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Comment by Andrew_nenakhov | Hacker News Reader

Wouldn't usercontent.github.com work just as well?

Script running on usercontent.github.com:

- is allowed to set cookies scoped to *.github.com, interfering with cookie mechanisms on the parent domain and its other subdomains, potentially resulting in session fixation attacks

- will receive cookies scoped to *.github.com. In IE, cookies set from a site with address "github.com" will by default be scoped to *.github.com, resulting in session-stealing attacks. (Which is why it's traditionally a good idea to prefer keeping 'www.' as the canonical address from which apps run, if there might be any other subdomains at any point.)

So if you've any chance of giving an attacker scripting access into that origin, best it not be a subdomain of anything you care about.

A completely separate domain is more secure because it's impossible to mess up. From the browser's point of view githubusercontent.com is completely unrelated to github.com, so there's literally nothing github could accidentally do or a hacker could maliciously do with the usercontent site that would grant elevated access to the main site. Anything they could do is equally doable with their own attacker-controlled domain.

I think one reason is that a subdomain of github.com (like username.github.com) might be able to read and set cookies that are shared with the main github.com domain. There are ways to control this but using a different domain (github.io is the one I'm familiar with) creates wider separation and probably helps reduce mistakes.

I read about this a while back but I can't find the link anymore (and it's not the same one that op pointed to).

client browsers have no "idea" of subdomains, either. if i have example.com login saved, and also a one.example.com and a two.example.com, a lot of my browsers and plugins will get weird about wanting to save that two.example.com login as a separate entity. I run ~4 domains so i use a lot of subdomains, and the root domain (example.com) now has dozens of passwords saved. I stand up a new service on three.example.com and it will suggest some arbitrary subset of those passwords from example.com, one.example.com, two.example.com.

Imagine if eg.com allowed user subdomains, and some users added logins to their subdomains for whatever reason, there's a potential for an adversarial user to have a subdomain and just record all logins attempted, because browsers will automagically autofill into any subdomain.

if you need proof i can take a screenshot, it's ridiculous, and i blame google - it used to be the standard way of having users on your service, and then php and apache rewrite style usage made example.com/user1 more common than user1.example.com.