The other day an interesting bug report was posted against the Firefox browser, and it caused some interesting discussions and blog posts on the subject of Man-In-The-Middle attacks and how current browsers etc make it (too?) easy to accept self-signed certificates and thus users are easily mislead. (Peter Burkholder wrote a great piece on SSL MITMing already back in 2002 which goes into detail on how this can be done.).
The entire issue essentially boils down to this:
To be able to really know that you're communicating with the true remote site (and not an impostor), you must have some kind of verification system.
In SSL land we have this system with CA certs for verifying certs and it works pretty good in most cases (I think). However, so many sites on the internet use HTTPS today without having the certificate signed by a party that is known to the browser already - most of them are so called self-signed which means there's nobody else that guarantees that they are who they claim to be, just themselves.
All current modern browsers want to give the users easy access to HTTP sites, to HTTPS sites with valid properly-signed certs but also allow users to connect to and browse on HTTPS sites with self-signed certs. And here comes the problem: how to tell users that HTTPS with self-signed certs is very insecure but still let them proceed? How do we tell them that the user may proceed but if this is a known popular site you really should expect a true and valid certificate as otherwise it is quite possibly a MITM you're seeing?
People are so used to just accept exceptions and click away nagging pop-ups so the warnings and alerts that are explicit and implied by the prompts you have to go through to accept the self-signed certificate. They don't seem to have much effect. As can be seen in this bug report, accepting an impostor's certificate for a large known site is far too easy.
In the SSH land however, we don't have the ca cert system and top-down trust hierarchy that SSL/TLS imposes. But does this matter? I'd say no, as most if not all users still don't reflect much over the fact when a server's host key is reported different than what you used before. Or when you connect to a host the first time you accept the host key without trying to verify it using a different channel. Thus you're subject to pretty much the same MITM risk. The difference is perhaps that less "mere end users" are using SSH this way.
Let me just put emphasis on this: SSL and SSH are secure. The insecureness here is not due to how the protocols work, but rather they are flaws that appear when we mix in real world users and UIs and so.
I don't have any sensible solutions to these problems myself. I'm crap at designing things for mere humans and UIs etc and I make no claims of understanding end users.
It seems there's a nice tool called ettercap that's supposedly a fine thing to use when you want to run your own MITM attacks on your LAN! And on the other side: an interesting take at improving the "accept this certificate" UI is offered by the Firefox's Perspectives plugin which basically also checks with N other sources' view to help you decide whether to trust a certificate.
I want to round off my rant with a little quote: