On Sun, Nov 21, 2010 at 10:41 PM, Mark Nottingham <m...@mnot.net> wrote: > I looked at the most recent specification and noted a few things. Apologies > if these are repeats of earlier feedback, but they're still outstanding. > > > 1) POST is listed as a "simple method," thereby exempting POST from the > "preflight" check. I understand that the rationale for this is that POST > already can be automatically generated (by form.submit() APIs), but this does > not justify opening a new security hole in the Web; it only makes closing the > existing ones more difficult.
As someone who would love to disable cross-site POSTing, and someone who actually has the ability to try to experiment with this on one of the most used user agents, I have to disagree. The amount of content out there that depends on cross-site POSTing using forms is enormous. The amount of content out there that depends on cross-site POSTing using CORS is insignificant. Trying to disable the former would be such an enormous undertaking that we wouldn't even notice the extra amount of work due to the latter. I've had this exact discussion at mozilla a couple of times and it basically always goes like this: other person: Hmm.. we might want to disable cross-site posting for forms some day, so is it such a good idea that cors enables it? me: If we do disable it for forms we'll just disable it for cors too. So much content will break for forms that the cors breakage won't be what we're concerned about. other person: Yeah, true. > 3) When a server changes the headers in a response based upon the value of > the incoming Origin header (as outlined in sections 5.1 and 5.2), it must > insert Vary: Origin into *all* responses for that resource; otherwise, > downstream caches will incorrectly store it. > > Be aware that doing so will cause many versions of IE not to cache those > responses at all. Another option would be to disallow varying the response > based upon the Origin header. Disallowing varying by origin seems like a bigger problem than IE not caching. > 5) Using a "preflight" check in combination with a cache exposes sites to DNS > rebinding, man-in-the-middle, and potentially other attacks that did not > exist before. This should be noted. The DNS rebinding issue is a quality of implementation issue. It's no problem simply rerequesting the preflight if the DNS resolves to a different IP between the preflight and the actual request. I agree that noting this in spec is a good idea. Could you describe the new man-in-the-middle attacks which did not exist before with cross-origin communications? > 6) Requiring a preflight check per-URI is not an efficient use of network > resources for applications that use a large number of URIs, as is becoming > more prevalent; effectively, it introduces another round-trip for each unsafe > request. Handling OPTIONS is also somewhat specialised on many servers. It's > also awkward to handle OPTIONS per-URI on many servers. I've raised this > several times before, and am still not convinced that the underlying > requirement (#8) justifies such a convoluted and ill-concieved design, or > indeed is effectively met by this design. > > Allowing a site to define a 'map' of where cross-origin requests are allowed > to go would be more efficient in most cases, would be vastly simpler to > implement for servers, and would be similar to many other site-wide policy > mechanisms on the Web. We had a design in place which allowed preflights to apply to multiple URIs. However there were too many issues with servers resolving URIs in weird ways which made us drop it. One concrete example was that some versions of IIS UTF8 decoded URIs and then ignored bits above the lower 8 bits. This made it treat URIs as if they contained ".." when the browser had no idea of this. In short, CORS felt like the wrong spec to start relying on servers not to do strange URI handling. / Jonas