If Russia lashes out at the United States over Ukraine-related sanctions by mounting aggressive cyberattacks, the most likely targets would be the financial sector, defense contractors, and unsuspecting but vulnerable small businesses, cybersecurity experts say.

President Joe Biden warned Monday of significant Russian cyberattacks against the U.S., calling for American companies to be on high alert due to new intelligence concerning the Russian government’s alleged digital attack plans. Biden urged companies to “harden your cyber defenses immediately.”