"We don't expect this trend to slow down. As hackers get more aggressive and more sites become outdated, hackers will continue to capitalize by infecting more sites," Google said in a post on its webmaster blog.
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Google, which inserts security warnings when it detects hacked sites, said most of those warned can clean up their pages, but that 61 per cent are not notified because their sites are not verified by the search engine.
"As always, it's best to take a preventative approach and secure your site rather than dealing with the aftermath," the blog said. "Remember a chain is only as strong as its weakest link."
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The news comes amid growing concerns over cybersecurity in the wake of massive hacks affecting Yahoo, the US government and major e-commerce firms.
Google said certain website hacks often follow similar patterns -- some insert "gibberish" on a page, while others create Japanese text that links to fake brand merchandise sites.
"Hacking behavior is constantly evolving, and research allows us to stay up to date on and combat the latest trends," Google said.
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