U.S. officials are racing to understand the full scope of a China-linked hack of major U.S. broadband providers, as concerns mount from members of Congress that the breach could amount to a devastating counterintelligence failure. From a report: Federal authorities and cybersecurity investigators are probing the breaches of Verizon Communications, AT&T and Lumen Technologies. A stealthy hacking group known as Salt Typhoon tied to Chinese intelligence is believed to be responsible. The compromises may have allowed hackers to access information from systems the federal government uses for court-authorized network wiretapping requests, The Wall Street Journal reported last week. Among the concerns are that the hackers may have essentially been able to spy on the U.S. government's efforts to surveil Chinese threats, including the FBI's investigations. The House Select Committee on China sent letters Thursday asking the three companies to describe when they became aware of the breaches and what measures they are taking to protect their wiretap systems from attack. Spokespeople for AT&T, Lumen and Verizon declined to comment on the attack. A spokesman at the Chinese Embassy in Washington has denied that Beijing is responsible for the alleged breaches. Combined with other Chinese cyber threats, news of the Salt Typhoon assault makes clear that "we face a cyber-adversary the likes of which we have never confronted before," Rep. John Moolenaar, the Republican chairman of the House Select Committee Committee on China, and Raja Krishnamoorthi, the panel's top Democrat, said in the letters. "The implications of any breach of this nature would be difficult to overstate," they said. Hackers still had access to some parts of U.S. broadband networks within the last week, and more companies were being notified that their networks had been breached, people familiar with the matter said. Investigators remain in the dark about precisely what the hackers were seeking to do, according to people familiar with the response.

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