Peiter Zatko is testifying on the company’s various security shortcomings.

Last month, former hacker and cybersecurity expert Peiter “Mudge” Zatko filed a whistleblower complaint to security agencies and US Congress about his former employer, Twitter. Zatko claims that, during his period of employment from 2020 to this year, he witnessed “extreme, egregious deficiencies by Twitter in every area of his mandate,” including unreported security breaches, out-of-date software and hardware, and regular targeting by foreign intelligence agencies.

Today, Zatko has officialyl begun his testimony to Congress about what, precisely, went on during his time at Twitter. “Mr. Zatko’s allegations of widespread security failures and foreign state actor interference at Twitter raise serious concerns,” said Senators Dick Durbin of Illinois and Chuck Grassley of Iowa in a statement. “If these claims are accurate, they may show dangerous data privacy and security risks for Twitter users around the world.”

Twitter has raised numerous concerns in recent years as a platform for both disinformation campaigns and radicalization of ideas. If their security is as lax as Zatko claims, it could be a major blow to the platform’s credibility. “Twitter has an outsized impact on global politics and events, and it even tried to reposition itself as a news app several years ago,” Jasmine Enber, a principal analyst at market research firm Insider Intelligence, told The Guardian. “The complaint has already caught the eye of regulators, and [Zatko’s] testimony could add fresh fuel to the fire.”