Wall Street’s top regulator faced questions on Thursday about its defenses against cyber criminals after admitting hackers breached its electronic database of corporate announcements and may have used it for insider trading. The incursion at the Securities and Exchange Commission struck at the heart of the U.S. financial system. The SEC’s EDGAR filing system is the central repository for market-moving information on corporate America with millions of filings ranging from quarterly earnings to statements on acquisitions. Accessing documents before they are released publicly would offer hackers a lucrative opportunity to trade on that information. The SEC has said it was...