As recently as last month, Marketwired employees were being sent phishing emails with links to malware, he said.
Yampolskiy suggested the breaches were a result of lax security at the newswire services. An outside scan of one of the services on Wednesday found multiple points of entry for a prospective attacker, he said.
The hackers "definitely knew what they were doing," Yampolskiy said. But compromising the services' networks "over multiple years seems ridiculously egregious to me. There's no defense or excuse for not being able to detect it."
PR Newswire, in a statement, said it takes security seriously. "As cybersecurity threats continue to evolve, so will our Information Security practices," CEO Robert Gray said in a statement.
Marketwired also places a high priority on protecting customer information, said Jason Maloni, the company's senior vice president.
"We found and fixed the issue at the heart of this matter and we are confident that Marketwired is protected by world-class security, monitoring and prevention practices," he said by email.
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