Living with HTTPS (19 Jul 2012)
(These are my notes from the first half of my talk at HOPE9 last weekend. I write notes like these not as a script, but so that I have at least some words ready in my head when I'm speaking. They are more conversational and less organised than a usual blog post, so please forgive me the rough edges.)
HTTPS tends to cause people to give talks mocking certificate security and the ecosystem around it. Perhaps that's well deserved, but that's not what this talk is about. If you want to have fun at the expense of CAs, dig up one of Moxie's talks. This talk deals with the fact that your HTTPS site, and the sites that you use, probably don't even reach the level where you get to start worrying about certificates.
I'm a transport security person so the model for this talk is that we have two computers talking over a malicious network. We assume that the computers themselves are honest and uncompromised. That might be a stretch in these malware-ridden times, but that's the area of host security and I'm not talking about that today. The network can drop, alter or fabricate packets at will. As a lemma, we also assume that the network can cause the browser to load any URL it wishes. The network can do this by inserting HTML into any HTTP request and we assume that every user makes some unencrypted requests while browsing.
If the average user typed mail.google.com into a browser and saw the following, what fraction of them do you think would login, none the wiser?
Can you even see what's terribly wrong here?
The problem is that the page isn't served over HTTPS. It should have been, but when a user types a hostname into a browser, the default scheme is HTTP. The server may attempt to redirect users to HTTPS, but that redirect is insecure: a MITM attacker can rewrite it and keep the user on HTTP, spoofing the real site the whole time. The attacker can now intercept all the traffic to this perfectly well configured and secure website.
This is called SSL stripping and it's terribly simple and devastatingly effective. We probably don't see it very often because it's not something that corporate proxies need to do, so it's not in off-the-shelf devices. But that respite is unlikely to last very long and maybe it's already over: how would we even know if it was being used?
In order to stop SSL stripping, we need to make HTTPS the only protocol. We can't do that for the whole Internet, but we can do it site-by-site with HTTP Strict Transport Security (HSTS).
HSTS tells browsers to always make requests over HTTPS to HSTS sites. Sites become HSTS either by being built into the browser, or by advertising a header:
Strict-Transport-Security: max-age=8640000; includeSubDomains
The header is in force for the given number of seconds and may also apply to all subdomains. The header must be received over a clean HTTPS connection.
Once the browser knows that a site is HTTPS only, the user typing mail.google.com is safe: the initial request uses HTTPS and there's no hole for an attacker to exploit.
(mail.google.com and a number of other sites are already built into Chrome as HSTS sites so it's not actually possible to access accounts.google.com over HTTP with Chrome - I had to doctor that image! If you want to be included in Chrome's built-in HSTS list, email me.)
HSTS can also protect you, the webmaster, from making silly mistakes. Let's assume that you've told your mother that she should always type https:// before going to her banking site or maybe you setup a bookmark for her. That's honestly more than we can, or should, expect of our users. But let's say that our supererogatory user enters https://www.citibank.com in order to securely connect to her bank. What happens? Well, https://www.citibank.com redirects her to http://www.citibank.com. They've downgraded the user! From there, the HTTP site should redirect back to HTTPS, but the damage has been done. An attacker can get in through the hole.
I'm honestly not picking on Citibank here. They were simply the second site that I tried and I was some surprised that the first site didn't have the problem. It's a very easy mistake to make, and everything just works! It's a completely silent disaster! But HSTS would have either prevented it, or would have failed closed.
HSTS also does something else. It turns this:
The “bypass this certificate error” button has gone. That button is a UI disaster. Asking regular people to evaluate the validity of X.509 certificates is insane. It's a security cop-out that we're saddled with, and which is causing real damage.
We've seen widespread MITM attacks using invalid certificates in Syria and, in recent weeks, Jordan. These attacks are invalid! This is squarely within our threat model for transport security and it shouldn't have been a risk for anybody. But we know that those bypass buttons are clicked 60% of the time by Chrome users. People are certainly habituated to clicking them and I bet that a large number of people were victims of attacks that we should have been able to prevent.
If you take only one thing away from this talk, HSTS should be it.
One we're sorted HSTS we have another problem. These snippets of HTML are gaping wide holes in your security:
<script src="http://... <link href="http://... <embed src="http://...
When you load sub-resources over HTTP, an attacker can replace them with content of their choosing. The attacker also gets to choose any page on your HTTPS site with the problem. That includes pages that you don't expect to be served over HTTPS, but happen to be mapped. If you have this problem anywhere, on any HTTPS page, the attacker wins.
With complex sites, it's very difficult to ensure that this doesn't happen. One good way to limit it is to only serve over HTTPS so that there aren't any pages that you expect to serve over HTTP. Also, HSTS might also save you if you're loading mixed script from the same domain.
Another mitigation is to use scheme-relative URLs everywhere possible. These URLs look like //example.com/bar.js and are valid in all browsers. They inherit the scheme of the parent page, which will be HTTP or HTTPS as needed. (Although it does mean that if you load the page from disk then things will break. The scheme will be file:// in that case.)
Yes, there's another of those damm bypass buttons. But we swear that it's just a transitional strategy and it already stops silent exploitation in a hidden iframe.
HTTP and HTTPS cookie jars are the same. No really: cookies aren't scoped to a protocol! That means that if you set a Cookie on https://example.com and then make a request to http://example.com, the cookies will be sent in the clear! In order to prevent this, you should be setting secure on your HTTPS cookies. Sadly this is a very easy thing to miss because everything will still work if you omit it, but without the secure tag, attackers can easily steal your cookies.
It's worth noting that HSTS can protect you from this: by preventing the HTTP request from being sent, the cookies can't be leaked that way, but you'll need to include all subdomains in the HSTS coverage.
There's a second corollary to this: attackers can set your HTTPS cookies to. By causing an request to be sent to http://example.com, they can spoof a reply with cookies that then override any existing cookies. In this fashion, an attacker can log a user in as themselves during their interaction with an HTTPS site. Then, say, emails that they send will be saved in the attacker's out-box.
There's no very good protection against this except HSTS again. By preventing any HTTP requests you can stop the attacker from spoofing a reply to set the cookies. Against, HSTS needs to cover all subdomains in order to be effective against this.
Get yourself checked out
You should go to https://www.ssllabs.com and run their scan against your site. It's very good, but ignore it if it complains about the BEAST attack. It'll do so if you make a CBC ciphersuite your top preference. Browsers have worked around BEAST and non-browser clients are very unlikely to provide the attacker enough access to be able to pull it off. You have a limited amount of resources to address HTTPS issues and I don't think BEAST should make the list.
Get a real certificate
You should get a real certificate. You probably already have one but, if you don't, then you're just training more people to ignore certificate errors and you can't have HSTS without a real certificate. StartSSL give them away for free. Get one.
If you've reached this far and have done all of the above, congratulations: you're in the top 0.1% of HTTPS sites. If you're getting bored, this is a reasonable point to stop reading: everything else is just bonus points from now on.
You should consider forward secrecy. Forward secrecy means that the keys for a connection aren't stored on disk. You might have limited the amount of information that you log in order to protect the privacy of your users, but if you don't have forward secrecy then your private key is capable of decrypting all past connections. Someone else might be doing the logging for you.
In order to enable forward secrecy you need to have DHE or ECDHE ciphersuites as your top preference. DHE ciphersuites are somewhat expensive if you're terminating lots of SSL connections and you should be aware that your server will probably only allow 1024-bit DHE. I think 1024-bit DHE-RSA is preferable to 2048-bit RSA, but opinions vary. If you're using ECDHE, use P-256.
You also need to be aware of Session Tickets in order to implement forward secrecy correctly. There are two ways to resume a TLS connection: either the server chooses a random number and both sides store the session information, of the server can encrypt the session information with a secret, local key and send that to the client. The former is called Session IDs and the latter is called Session Tickets.
But Session Tickets are transmitted over the wire and so the server's Session Ticket encryption key is capable of decrypting past connections. Most servers will generate a random Session Ticket key at startup unless otherwise configured, but you should check.
I'm not going to take the time to detail how to configure this here. There are lots of webservers and it would take a while. This is more of a pointer so that you can go away and research it if you wish.