About 20,000 websites may have been hacked, Google announced this week. The Google Search Quality team - made up of employees with a cyber crime degree or online cyber crime degree - sent out the warnings to the operators of websites it believes are affected.

"Specifically, we think that JavaScript has been injected into your site by a third party and may be used to redirect users to malicious sites," the warning said, adding that server configuration files also may have been compromised. "As a result of this, your site may be cloaking and showing the malicious content only in certain situations."

Operators of the affected websites must be sure to remove the malware, fix the vulnerability, keep software up to date and contact web hosts for thorough technical support, Google said. If these fixes aren't made, visitors likely will stay away, driving down traffic and harming a website's reputation.

Paul Roberts, editor of Kaspersky Lab's Threat Post blog, said these malicious websites often use search engine optimization to evade Google's filters to appear on search results. He added that because of those tactics, Google is only able to flag half of all malicious websites.

Google spokesman Mark Jansen said the warning was part of the company's efforts to be transparent with its webmasters to cut down on spam. In July, the tech giant blocked more than 11 million URLs with the ending "co.cc" because the domain name is typically used by cyber criminals to spread antivirus programs and commit sneak attacks. Some bulk providers at the time could host more than 50,000 malware domains, which could affect a large number of internet users.