VeriSign said many major Web sites were temporarily blocked by a mysterious flood of bogus messages in January and February, raising questions about the Internet’s defenses against one potent form of attack. Though the attacks have abated, VeriSign said it may need to sharply increase spending on additional computing and communications capacity to keep operating in future onslaughts. The claims come following the company’s proposed settlement with ICANN, still subject to Department of Commerce approval, that would allow it to raise the price of dot-com domain registrations.