A. Most big brokerage firms such as Fidelity, Vanguard, and Schwab cover 100% of any losses in your brokerage accounts due to unauthorized activities.
However, if you share your user name, password or answers to security questions with a person or a service, such as a financial adviser or an aggregator site (e.g. Mint.com), you may have a tougher time to get the brokerage firm to pay your claim if money is stolen through a data breach at a third party.