Server problems caused by 'transparent' self-signed SSL certificates

November 30, 2008

One of the issues with allowing self-signed SSL certificates to be automatically used for transparent encryption over https URLs is the problems that they cause for server-side applications.

Right now, a server-side program that care can more or less assume that a SSL-encrypted connection means either that it is securely talking to the real end user's machine or that the end user has been actively compromised (which it can't do anything about). It may thus use things like 'you can only talk to this service over https' as a reasonable security precaution, one that means something.

If you allow self-signed SSL certificates to be used transparently for encryption in https URLs, you enable relatively trivial man in the middle attacks. This negates all of the security properties of SSL connections. Because the browser knows what level of security the certificate it accepted had, browsers can recognize this and lie to things like Javascript so that nothing thinks it's getting more security than it actually is.

However, the web server does not know and cannot know this information, since it has no way of finding out if it is talking to the real end machine or to a man in the middle. As such, server side applications can no longer assume that a https URL and a SSL connection means actual security; it might be secure and it might not be, and they can't tell the difference. This leaves them with no way to require something approximating actual security from clients, and it breaks current applications (to the extent that they assume that SSL means security).

(Server side applications cannot do things like feed Javascript to the client browser to see if it is really connecting directly to them, because the man in the middle can rewrite that Javascript on the fly.)

(In theory the one way out of this is to require everything to have client side certificates. However, I do not believe that this is any more practical than trying to make all server side applications instantly give up any 'SSL means security' beliefs that they may have.)

Written on 30 November 2008.
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Last modified: Sun Nov 30 03:15:45 2008
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