The Russia-linked hackers behind last year’s compromise of a wide swath of the U.S. government and scores of private companies, including SolarWinds Corp. , have stepped up their attacks in recent months, breaking into technology companies in an effort to steal sensitive information, cybersecurity experts said.

In a campaign that dates back to May of this year, the hackers have targeted more than 140 technology companies including those that manage or resell cloud-computing services, according to new research from Microsoft Corp. The attack, which was successful with as many as 14 of these technology companies, involved unsophisticated techniques like phishing or simply guessing user passwords in hopes of gaining access to systems, Microsoft said.