Salt Typhoon's breach compromises U.S. telecom giants, including T-Mobile, AT&T, and Verizon. The hack exposes vulnerabilities in U.S. wiretapping systems, threatening national security and potentially the data of millions of Americans. T-Mobile claims limited damage, but the full scope of the breach is still unclear.

The Chinese government espionage campaign that has deeply penetrated more than a dozen U.S. telecommunications companies is the “worst telecom hack in our nation’s history — by far,” a senior U.S. senator told The Washington Post in an interview this week.

All the major U.S. carriers, including AT&T, Verizon, and T-Mobile, were impacted, according to the Post. Incredibly, Warner says the hackers are still inside the U.S. system and there’s no obvious way to get them out that doesn’t involve physically replacing old equipment, according to Warner.

In an interview with The Washington Post, the Virginia Democrat and Senate Intelligence chairman provided more disturbing details about the cyber espionage effort, which was allegedly carried out by a Chinese hacking group known as Salt Typhoon.