Dan Griffin's Blog
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Lock Down Your CA!
November 16, 2006
Just saw an interesting talk entitled “Securing Your Certificate Authorities [CA] Private Keys” by Brian Komar of IdentIT, Inc. Included was a demo of installation and configuration of two vendors’ Hardware Security Module (HSM) and accompanying Cryptographic Service Provider (CSP). The demo was quite smooth.
During the demo, one thing in particular caught my attention from a security perspective: while interacting with one of the CAs, I’m pretty sure I saw one of the HSM CSPs display a dialog box. What are the ramifications of this behavior? Well, the Certificate Authority service is probably running as Local System. The CSP is loaded into that process. Hence, the dialog box is system, yet interacting with the user desktop. This exposes the CA process to a so-called “Shatter Attack,” referring to a potential Elevation of Privilege accessible to processes within a given session.
Unfortunately, due to the architecture of Microsoft Crypto API and the behavior of session zero, there’s really no way to avoid this. What to do? Easy – lock down your CA server. There’s a good summary of how to do this here. Note that even non-admin users, if given logon privilege to the console session of your server, can launch a Shatter Attack. So think about who has logon rights to your CA!
By the way – in Longhorn Server, this situation improves dramatically, since interactive logons to session zero are disallowed. How will legacy CSPs in the above scenario function in this environment? Vista client has the built-in capability to notify a logged-on user that a session zero process is attempting to display a dialog. In response, the user can switch to the session zero desktop, respond to the dialog, and switch back.
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