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December 3, 2019

Programmatically Accessing Your Customers' Google Cloud Accounts (While Avoiding the Confused Deputy Problem)

SaaS applications often need to access their customers' cloud resources at providers like Amazon Web Services and Google Cloud Platform. For instance, a monitoring service might require read-only access to their customers' AWS accounts so it can inventory resources. At SSLMate, we request access to our customers' DNS zones so we can publish DNS records to automatically validate the certificates that they request.

Doing this with AWS was easy, thanks to their detailed documentation for precisely this use case. However, when it came to Google Cloud, the only hint of a solution that I could find was buried deep in this FAQ:

How can I access data from my users' Google Cloud Platform project using Cloud APIs?

You can access data from your users' Google Cloud Platform projects by creating a service account to represent your service, and then having your customers grant that service account appropriate access to their cloud data using IAM policies. Note that you might want to create a service account per customer if you need to avoid confused deputy problems.

The first part sounds pretty easy: create a service account which SSLMate uses when making DNS changes and ask customers to grant this account access to Cloud DNS in their Google Cloud project. What isn't as easy, unfortunately, is solving the confused deputy problem. Contrary to the FAQ, this isn't something that we "might" want to do - it's something we absolutely must do. If SSLMate used just a single service account and all of our customers authorized it, then a malicious customer would be able to request a certificate for any of our customer's domains. SSLMate would access the victim's project using the single SSLMate service account and publish the validation record for the attacker's certificate request. This would succeed since the victim had authorized the single SSLMate service account. I can't think of any application that would not also be vulnerable if it did not address the confused deputy problem.

AWS provides a nice solution to the problem. The customer doesn't just authorize SSLMate's AWS account; they authorize the AWS account plus an "external ID", which for SSLMate is the same as their SSLMate customer ID. When SSLMate connects to AWS to add a DNS record, it sends the ID of the customer on whose behalf it is acting. If it doesn't match the authorized external ID, AWS blocks the request.

But with Google Cloud we're stuck creating a different service account for each SSLMate customer. The customer would authorize only that service account, and SSLMate would use it to access their Google Cloud project. Creating all these service accounts isn't hard - there's an API for that - but what's hard is giving each of these service accounts their own key. I'd have to securely store all those keys somewhere, and rotate them periodically, which is a pain.

Fortunately, Google Cloud has an API to generate short-lived OAuth2 access tokens for a service account. SSLMate invokes this API from a single master service account to get an access token for the customer-specific service account, and uses the access token to make the DNS change requests. When it's done, SSLMate discards the credentials. The customer-specific accounts have no long-term keys associated with them; only the single master service account does, making key management significantly easier, and equivalent to AWS.

Here's how you can set this up for your SaaS application...

1. Create the Master Service Account

This service account will be used by your app to create customer-specific service accounts, and to create short-lived access credentials for accessing those accounts.

  1. Create the service account, giving it a name of your choosing.

  2. Assign the service account the following roles:

    • Service Account Admin - this is needed to create customer-specific service accounts
    • Service Account Token Creator - this is needed to get the short-lived access credentials
  3. Create a key for this service account and save a copy. This key will be used by your app.

2. Onboarding a Customer

Your app needs to do this every time you onboard a new customer (or an existing customer wants to integrate their Google Cloud account with your service).

  1. Create a service account for the customer using the IAM API, authenticating with the key for your master service account. I suggest deriving the service account ID from the customer's internal ID. For instance, if the customer ID is 1234, use customer-1234 as the service account ID.

  2. Instruct your customer to authorize this service account as follows:

    1. Visit the IAM page for their project.
    2. Click Add.
    3. In the New Member box, your customer must enter the email address of the service account created in step 1. The email address will look like: SERVICE_ACCOUNT_ID@YOUR_PROJECT_ID.iam.gserviceaccount.com
    4. In the Role box, choose the roles necessary for your app to function (in SSLMate's case, this is "DNS Administrator").
    5. Click Save.
  3. After your customer authorizes the account, I recommend testing the access by making a simple read-only request as described in part 3 below and displaying an error if it doesn't work.

3. Accessing Your Customer's Account

  1. Use the generateAccessToken API to create an access token for the customer-specific service account. Authenticate to this API using the master service account's key. The name parameter must contain the email address of the customer-specific service account, which you can derive from the customer ID if you follow the naming convention suggested above. The scope array must contain the OAuth scopes you need to access. (In SSLMate's case, this is https://www.googleapis.com/auth/ndev.clouddns.readwrite)

  2. Use the token returned by the API call as a Bearer token for accessing your customer's account.

If you're using Go, it's helpful to put the above logic in a type that implements the oauth2.TokenSource interface, which you can pass to oauth2.NewClient to create the http.Client that you use with the Google Cloud API library. Here's a drop-in token source type you can use, and here's an adaptable example of how to use it.

Conclusion

While it's more complicated to set up than AWS, in the end this solution has the same desirable properties as AWS: protection against confused deputy attacks, and just one long term credential for your application. If only Google Cloud's documentation for this was as helpful as AWS's!

Addendum: Why I Didn't Use Three-Legged OAuth

My first attempt at implementing this feature used three-legged OAuth. SSLMate would redirect the customer to Google, they'd click a single button to authorize SSLMate for read/write access to Google Cloud DNS, and then Google would redirect back to SSLMate with an access code. This worked well and provided a great user experience since the customer didn't need to do any configuration. Unfortunately, the access was linked to the customer's Google account, rather than their Google Cloud project. I thought this was a bad idea because if the Google Account ever lost access to the project, it would break the SSLMate integration. The integration needs to continue working even if the employee who set it up leaves the company which owns the Google Cloud project.

Another thing which turned me off three-legged OAuth was that Google wanted to subject my integration to a lengthy review because accessing DNS is considered a "sensitive scope." This sounded like a hassle, and I assume it is only going to get more restrictive in the future as Google tries to stop malicious OAuth apps like last year's viral Google Docs attack. For example, if they decided one day to classify DNS access as a "restricted scope," I would be subjected to a 5 figure security audit.

Consequentially, I ditched three-legged OAuth and turned to the solution described above. The user experience is not quite as nice but it's much more robust. SSLMate still uses three-legged OAuth with other DNS providers which support it, like DNSimple and Digital Ocean.

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Hi, I'm Andrew. I run SSLMate, which makes SSL certificates easy through automation, great software, and friendly support.

I blog about security, PKI, Linux, and more. If you liked this post, check out my other posts or subscribe to my RSS feed.

My email address is andrew@agwa.name. I'm AGWA at GitHub and @__agwa on Twitter.

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