Iranians Compromised a Highly Sensitive CIA Covert Communications System in 2011 by Using Google Search: Report (yahoo.com)
In 2011, Iran was able to use Google's search functionality to hack into a secret CIA communication network that was being used to contact agents and informants around the world -- a breach that appears to have triggered the exposure and execution of Agency sources in China and Iran, Yahoo News reported Friday.
Most of the methods Iranians used would have been familiar to George Smiley. They looked at what the Americans obviously knew about Iran and figured out who could have told them. Then they leaned on those people and found out how they were communicating with the CIA.
This is where Google came in. These people were using phony websites to communicate with the CIA, and Iranian intelligence used Google to uncover similar websites. Then they hacked into those websites after which they had the keys to the kingdom.
It was the CIA's reliance on a bodged-together, vulnerable system that killed those assets. They used it even after they'd been warned by their own analysts in 2008 that it had been compromised.
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This is a really long article that can be summarized in about two paragraphs:
Well, plus one more very important paragraph:
They were told there was a problem. They ignored it, and fired the person who told them.