Thoughts, stories and ideas.

  • Amazon S3 — Amazon S3 was briefly mentioned in previously. The default base domain used to access the bucket is not always the same and depends on the AWS region that is used. The full list of Amazon S3 base domains is available in AWS documentation. Similarly to CloudFront, Amazon S3 allows specifying the alternate (custom) domain name to access the bucket's content.
  • Heroku — Heroku is a Platform-as-a-Service provider which enables deployment of an application using simple workflow. Since access to the application is needed, Heroku exposes the application using subdomain formed on herokuapp.com. However, it is also possible to specify the custom domain name to access the deployed application.
  • Shopify — Shopify provides a way of creating and customizing e-commerce stores in the cloud. The default subdomain to access the store is built on myshopify.com. As services described before, Shopify allows specifying alternate domain names. Noteworthy is that Shopify verifies correct CNAME record configuration. However, this verification is not domain ownership verification. Shopify only checks for accurate CNAME record that is present in the alternate domain's DNS zone. This verification, therefore, does not prevent subdomain takeovers.
  • GitHub — GitHub is a version control repository for Git. GitHub also allows free web hosting using their GitHub Pages project. This web hosting is usually used for project's documentation, technical blogs, or supporting web pages to open-source projects. GitHub Pages supports custom domain name in addition to default domain name under github.io.
  • Microsoft Azure — Microsoft Azure is a more prominent cloud provider, similar to AWS. It is different compared to the cloud services mentioned above in that it does not provide a virtual hosting architecture. Simply put, for each cloud service, Azure creates own virtual machine with own IP address. Therefore the mapping between a domain name and IP address is unambiguous (one-to-one mapping). Noteworthy is that since this is not a regular virtual hosting setup, configuring CNAME record does not necessarily have to be explicitly defined in the resource settings. Azure provides multiple cloud services but the ones discussed in this thesis have default domains of cloudapp.net and azurewebsites.net. Its documentation describes setting the link between the domain name and Azure resource using A or CNAME records (pointing to one of the two domains mentioned previously). An interesting observation is that for A records, Azure does a domain ownership verification using TXT records. However, it is not the case for a CNAME record, and subdomain takeover is, therefore, possible even in the case of Microsoft Azure.

For an extended listing of affected cloud providers, I highly recommend checking "Can I take over XYZ?" guide.

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