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Sub-Prime PKI: Attacking Extended Validation SSL

Jackson and Barth[1], in their paper "Beware of Finer-Grained Origins" (May 2008), describe a number of HTTP session attributes beyond Same Origin Policy(SOP) that are used by web browsers to make security decisions. One of the attributes that the paper discusses is the type of SSL certificate presented by a web server when negotiating a secure connection. Modern web browsers support both domain validated (DV) and extended validation (EV) SSL certificates. EV SSL certificates were created to combat phishing and other web based spoofing attacks which succeed in spite of the DV SSL model for web site identification. The EV SSL model improves upon the DV model in two ways. First, certificate authorities enforce a more thorough off-line validation process in order to more accurately identify the agent requesting the certificate as the owner of the domain which the certificate is for. Second, when a certificate chains up to an EV root certificate authority (CA), EV-capable web browsers display the web sites identity information next to, or in, the address bar. This identity information is usually contained in a green badge, showing what certificate authority Verisign refers to as the “green glow.” While Jackson and Barth describe a specific flaw in the Same Origin Policy that could allow an attacker to inject arbitrary JavaScript into an EV SSL page, this paper describes two other short comings in browser design that allow an attacker to silently Man-In-The-Middle (MITM) EV SSL protected web sites. In addition to these design flaws, this paper introduces a new attack, called SSL Rebinding, and revisits an old attack, browser cache poisoning. Between SSL rebinding and cache poisoning attacks, this paper will show that the described design flaws in modern web browsers create a “weakest link” scenario, where high-assurance EV SSL certificates are permanently handicapped by their low-assurance DV counterparts. After reading this paper, the reader will see that until our browser vendors address these design issues, the “green glow” of EV SSL is less of an assurance than promised by the marketing materials of EV SSL vendors. Background Jackson and Barth disclosed their research at the Web 2.0 Security & Privacy conference on 22 May 2008. This period was the “calm before the storm” for the information security community, as speculation about the yet-to-be-disclosed Dan Kaminsky DNS flaw was about to begin running rampant. When the paper was originally released, a key requirement to launch the attacks described by Jackson and Barth was for an attacker to have a foot-hold between a client and server as a Man-In-The-Middle (MITM). At the time, such MITM attacks were mostly limited to insecure wireless networks and attacker controlled rogue access points. However, when the results of the Kaminsky DNS bug came to light weeks later in July, the long-held security expert assertion that DNS could be easily subverted on a mass scale was shown to be an easily implemented reality. Suddenly, a realistic defense was required for attacks that could be easily launched against large sections of the Internet, and targeted at prominent financial and ecommerce web properties.