Chinese espionage hackers behind an eight-month campaign to hack Barracuda email security appliances intensified their focus on high-priority targets around the time the company moved to fix the zero-day flaw behind the campaign.

Within roughly a week of Barracuda’s late-May public disclosures of the zero-day flaw affording Chinese hackers access to its ESG line of products, the threat actor behind the hacking spree deployed an additional backdoor to select a sliver of targets, mainly U.S. and foreign government agencies and high-tech companies, said researchers from Mandiant.