Vulnerabilities in browser extensions put users at risk by providing a way for website and network attackers to gain access to users’ private data and credentials. Extensions can also introduce vulnerabilities into the websites that they modify. In 2009, Google Chrome introduced a new extension platform with several features intended to prevent and mitigate extension vulnerabilities: strong isolation between websites and extensions,privilege separation within an extension, and an extension permission system. We performed a security review of 100 Chrome extensions and found 70 vulnerabilities across 40 extensions. Given these vulnerabilities,we evaluate how well each of the security mechanisms defends against extension vulnerabilities. We find that the mechanisms mostly succeed at preventing web attacks,new security mechanisms are needed to protect users from network attacks on extensions, website metadata attacks on extensions, and vulnerabilities that extensions add to websites. We propose and evaluate additional defenses, and we conclude that banning HTTP scripts and inline scripts would prevent 47 of the 50 most severe vulnerabilities with only modest impact on developers.
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