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Existing Web browsers handle security errors in a manner that often confuses users. In particular, when a user visits a secure site whose certificate the browser cannot verify, the browser typically allows the user to view and install the certificate and connect to the site despite the verification failure. However, few users understand the risk of man-in-the-middle attacks and the principles behind certificate-based authentication. We propose context-sensitive certificate verification (CSCV), whereby the browser interrogates the user about the context in which a certificate verification error occurs. Considering the context, the browser then guides the user in handling and possibly overcoming the security error. We also propose specific password warnings (SPW) when users are about to send passwords in a form vulnerable to eavesdropping. We performed user studies to evaluate CSCV and SPW. Our results suggest that CSCV and SPW can greatly improve Web browsing security and are easy to use even without training. Moreover, CSCV had greater impact than did staged security training.