alt Hacker NewsWhy is this so, so common? They're subdomains. They're free. It's not hitting anybody's budget to publish a new DNS entry. If someone has permission to publish anything in your name, they probably should be able to go make themselves a subdomain.
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Because corporate security has nothing to do with security, and everything to do with adding bureaucracy roadblocks to legitimate activity.
Because they point foo.example.com to AWS. They then let whatever the CNAME is pointing to lapse. Then an attacker registers the lapsed AWS and can now put their content on your trusted domain.
https://aws.amazon.com/blogs/security/threat-tactic-spotligh...
(AWS have since fixed this problem, but it exists on other services.)
Counter-argument is: do you really want some team of 90% marketers and PMs throwing up a MVP/WIP codebase for some ancillary product not related to your core business on your core domain? At a minimum you'd want a thorough security review and risk assessment, and that goes against the ethos of "ship fast and pivot as needed".
Because web security has a lot of legacy crap. Such as cookies. Oh you added Domain=foo.com to your cookie? Now marketingvibecodedapp.foo.com can access the cookie from your main domain. What do you mean you thought adding the domain field restricts the cookie to just that domain? Lol no, it's the exact opposite, gotcha, dumbass.