The U.S. Securities and Exchange Commission (SEC) is examining the exposure of stock exchanges, brokerages, and other Wall Street firms to cyber attacks that have been called a threat to financial stability.

The SEC is holding a roundtable discussion of those risks in Washington today as it weighs a new rule proposal asking whether stock exchanges should be required to tell members about breaches of critical systems. More than half of exchanges surveyed globally in 2012 said they experienced a cyber attack, while 67 percent of U.S. exchanges said a hacker tried to penetrate their systems.

The agency also will probe how companies are disclosing cyber threats to investors in public filings. Businesses including Target Corp., from which hackers stole payment-card data for millions of shoppers in December, are required to disclose such threats when the information would affect an investor's willingness to own the company's shares.