Mr Zatko was giving evidence following an 84-page long whistleblowing complaint he made about security practices inside the social network.
He was fired by the firm in January.
He also said “one-time fines” imposed by regulators over breaches of rules on data protection “didn’t bother Twitter at all”.
In his damning testimony, Mr Zatko described an organisation prioritising revenue generation above everything else.
At the start of the hearing he grew tearful about his role as a whistleblower, saying it was not a decision he had taken lightly.
“I’m risking my career and reputation… if something good comes out of it five or ten years down the line, it will be worth it,” he said later on.
He also said he still thought Twitter offered a good service but laughed when asked whether he would buy it – a wry nod to the saga of Elon Musk’s deal.
During his questioning, Mr Zatko said that employees had expressed concerns to him that Twitter was carrying advertising from “organisations which may or may not be associated with the Chinese government”, a potential national security risk.
When he raised concerns with Twitter executives he was told it would be “problematic” to lose that revenue stream, he said.
He also said he was troubled by Twitter’s attitude to other national security issues he had raised. He said “half the company” were engineers and they all had access to users’ personal information.
It is believed around 4,000 employees had access to this data. He said he was worried that rogue employees had the power to take information without leaving a trace.
He added that there was a danger that employees could “dox” users, where private information is posted online, though he had not seen this happen.
He said Twitter does not log the activity of employees who access private data – which surprised him.
He also said that Twitter’s security systems made it difficult to monitor potential espionage. In a previous statement Mr Zatko said that an Indian agent had been employed by the company .
“The company did not in fact disclose to users that it was believed by the executive team that the Indian government had succeeded in placing agents on the company payroll,” Mr Zatko said last month.