Why both no-cache and no-store should be used in HTTP response?

Question

I'm told to prevent user-info leaking, only "no-cache" in response is not enough. "no-store" is also necessary.

Cache-Control: no-cache, no-store

After reading this spec http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html, I'm still not quite sure why.

My current understanding is that it is just for intermediate cache server. Even if "no-cache" is in response, intermediate cache server can still save the content to non-volatile storage. The intermediate cache server will decide whether using the saved content for following request. However, if "no-store" is in the response, the intermediate cache sever is not supposed to store the content. So, it is safer.

Is there any other reason we need both "no-cache" and "no-store"?

Answer 1

I must clarify that no-cache does not mean do not cache. In fact, it means "revalidate with server" before using any cached response you may have, on every request.

must-revalidate, on the other hand, only needs to revalidate when the resource is considered stale.

If the server says that the resource is still valid then the cache can respond with its representation, thus alleviating the need for the server to resend the entire resource.

no-store is effectively the full do not cache directive and is intended to prevent storage of the representation in any form of cache whatsoever.

I say whatsoever, but note this in the RFC 2616 HTTP spec:

History buffers MAY store such responses as part of their normal operation

But this is omitted from the newer RFC 7234 HTTP spec in potentially an attempt to make no-store stronger, see:

http://tools.ietf.org/html/rfc7234#section-5.2.1.5

Answer 2

Under certain circumstances, IE6 will still cache files even when Cache-Control: no-cache is in the response headers.

The W3C states of no-cache:

If the no-cache directive does not specify a field-name, then a cache MUST NOT use the response to satisfy a subsequent request without successful revalidation with the origin server.

In my application, if you visited a page with the no-cache header, then logged out and then hit back in your browser, IE6 would still grab the page from the cache (without a new/validating request to the server). Adding in the no-store header stopped it doing so. But if you take the W3C at their word, there's actually no way to control this behavior:

History buffers MAY store such responses as part of their normal operation.

General differences between browser history and the normal HTTP caching are described in a specific sub-section of the spec.

Answer 3

From the HTTP 1.1 specification:

no-store:

The purpose of the no-store directive is to prevent the inadvertent release or retention of sensitive information (for example, on backup tapes). The no-store directive applies to the entire message, and MAY be sent either in a response or in a request. If sent in a request, a cache MUST NOT store any part of either this request or any response to it. If sent in a response, a cache MUST NOT store any part of either this response or the request that elicited it. This directive applies to both non- shared and shared caches. "MUST NOT store" in this context means that the cache MUST NOT intentionally store the information in non-volatile storage, and MUST make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it. Even when this directive is associated with a response, users might explicitly store such a response outside of the caching system (e.g., with a "Save As" dialog). History buffers MAY store such responses as part of their normal operation. The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about accidental releases of information via unanticipated accesses to cache data structures. While the use of this directive might improve privacy in some cases, we caution that it is NOT in any way a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.

Answer 4

If you want to prevent all caching (e.g. force a reload when using the back button) you need:

• no-cache for IE

• no-store for Firefox

There's my information about this here:

http://blog.httpwatch.com/2008/10/15/two-important-differences-between-firefox-and-ie-caching/

Answer 5

no-store should not be necessary in normal situations, and can harm both speed and usability. It is intended for use where the HTTP response contains information so sensitive it should never be written to a disk cache at all, regardless of the negative effects that creates for the user.

How it works:

• Normally, even if a user agent such as a browser determines that a response shouldn't be cached, it may still store it to the disk cache for reasons internal to the user agent. This version may be utilised for features like "view source", "back", "page info", and so on, where the user hasn't necessarily requested the page again, but the browser doesn't consider it a new page view and it would make sense to serve the same version the user is currently viewing.

• Using no-store will prevent that response being stored, but this may impact the browser's ability to give "view source", "back", "page info" and so on without making a new, separate request for the server, which is undesirable. In other words, the user may try viewing the source and if the browser didn't keep it in memory, they'll either be told this isn't possible, or it will cause a new request to the server. Therefore, no-store should only be used when the impeded user experience of these features not working properly or quickly is outweighed by the importance of ensuring content is not stored in the cache.

My current understanding is that it is just for intermediate cache server. Even if "no-cache" is in response, intermediate cache server can still save the content to non-volatile storage.

This is incorrect. Intermediate cache servers compatible with HTTP 1.1 will obey the no-cache and must-revalidate instructions, ensuring that content is not cached. Using these instructions will ensure that the response is not cached by any intermediate cache, and that all subsequent requests are sent back to the origin server.

If the intermediate cache server does not support HTTP 1.1, then you will need to use Pragma: no-cache and hope for the best. Note that if it doesn't support HTTP 1.1 then no-store is irrelevant anyway.

Answer 6

If a caching system correctly implements no-store, then you wouldn't need no-cache. But not all do. Additionally, some browsers implement no-cache like it was no-store. Thus, while not strictly required, it's probably safest to include both.

Answer 7

For chrome, no-cache is used to reload the page on a re-visit, but it still caches it if you go back in history (back button). To reload the page for history-back as well, use no-store. IE needs must-revalidate to work in all occasions.

So just to be sure to avoid all bugs and misinterpretations I always use

Cache-Control: no-store, no-cache, must-revalidate

if I want to make sure it reloads.

Answer 8

Note that Internet Explorer from version 5 up to 8 will throw an error when trying to download a file served via https and the server sending Cache-Control: no-cache or Pragma: no-cache headers.

See http://support.microsoft.com/kb/812935/en-us

The use of Cache-Control: no-store and Pragma: private seems to be the closest thing which still works.

Answer 9

Originally we used no-cache many years ago and did run into some problems with stale content with certain browsers... Don't remember the specifics unfortunately.

We had since settled on JUST the use of no-store. Have never looked back or had a single issue with stale content by any browser or intermediaries since.

This space is certainly dominated by reality of implementations vs what happens to have been written in various RFCs. Many proxies in particular tend to think they do a better job of "improving performance" by replacing the policy they are supposed to be following with their own.

Answer 10

Just to make things even worse, in some situations, no-cache can't be used, but no-store can:

http://faindu.wordpress.com/2008/04/18/ie7-ssl-xml-flex-error-2032-stream-error/

Answer 11

OWASP discusses this:

What's the difference between the cache-control directives: no-cache, and no-store?

The no-cache directive in a response indicates that the response must not be used to serve a subsequent request i.e. the cache must not display a response that has this directive set in the header but must let the server serve the request. The no-cache directive can include some field names; in which case the response can be shown from the cache except for the field names specified which should be served from the server. The no-store directive applies to the entire message and indicates that the cache must not store any part of the response or any request that asked for it.

Am I totally safe with these directives?

No. But generally, use both Cache-Control: no-cache, no-store and Pragma: no-cache, in addition to Expires: 0 (or a sufficiently backdated GMT date such as the UNIX epoch). Non-html content types like pdf, word documents, excel spreadsheets, etc often get cached even when the above cache control directives are set (although this varies by version and additional use of must-revalidate, pre-check=0, post-check=0, max-age=0, and s-maxage=0 in practice can sometimes result at least in file deletion upon browser closure in some cases due to browser quirks and HTTP implementations). Also, 'Autocomplete' feature allows a browser to cache whatever the user types in an input field of a form. To check this, the form tag or the individual input tags should include 'Autocomplete="Off" ' attribute. However, it should be noted that this attribute is non-standard (although it is supported by the major browsers) so it will break XHTML validation.

Source here.