Personal friends of employees at Google, Adobe and other high -technology companies were targeted by hackers in a string of recently disclosed cyber attacks, raising privacy concerns and pointing to a highly sophisticated operation, security experts said.

Cybersecurity experts analysing the attacks said the hackers spied on individuals and used other sophisticated techniques making them extremely difficult to stop. The disclosures come amid renewed alarm over cybersecurity after Google said it had been the target of a series of cyberattacks from China.

The most significant new discovery is that the attackers had selected employees at the companies with access to proprietary data, then learned who their friends were. The hackers compromised the social network accounts of those friends, hoping to enhance the probability that their final targets would click on the links they sent.