Google shows “This site may harm your Computer” warning for malicious sites for users in their search results which you may know, today Google added another label “This site may be compromised” which appears for hacked or compromised sites. If you see this warning you can stop visiting that site or proceed to it at your own risk.
Clicking on “ This site may be compromised “ link takes you to a article in Google help Center which explains more information about the notice. Users can contact that hacked site webmaster to notify the same so that he can investigate and clean up the site if it’s hacked. Meanwhile Google also tries to contact the site’s webmaster via his webmaster tools account and if any email addresses found on webpage.
Google uses various automated tools to detect hacked sites, when something suspicious is detected Google adds the above notification to their results. This label may impacts traffic to the hacked site, but once the owner fixes his site, this warning label will automatically disappears from Google search results. Affected webmaster can request a review to speed up the removal of notice.
Source:Google Webmaster Central Blog
Update: In facts it’s old news, we’ve covered it as Matt Cutts talked about it on Google+.