A dangling CNAME is a DNS CNAME record that resolves to a hostname whose underlying resource has been deleted or de-provisioned. The DNS still exists, but the target no longer belongs to anyone — until an attacker claims it. Dangling CNAMEs are the raw material of most subdomain takeovers.
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What is a dangling CNAME?
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What Is a Subdomain Takeover? (And Why Microsoft Had 670 of Them)
Subdomain takeover is the class of bug where a `CNAME` points to a service you already gave up — like an old S3 bucket, an expired Azure Cloud App, or a decommissioned Heroku dyno. An attacker registers the abandoned resource, and now they serve traffic from your subdomain.