US regulators contacted banks about possible system vulnerabilities after last week's $6.3 billion Binance hack, according to sources familiar with the matter. Several sources said Federal Reserve and Office of the Comptroller of the Currency supervisors began asking banks on Monday morning about unusual activity or service disruptions. Two other people familiar with banks' efforts said banks are examining the attack and testing defenses against similar attacks. A source at one contacted bank said the bank experienced no service disruptions. Binance said the company did not know whether the hack affected US banks, while the Fed and OCC declined to comment.