Eight leading browsers, including three from China, were tested against the Security Stack Test Methodology V1.5 using 657 samples of socially engineered malware (SEM) that were captured over 14 days in NSS Labs’ unique live testing harness. SEM attacks use several different methods to deceive users into downloading malicious software, but the browser is the primary vector for delivery of SEM and therefore is the first line of defense against such attacks.