Hacker News Reader

Comment by pak | Hacker News Reader

The difference between GET and POST is interface-related, not security-related. A better example for why you avoid GET for non-idempotent operations is to prevent a webcrawler from coming along and innocently making massive changes to your site just by following links. Security-wise, there's little difference between the two, it's trival for the client to change the type of request. (If I can't get some guy on a forum by embedding a picture, I'll send him a link to some website I control where there's a hidden <form> that posts to the site I want.) CSRF is only preventable by putting secret tokens in your forms that get echoed in the submission request (or by double-submitting the cookie as I do, but tptacek will be in here shortly to tell me why that is so silly).

View on HN

Nope, that's exactly what I came here to say. Thanks!

People conflate CSRF-ability with destructive GET operations because a lot of apps that are pervasively CSRF-able also happen to have lots of inadvertant destructive GET's, but the bad GET endpoints are a reliability flaw more than a security flaw.

There is one security difference. POST doesn't leave the data in the URL bar at the top of the browser. I sure would rather a site submit my credit card via POST than via GET.

In addition to tzs' comment above, POST parameters do not get logged by the web server either (default Apache logging, etc), unless you're using a module like mod_security to log POST data.

Besides, don't use GET for non-idempotent requests (those which change state). That's just webapp development 101.

Edited for clarity.

Either I'm completely misunderstanding your grammar, or you don't understand what idempotent means... idempotent and state-changing are almost opposites.

You don't understand what idempotent means. An action is idempotent if repeating it does not change the result. For example, multiplying by zero is idempotent, 0 * x is the same as 0 * 0 * 0 * 0 * x. But of course it changes something, as 0 * x is clearly not x in the general case.

In the http case, GET and HEAD should not change any state (be safe and idempotent), PUT and DELETE should change state but be idempotent, and POST can do whatever it wants. http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html

Maybe you saw the text after @marcinw changed his text? I agree with you, of course I will say that most http GET actions I see are trivially idempotent -- that is to say they are idempotent because they don't change the state at all.

No, I saw the original version that cautioned against state changing idempotent GET requests. The changed version is actually worse, as it only cautions against non-idempotent GETs, which is not enough. State changing idempotent requests via GET are quite as bad.

In fact, idempotency is irrelevant for GET, as GET requests should be safe, which trivially implies that they are idempotent. So requiring idempotency in addition to safety does nothing except making the prose harder to read.

These comments seem at odds:

  > GET and HEAD should not change any state
  > (be safe and idempotent)

  > But of course it changes something, as
  > 0 * x is clearly not x in the general case.

If 0 * x is idempotent, but it changes things then how can 'should not change state' be equivalent to idempotent? If I use a GET request to submit a shopping cart (for example), as long as submitting that URL a second time doesn't placee a second order for a copy of the original shopping cart (maybe it has a unique identifier so the backend can tell that it's already been submitted), then that should be idempotent, right?

Using GET to submit a shopping cart can be "idempotent", but it is not "safe" since it changes state on the server. GET should always be both safe and idempotent.

Right, but the idempotence is a side effect of the safety here. It's not the main point.

Sorry for the confusion, I meant "non-idempotent". I had two separate thoughts in my head at once and munged the two, leaving out the "non-".

Sorry, but your "correction" made your statement even more inaccurate. You should have elided the scary latin "idempotent" and kept "state changing", as the latter is what GET requests must avoid. GET http://example.com/index?action=delete;recursive=true is idempotent, but usually not what you want to see in action on your site...

That's true 99.99% of the time.

However, just to play devil's advocate, there is a class of attacks based around injecting code into logfiles since they'll log the query string of a GET request. If you can then get the server to say include that logfile as a php file, it will execute the embedded PHP.

Of course the real vuln isn't logging GET params (it's convincing the server to include the logfile as .php), but I thought it was worth pointing out.

Your `hidden form` exploit is moot for the example in the post

I think that in 2011, checking referer headers can be considered an acceptable form of CSRF prevention. Some very old browsers and plugins allow referer headers to be spoofed, but by now so many other vulnerabilities have been found in all those that if you're still using them then CSRF is the least of your problems.

Could someone explain to me briefly the method with which a token would be used, either in GET or POST?

Indeed. I actually just finished implementing the secret token method throughout my app less than a minute ago. I have fullscreen and mobile versions... and without going into too much detail on exactly why in my particular case, it's cleaner and easier to use simple links (i.e., GET) for something like removing items from lists... so the token method is the only feasible solution to prevent something like I mentioned in my other comment (fake images and what not). But with the fullscreen version, since it's all AJAX and to make things easy on myself, I converted everything (except file uploads) to send data via POST (all links are generated by a function so instead of href it's onclick+ajax)... along with the token too of course... which was all a win-win because I lifted the data limit that GET used to impose as well.

But anyway... today I learned that even though GET and POST are no different security-wise... always use a secret token when security is an issue... and it's best to use POST because it allows MUCH more data than GET.

Reply on news.ycombinator.com