefox, Internet Explorer and other browsers, he said.

This type of malicious software is known as click fraud malware and can target multiple websites at once. In addition to injecting ads into Web pages, such rogue extensions are also known to hijack search queries in order to earn their creators affiliate revenue, said Graham Cluley, a senior technology consultant at Sophos, in a blog post Tuesday.

Spotting this type of rogue behavior on Wikipedia is easier than on other websites because the site doesn't run any commercial advertisements. "We're here to distribute the sum of human knowledge to everyone on the planet -- ad-free, forever," Beaudette said.

Wikipedia's operating costs are covered by donations. An online fundraiser is organized every year, and that's usually the only time a banner is displayed on the site's pages.

Users who are seeing commercial ads on Wikipedia should disable all their browser add-ons to determine if they are the source of the problem, Beaudette said.

Even if this makes the ads disappear, however, it is not necessarily a permanent solution and does not fix the underlying issue. The malware might have other components running on the system that could reinfect the browser.

"Run an up-to-date anti-virus to make sure that whatever might have introduced the unwanted ads isn't also up to other malicious behavior behind the scenes," Cluley wrote.

If disabling the browser add-ons and running an antivirus scan does not solve the issue, it's likely that the ads are automatically being injected into Web sessions at the network level. Some Internet caf é s and free Wi-Fi providers are in the habit of doing this, Beaudette said.

Visiting the affected website over HTTPS (HTTP Secure) might sometimes remove the rogue ads, because HTTPS sessions are encrypted. There are browser extensions like the Electronic Frontier Foundation's HTTPS Everywhere that enable HTTPS by default on websites that support it.