Understanding "same-site" and "same-origin"
"same-site" and "same-origin" are frequently cited but often misunderstood terms. For example, they are mentioned in the context of page transitions, fetch() requests, cookies, opening popups, embedded resources, and iframes.
Origin #
"Origin" is a combination of a scheme (also known as the protocol, for example HTTP or HTTPS), hostname, and port (if specified). For example, given a URL of https://www.example.com:443/foo , the "origin" is https://www.example.com:443.
"same-origin" and "cross-origin" #
Websites that have the combination of the same scheme, hostname, and port are considered "same-origin". Everything else is considered "cross-origin".
| Origin A | Origin B | Explanation of whether Origin A and B are "same-origin" or "cross-origin" |
|---|---|---|
| https://www.example.com:443 | https://www.evil.com:443 | cross-origin: different domains |
| https://example.com:443 | cross-origin: different subdomains | |
| https://login.example.com:443 | cross-origin: different subdomains | |
| http://www.example.com:443 | cross-origin: different schemes | |
| https://www.example.com:80 | cross-origin: different ports | |
| https://www.example.com:443 | same-origin: exact match | |
| https://www.example.com | same-origin: implicit port number (443) matches |
Site #
Top-level domains (TLDs) such as .com and .org are listed in the Root Zone Database. In the example above, "site" is the combination of the TLD and the part of the domain just before it. For example, given a URL of https://www.example.com:443/foo , the "site" is example.com.
However, for domains such as .co.jp or .github.io, just using the TLD of .jp or .io is not granular enough to identify the "site". And there is no way to algorithmically determine the level of registrable domains for a particular TLD. That's why a list of "effective TLDs"(eTLDs) was created. These are defined in the Public Suffix List. The list of eTLDs is maintained at publicsuffix.org/list.
The whole site name is known as the eTLD+1. For example, given a URL of https://my-project.github.io , the eTLD is .github.io and the eTLD+1 is my-project.github.io, which is considered a "site". In other words, the eTLD+1 is the effective TLD and the part of the domain just before it.
"same-site" and "cross-site" #
Websites that have the same eTLD+1 are considered "same-site". Websites that have a different eTLD+1 are "cross-site".
| Origin A | Origin B | Explanation of whether Origin A and B are "same-site" or "cross-site" |
|---|---|---|
| https://www.example.com:443 | https://www.evil.com:443 | cross-site: different domains |
| https://login.example.com:443 | same-site: different subdomains don't matter | |
| http://www.example.com:443 | same-site: different schemes don't matter | |
| https://www.example.com:80 | same-site: different ports don't matter | |
| https://www.example.com:443 | same-site: exact match | |
| https://www.example.com | same-site: ports don't matter |
"schemeful same-site" #
The definition of "same-site" is evolving to consider the URL scheme as part of the site in order to prevent HTTP being used as a weak channel. As browsers move to this interpretation you may see references to "scheme-less same-site" when referring to the older definition and "schemeful same-site" referring to the stricter definition. In that case, http://www.example.com and https://www.example.com are considered cross-site because the schemes don't match.
| Origin A | Origin B | Explanation of whether Origin A and B are "schemeful same-site" |
|---|---|---|
| https://www.example.com:443 | https://www.evil.com:443 | cross-site: different domains |
| https://login.example.com:443 | schemeful same-site: different subdomains don't matter | |
| http://www.example.com:443 | cross-site: different schemes | |
| https://www.example.com:80 | schemeful same-site: different ports don't matter | |
| https://www.example.com:443 | schemeful same-site: exact match | |
| https://www.example.com | schemeful same-site: ports don't matter |
How to check if a request is "same-site", "same-origin", or "cross-site" #
Chrome sends requests along with a Sec-Fetch-Site HTTP header. No other browsers support Sec-Fetch-Site as of April 2020. This is part of a larger Fetch Metadata Request Headers proposal. The header will have one of the following values:
cross-sitesame-sitesame-originnone
By examining the value of Sec-Fetch-Site, you can determine if the request is "same-site", "same-origin", or "cross-site" ("schemeful-same-site" is not captured in Sec-Fetch-Site).