Extended Validation certificates and XSS considered harmful

A cross-site scripting vulnerability on the popular SourceForge.net website shows how Extended Validation SSL certificates could be exploited by fraudsters. Piggybacking on the anticipated extra trust instilled by the presence of an EV SSL certificate, arbitrary content could be injected onto the secure page at SourceForge to create a very convincing phishing attack. The green address bar displayed by the web browser would assure users that they are looking at a website that can be trusted, even though the page they are looking at may contain scripts or HTML created by a remote attacker.

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The vulnerable page at SourceForge, showing the green address bar and injected JavaScript being executed

Extended Validation SSL certificates were originally created as a direct response to the rise in internet fraud, with additional verification processes reducing the likelihood of erroneously issuing a certificate to an unauthorised party. Modern web browsers treat EV SSL certificates differently to ordinary SSL certificates, typically turning the address bar green to show that a site can be trusted. Once users are conditioned into thinking that green means good, this could prove harmful when an EV SSL site contains a cross-site scripting vulnerability.

The number of EV SSL certificates in use worldwide is still relatively small and has only recently risen above 4,000. SourceForge is a large open source software development website, with a high ranking amongst users of the Netcraft Toolbar, and uses a VeriSign Class 3 Extended Validation SSL certificate for its main secure site at https://sourceforge.net.

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Nightly builds of Firefox also display the green address bar element

Both Internet Explorer 7 and recent nightly builds of the Mozilla Firefox web browser display a green address bar when accessing the vulnerable page at SourceForge, even when it is used to inject content that may have been created by a fraudster. Netcraft has informed SourceForge about this issue, although the xssed.com mirror, where this vulnerability was first discovered, suggests that it has remained unfixed since last year.

This discovery (believed to be the first documented case of XSS on an EV SSL website) highlights the need to remain wary of web application security, even when delivered with the most secure and trusted option of Extended Validation SSL certificates.

Netcraft offers extensive web application penetration and security testing services to identify vulnerabilities such as cross-site scripting.