In the last several weeks Google has begun to show “This site may be compromised.” warnings, for websites they “believe may have been hacked or otherwise compromised”, in their search results. According to Google’s article about of the warning they have been added “To protect the safety of our users” and they recommend users “should be careful about providing personal information to the site” being flagged.
In the past when Google has detected websites they believe to be hacked and violate their Webmaster Guidelines, they have removed the websites from their index and placed a “Notice of Suspected Hacking” message in their Webmaster Tools to let the webmaster know. It’s unclear at this point if Google has replaced doing that with the new warning or if the warning is only for websites that have been hacked in such a way that does not warrant being removed for their search index. Unlike the malware warning (“This site may harm your computer.”) Google places in their search results, which sends users to an interstitial page when they click search result for an affected website, users are still able to directly access the website.
For websites which display the warning, after the hack has been removed reconsideration needs to be requested from Google to have the warning message removed. According to a post by Google employee John Mueller “These requests are processed fairly quickly (usually within a day, though it’s not possible to give an exact timeframe). “