REST API GET with sensitive data
I'm designing api with method that should be an idempotent, and should not modify any data on the server. It should be method that process request and return response for given parameters.
One of the parameters is sensitive data. It's not an option to use additional encryption. Data is already encrypted, but security requirements are very demanding and even encrypted data should be treated very carefully.
According to REST spec, idempotent query method should be implemented as a GET HTTP method. Problem in this case is sensitive data that shouldn't be pass as a GET parameter in URL. Only option in HTTP standard is to pass sensitive data in a body part of HTTP request.
My question is what is better? Broke rest api design, and send query request as a POST, or pass encrypted data in URL? Maybe is there better solution I don't see?
Answer
According to REST spec, idempotent query method should be implemented as a GET HTTP method.
2016
As far as I can tell with my limited English SHOULD != MUST. You won't break REST API design by sending a POST in this case. You can send your sensitive data in a HTTP header if that is possible. And ofc. you should use HTTPS if you want to send sensitive data to anywhere.
2019
I checked the HTTP 1.1 standard meanwhile. They don't explicitly use the MUST or SHOULD words in the specs for idempotency, but I got the impression they mean SHOULD. Another HTTP related thing here, that we use GET mostly because we can cache response with it. You don't necessarily want to cache sensitive data, so it might not make sense to insist on using GET on retrieval when security is more important by the parameters. You can find some tips about how to set cache-control headers here, but you can read the HTTP standard for that too.
From security perspective my non-expert opinion is the following:
Normally query parameters are not that sensitive, usually they are just random ids or keywords. So maybe the problem is with your design and you should hide these sensitive parameters (e.g. social security number) behind random ids instead of querying them explicitly. Another thought here, that user credentials must be in the Authorization header for example, not in the query string, so if the sensitive data is that kind, then you are doing it wrong.
As far as I understand the issue about sending sensitive data in URLs is that it can show up in browser history, cache, address bar and in server logs unencrypted. Even though many people call REST webservices directly from browser via AJAX (or the fetch API), that is not the intended way they should be used. Webservices are mostly for server side usage to scale out your application to multiple threads, cores or servers. So if you use a server side HTTP client which does not have history or cache to call the REST webservice programmatically, then all you need to do is encrypting your logs. If the client has cache, then you can encrypt that too if you feel it necessary. I think it is possible to filter these params from logs and store the cached content based on the salted hash of the URL, but I don't have much experience with that.
If you have a 3rd party client or a browser where you don't have that kind of control, then you can still assume that it follows the HTTP standard. So you can use the cache-control headers to disable cache for sensitive content. The address bar and history is not a problem by single page applications unless they move the sensitive data to there with the history API, but that can happen no matter what you do. It is possible to disable the Referrer header too. Only if you serve HTML with your webservice will you have a problem with browsers, because that assumes that javascript is disabled (so you cannot use location.replace to override browser history along with the sensitive querystring) and that the browser is your REST client. I think that is a very unlikely scenario, though it is possible to do it relative well with XML+XSL reusing most of the code or nowadays maybe with nodejs or some sort of transpiler on different languages.
So I think this can be solved even without POST if you do everything right. But this is just an opinion, I wait for security expert to correct me...