Protecting computer users from online frauds, such as phishing and pharming. A client computer may include a page signature extractor and a policy enforcer. The page signature extractor may encode a web page to generate its signature, which may be provided to a remote server computer for comparison with signatures of phishing pages. The client computer and the server computer may communicate using the DNS protocol. The policy enforcer may perform one or more predetermined actions when a match is found. Examples of such actions include replacing the web page with a blocking page, displaying a warning message, or both. The policy enforcer may be configured to determine if the web page is part of a phishing or pharming attack by comparing the URL of the web page to URLs of legitimate web pages.