Senator Ron Wyden, of Oregon said the hack “appears to be significant”, including through the compromise of dozens of email accounts and access to the departmental offices division of the Treasury Department, which was home to its highest-ranking officials.
Wyden, the most senior Democrat on Senate Finance Committee, said that, "Treasury still does not know all of the actions taken by hackers, or precisely what information was stolen."
The US Treasury was among the earliest known agencies to have been affected in the massive breach that now encompasses a broad spectrum of US departments.
The hackers seized upon a Microsoft flaw to infiltrate the email system used by the Treasury Department’s senior leadership, The New York Times reported.
A Wyden aide said the hackers were able to access the Treasury officials' Microsoft-hosted inboxes after taking control of the cryptographic key used by Treasury's "single sign on" infrastructure - a service used in many organizations so that employees can access a variety of services with a single username and password, US media reported.
The senator said the breach began in July, but experts believe the overall hacking operation began months earlier.
The Treasury Department learned of the breach not from any of the government agencies whose job is to protect against cyber-attacks, but from Microsoft, which runs much of Treasury’s communications software, Wyden noted.
The US government and cyber-security experts are still struggling to find out the scope and consequences of the cyber-attack, which began when hackers subverted the Texas-based software company SolarWinds Corp and used the company as a springboard to jump deep into government and corporate networks.
SolarWinds Corp said up to 18,000 of its customers had downloaded a compromised software update that allowed hackers to spy unnoticed on businesses and agencies for almost nine months.
The Department of Homeland Security’s cyber-security arm said in a statement that the intrusion posed a “grave” risk to the US government and private networks.
US media have blamed Russia for the alleged espionage operation, while President Donald Trump downplayed the breach and raised the possibility that China might be involved. Top US officials - including Secretary of State Mike Pompeo - have blamed Russia, a charge the Kremlin strongly denies.