Full Title or Meme
SameSite is an attribute in HTTP that is applied to the Cookie header.
- New HTTP headers focus on Cross-Origin iFrames which complicates the terms sites and origins which is further explained on that wiki page.
Description of Impact
This is largely from Brock Allen (see below) and D Waite at Ping.
There is another leg to this workflow, though, which is affected by the scenario you describe Brian. And it's what breaks the login workflow (cross-site). My observations are from trying to reverse engineer what the browser is doing, so I don't know if it's intended. Also, it's complicated and I've always had a hard time explaining what I think is happening, so apologies in advance.
During this workflow, if the RP then issues its own SameSite=Strict (and Lax too IIRC) session cookie on that exact POST response and also emits a 302 elsewhere in the RP, then on that next request the browser makes it will omit sending the cookie. I think it does this because it detects that you're in the middle of a long redirect chain which originated at the OP and has crossed the line into the RP (cross-site). The end result is that it won't send the RP session cookie on that final request, and from the end user's (and RP's) perspective this last request is anonymous. I know this is complicated to follow, so apologies.
Interestingly, though, if the user were to just hit refresh in the browser or manually navigate to somewhere in the RP, then the cookie is sent normally since it's the user that initiates the request. IOW, on the POST cross-site, the browser accepts the cookie just fine. It's just that it doesn't send it on the request because the request chain looks initiated from cross-site.
SameSite applies in navigation capacities as well, e.g. the first HTTP request to the new site on link, redirect, or post. None and the temporary chromium default workaround behavior would be the only ones that would send state along with a HTTP POST. A world with just lax and strict would require more complex server acrobatics.
I’m (also) not Sam, but I can try to supply a bit of color:
SameSite is meant to limit state outside first-party contexts, e.g. limit XSRF-style attacks by having the browser not send state for request originating from outside your domain. It was NOT intended as a privacy restriction.
The current behaviors of SameSite are:
|None||always send cookies|
|Lax|| Send on ’Safe’ requests (GET, HEAD, OPTIONS)