Security researchers say hackers compromised popular computer-cleaning software as part of a sophisticated attack that targeted several high-profile technology companies.

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The company that sells CCleaner software, Prague-based Avast Software, says the attack affected more than 2 million computers in late August and early September.

San Jose-based Cisco Systems and its Talos cybersecurity group discovered and disclosed the breach after hackers targeted Cisco’s own internal domain, as well as domains of Microsoft, Google, Samsung and other tech companies.

Cisco cybersecurity expert Craig Williams says attackers used an “advanced reconnaissance system” on hundreds of thousands of PCs with the aim of penetrating the tech companies’ domains and extracting valuable intellectual property.

Cisco says at least 20 targeted machines were infected with the secondary malware. It hasn’t disclosed which companies were affected.