Sam Bankman-Fried has on two occasions used a virtual private network (VPN) to mask his web connection while under house arrest, federal prosecutors have said.

In a letter to Judge Lewis Kaplan, who's overseeing Bankman-Fried's criminal case, US attorney Damian Williams says the government "informed defense counsel and raised concerns" about the defendant's use of a VPN on January 29 and February 12.

A VPN is software that masks a user's web-browsing activity and location.