In my last post, I mentioned how warning users when a previously-seen cert changes may generate false positives for some sites. If a website has a multiple servers with different certs, the browser may often generate spurious errors for that site. But could this be a symptom of a genuine security problem?
Citibank appears to have one certificate per server. You can verify this yourself by going to their website and multiple times, clearing your browser each time. Clicking on the SSL icon to the left of the URL will show a different cert.
Here are the first 4 bytes of three serial numbers of certs observed at Citibank:
The Citibank certs are all identical except for a few fields. As you would expect, the domain name (CN) field is identical for each. The organizational unit (OU) differs (e.g., “olb-usmtprweb3″ versus “…web1″), but this field is not interpreted by browsers and is more of a convenience. The web server’s public key is different in each cert. And, of course, the serial number and signature fields also differ, as they should for all certs.
On the other hand, Wells Fargo appears to have only one cert. This cert (serial 41:c5:cd:90) is the same even after accessing their site via a proxy to ensure some load-balancing magic isn’t getting in the way. It’s easy to ignore this difference, but there might be something else going on.
Protecting the web server’s private key is one of the most important operational security duties. If it is discovered, all past and present encrypted sessions are compromised. (Yes, I know about DHE but it’s not widely used). After cleaning up the mess, the organization needs to get a new certificate and revoke the old one. This is no easy task as CRLs and OCSP both have their downsides.
One key question to ask an opsec department is “have you ever done a live cert revocation?” It’s one of those things that has to be experienced to be understood. In the recent Comodo fiasco, leaf cert revocations were embedded in browser software updates because the existing revocation mechanisms weren’t reliable enough.
Since web servers run commodity operating systems, most big sites use a hardware security module (HSM) to protect the private key. This is a dedicated box with some physical tamper resistance that is optimized for doing private key operations. By limiting the API to the server, HSMs can be hardened to prevent compromise, even if the server is hacked. The main downsides are that HSMs are expensive and may not live up to the original security guarantees as the API surface area grows.
Now, back to the two banks. Why would one have multiple certs but not the other? Certificates cost money, so if you’re offloading SSL to a single accelerator, there’s no reason to give it multiple certs. If each server has a dedicated HSM, you could use separate certs or just generate one and export it to all the others. You need to do this anyway for backup purposes.
This is just supposition, but one thing this could indicate is a different approach to securing the private key. Instead of generating one cert and private key, you create one per server and store it without an HSM. If a server gets compromised, you revoke the private key and move on. This might seem like a good idea to some since the cost of a cert must be lower than an HSM. However, the ineffectiveness of revocation today shows this to be a dangerous choice.
There may be other explanations for this. Perhaps Citi uses individual HSMs and Wells Fargo has a single SSL accelerator with plaintext HTTP in the backend. Perhaps they got a bargain on certs by buying in bulk. However, any time a system has more keys than necessary, it can lead to complicated key management. Or worse, it may indicate a weaker system design overall.
There’s no way to know the real story, but it’s good food for thought for anyone else who might be considering multiple certs as a substitute for strong private key protection. Cert revocation doesn’t currently work and should not be relied on.