The OpenSSL heartbleed bug was a serious kick to the Internet's collective ass. This video provides a quick overview if you want the details. In summary, an attacker could craft a payload with a fake size (up to 64k) and trick openssl into sending a random chunk of server memory. WTF?! To understand how bad this was I spent a minute hacking on this script that was going around. I pointed it at login.yahoo.com (which is no longer vulnerable) and tried to see if I could catch a username and password flying by. I had one within 30 seconds. That's how bad it was; you could read random parts of the server's memory which may contain passwords, private keys, or whatever else OpenSSL was processing for current site visitors.
I had stolen someone's credentials. Game over, right? How do you protect yourself against something as bad as this?
So, I logged into Yahoo (just for research purposes) and first I saw a captcha. Ah, nice, Yahoo detected that the user was logging in from somewhere new. Naturally I entered the captcha (just doing research!) but next it presented me with one of the user's security questions. Cool. I did not try to guess the answser :)
This is how you protect yourself against security vulnerabilities. Put up as many barriers as possible (as long as it doesn't compromise usability). Heartbleed wasn't the first serious SSL/TLS vulnerability and it won't be the last. Developers make stupid mistakes. Software is written by humans. It's easy to forget to check on something or to miscalculate another thing when designing the security of a system. You need to protect yourself from your own stupidity.
I've been working on an internal API at Mozilla that signs APKs (Android packages) for the Firefox Marketplace. It would be very bad if someone compromised this because they could spread malware like wildfire. With help from our security team I arrived at a loosely coupled system where each service has a specific role. You'd have to compromise several parts of the system to start doing real damage. Also, not all developers work on all systems which is important since a typical way to break into a system is to break into employee machines.
Even though the signer is an internal API that we can firewall off we decided to use Hawk (based on Oauth 1.0) for communication; it has a lot of useful security properties. (Out of this grew Mohawk for Python.) In Hawk, two parties who want to communicate use a shared private key to sign requests. When you make a request you have to sign it. The receiving end will ignore it if the signature doesn't match. Someone asked me, well, how is that different than logging in and passing a session token around over SSL? It's safer because an intercepted signature is useless. There's nothing to intercept. Besides the signature which prevents tampering, there is a timestamp and nonce to protect against replay attacks. Hawk also lets the client verify the signature of the response making it a two-way secure channel. In other systems, if you were to somehow intercept a username, a password, or even a session token, you could do a lot of damage.
Of course, something could always go wrong at each step of the way. A bug like heartbleed would still be catastrophic. It just helps to have many barriers in many different places because most vulnerabilities only allow partial access to a system. Putting up many barriers like this is known as defense in depth.
It's easy to go overboard and add complexity to your project, though. In our payments system we made a dedicated service whose sole responsibility is to handle sensitive payment API credentials (i.e. it makes the money flow). This is good. It's isolated; only a few other internal servers can talk to it. For the few moments when the service actually sends login credentials to payment APIs we added a feature where the service proxies itself, parses the result, and responds to the client as if no proxying occurred. Did that even make sense? Probably not because it's confusing. This self-proxying feature makes the system hard to understand. However, for something like heartbleed it would have been nice since the ultra sensitive credentials are NOT held in the main server's memory. They are held in a separate, internal instance of the server whose sole purpose is to transmit credentials. Hmmm.