MainStreet Bank reports vendor cyber incident that leaked customer info. In regulatory filings with the Securities and Exchange Commission, MainStreet Bank’s holding company said a cyber incident connected to a third-party vendor had exposed information. MainStreet Bank said a cyberattack affecting one of its vendors exposed the sensitive information of about 5% of its customers. In regulatory filings with the Securities and Exchange Commission (SEC) on Friday afternoon, MainStreet Bancshares said it was informed in March that the vendor was compromised.