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Subdomain takeover is a form of cyberattack in which an attacker gains control over a subdomain of a target domain. An attacker’s objectives in a subdomain takeover might include serving content on the vulnerable subdomain, reading cookies from the primary domain, performing cross-site scripting, or circumventing the primary domain’s content security policies. Subdomain takeover attacks most often happen when the subdomain has a canonical name (i.e., a CNSME) in the DNS record, but no host provides content. For example, this situation can happen when setting up or removing a subdomain if the content has not yet been published or removed (without also removing the CNAME reference). Under these circumstances, an attacker can execute a subdomain takeover by providing their own virtual host and then hosting their own content for it.
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