The U.S. Securities and Exchange Commission hails its database of company filings as an innovation that has dramatically boosted corporate transparency. But a hack that led to the theft of market-moving secrets is the latest sign that the technology also brings dangers the SEC is struggling to combat.

The breach adds to a growing list of SEC embarrassments over Edgar, a massive online system where companies are required to disclose everything from stock sales by top executives to regulatory investigations. Past setbacks include fraudsters posting fake takeover announcements and allegations that some traders were getting access to market-moving news before others.

The cyberattack that occurred last year—but wasn't disclosed until Wednesday—could be the most problematic incident, because it casts doubt on the SEC's ability to safeguard data that fuels billions of dollars in daily financial transactions. The regulator was already grappling with hackers infiltrating companies to profit from insider trading, and now it turns out its own systems are a target.