The protocolIs function in platform/KURLGoogle.cpp in WebCore in WebKit before r55822 as used in Google Chrome before 4.1.249.1036 and Flock Browser 3.x before 3.0.0.4112 does not properly handle whitespace at the beginning of a URL which allows remote attackers to conduct cross-site scripting (XSS) attacks via a crafted javascript: URL as demonstrated by a \\x00javascript:alert sequence.