Microsoft Research published a paper sometime last month analyzing Single Sign On services hosted by various commercial entities.
The paper had been sitting on my desk for a couple weeks (literally) before I had a chance to read through it. It actually made it’s rounds through the company before I had a chance to read it.
In any case, I thought it would be good to post a link for people to read because it outlines some very important implications of using a Single Sign On service.
With the boom of software-as-a-service and social networking, web-based single sign-on (SSO) schemes are being deployed by more and more commercial websites to safeguard many web resources. Despite prior research in formal verification, little has been done to analyze the security quality of SSO schemes that are commercially deployed in the real world. Such an analysis faces unique technical challenges, including lack of access to well-documented protocols and code, and the complexity brought in by the rich browser elements (script, Flash, etc.). In this paper, we report the first “field study” on popular web SSO systems. In every studied case, we focused on the actual web traffic going through the browser, and used an algorithm to recover important semantic information and identify potential exploit opportunities. Such opportunities guided us to the discoveries of real flaws. In this study, we discovered 8 serious logic flaws in high-profile ID providers and relying party websites, such as OpenID (including Google ID and PayPal Access), Facebook, JanRain, Freelancer, FarmVille, Sears.com, etc. Every flaw allows an attacker to sign in as the victim user. We reported our findings to affected companies, and received their acknowledgements in various ways. All the reported flaws, except those discovered very recently, have been fixed. This study shows that the overall security quality of SSO deployments seems worrisome. We hope that the SSO community conducts a study similar to ours, but in a larger scale, to better understand to what extent SSO is insecurely deployed and how to respond to the situation.
The gist of the paper is that when it comes to verification and validation of the security of SSO protocols, we tend to do formal tests of the protocols themselves, but we don’t ever really test the implementations of the protocols. Observation showed that most developers didn’t fully understand the security implications of the most important part in an SSO conversation – the token exchange:
Our success indicates that the developers of today’s web SSO systems often fail to fully understand the security implications during token exchange, particularly, how to ensure that the token is well protected and correctly verified, and what the adversary is capable of doing in the process.
Think about it. The token received from the IdP is the identity. The relying party trusts the validity of the identity by verifying the token somehow. If verification isn’t done properly an attacker can inject information into the token and elevate their privileges or impersonate another user. This is a fundamental problem:
For example, we found that the RPs of Google ID SSO often assume that message fields they require Google to sign would always be signed, which turns out to be a serious misunderstanding (Section 4.1).
Not all of the data in a token needs to be signed. In fact, if the IdP isn’t the authoritative source of the particular piece of data it may not want to sign that data. If the IdP can’t or wont sign the data, do you really want to trust it?
What’s the rule that’s always hammered into us when writing code? Do not trust user input. Even if it’s supposed to have come from another machine:
[…] when our browser (i.e., Bob’s browser) relayed BRM1 [part 1 of the message exchange], it changed openid.ext1.required (Figure 8) to (firstname,lastname). As a result, BRM3 [part 3 of message exchange] sent by the IdP did not contain the email element (i.e., openid.ext1.value.email). When this message was relayed by the browser, we appended to it email@example.com as the email element. We found that Smartsheet accepted us as Alice and granted us the full control of her account.
If you receive a message that contains something you need to use you, not only do you have to validate that it’s in the right format, but you have to validate that it hasn’t been modified or tampered with before it hits your code.
This is something I’ve talked about before, but in a more generalized nature. Validate-validate-validate!
As an aside, an interesting observation made in the research is that all of this was done through black-box testing. The researchers didn’t have access to any source code. So if the researchers could find problems this way, the attackers could find problems the same way:
Our study shows that not only do logic flaws pervasively exist in web SSO deployments, but they are practically discoverable by the adversary through analysis of the SSO steps disclosed from the browser, even though source code of these systems is unavailable.
This tends to be the case with validation problems. Throw a bunch of corrupted data at something and see if it sticks.
They also realized that their biggest challenge wasn’t trying to understand the protocol, but trying to understand the data being used within the protocol.
For every case that we studied, we spent more time on understanding how each SSO system work than on reasoning at the pure logic level.
The fundamental design of a Single Sign On service doesn’t really change between protocols. The protocols may use varying terms to describe the different players in the system, but there are really only three that are important: the IdP, the RP, and the client. They interact with each other in fundamentally similar ways across most SSO protocols. It’s no surprise that understanding the data was harder than understanding the logic.
They didn’t go into much detail about why they spent more time studying data, but earlier they talked about how different vendors used different variations on the protocols.
[…] the way that today’s web SSO systems are constructed is largely through integrating web APIs, SDKs and sample code offered by the IdPs. During this process, a protocol serves merely as a loose guideline, which individual RPs often bend for the convenience of integrating SSO into their systems. Some IdPs do not even bother to come up with a rigorous protocol for their service.
In my experience the cost of changing a security protocol for the sake of convenience is usually protocol security. It usually doesn’t end well.