What is the difference between CORS and CSPs?

Question

From my perspective, the technologies referred to as Cross-Origin Resource Sharing (CORS) and Content Security Policies (CSPs) seem to be very similar in purpose and implementation.

Both seem to allow you to whitelist the origins of resources which an uncompromised version of your webpage incorporates, via HTTP response headers. The only difference I can see is that CSPs seem to be more fine-grained in what you can approve in your HTTP response.

Solution 1

CORS allows the Same Origin Policy to be relaxed for a domain.

e.g. normally if the user logs into both example.com and example.org, the Same Origin Policy prevents example.com from making an AJAX request to example.org/current_user/full_user_details and gaining access to the response.

This is the default policy of the web and prevents the user's data from being leaked when logged into multiple sites at the same time.

Now with CORS, example.org could set a policy to say it will allow the origin https://example.com to read responses made by AJAX. This would be done if both example.com and example.org are ran by the same company and data sharing between the origins is to be allowed in the user's browser. It only affects the client-side of things, not the server-side.

CSPs on the other hand set a policy of what content can run on the current site. For example, if JavaScript can be executed inline, or which domains .js files can be loaded from. This can be beneficial to act as another line of defence against XSS attacks, where the attacker will try and inject script into the HTML page. Normally output would be encoded, however say the developer had forgotten only on one output field. Because the policy is preventing in-line script from executing, the attack is thwarted.

Solution 2

CORS allows a site A to give permission to site B to read (potentially private) data from site A (using the visitor's browser and credentials).

CSP allows a site to prevent itself from loading (potentially malicious) content from unexpected sources (e.g. as a defence against XSS).

Solution 3

@JodySowald's comment, which I find more succinct:

Content-Security-Policy prevents calls to external resources and Cross-Origin-Resource-Sharing prevents calls from external sources. To provide an example. For example.com to show example.net in an iframe, example.com must not block example.net with its CSP settings and example.net must not block example.com with its CORS settings.

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Original answer:

None of the answers above give a clear and concise difference between CSP and CORS. Here is my way of thinking about them:

Let's say we have example.com website that wants to send a request to example.net.

1. When user visits example.com in browser, example.com server returns example.com HTTP response, CSP restriction within this response can prevent example.com in browser from issuing request to example.net.
2. If there is no CSP restriction within example.com HTTP response, then example.com in browser can send a request to example.net.
3. Upon receiving the request, example.net server responds with example.net HTTP response, CORS restriction within this response can prevent example.com in browser from loading it. (Note that by default, Same-origin policy will restrict the response from loading, unless otherwise specified by CORS)

So CSP protects example.com and same-origin policy (the lack of CORS) protects example.net in the example above.

Solution 4

CORS checks with the third party for authorization to use its services. So, the third party provides or denies authorization.

So for example if a page in www.example.com needs to make a request to www.example.org we need to send an OPTIONS request sent to www.example.org with Origin:www.example.com as a precursor to request for authorization. Now, www.example.org provides or denies authorization.

CSP prevents a webpage from inadvertently loading malicious content from a third party by specifying where a particular type of content can be loaded from. So, for example you can provide a valid source for each of the following scripts, css, media etc. by using directives

Example:

Content-Security-Policy: default-src 'none'; script-src 'self' www.google-analytics.com ajax.googleapis.com; connect-src 'self'; img-src 'self'; style-src 'self';

View more solutions

Comments

[JepZ]

Much clearer and to the point than all mozilla/html5rocks/w3c documents together. Thx.

[Vlas Bashynskyi]

can CSP be used on site A to prevent site A from reading data from site B? I am talking mainly about xhr requests.

[mathk]

Can I say that CORS is a subset of what can be done with CSP ?

[Quentin]

@mathk — No. As I said in the answer, they are completely different things.

[mathk]

I mean if you do not allow the browser to download content from site B you will also guaranty that nothing will be sent to site B using CSP ?

[Quentin]

@mathk — Site A preventing data being sent to Site B (which can be achieved with CSP) is completely different to Site B preventing Site A from reading data from it (which is the default behaviour but which can be relaxed with CORS).

[mathk]

Are we agree that: Site A preventing data being sent to Site B -> Browser can not do a GET[other verb] A. and ` Site B preventing Site A from reading data from it ` Browser can not do a POST B <some data> ?

[Quentin]

@mathk — No. Nothing is stopping JS on site A triggering a post request to site B. The Same Origin Policy just stops site A's JavaScript from reading the response.

[mathk]

Let us continue this discussion in chat.

[Steve]

In this example, wouldn't the Same Origin policy protect def.net, whereas CORS removes this protection if enabled?

[paradite]

@Steve you are right, it's kind of a misnomer, I've updated my answer to reflect it.

[Michael Terry]

When folks say these things are "completely different" that just makes the differences harder to understand because they aren't completely different. Same-origin policy (to which CORS is closely related) prevents reading cross-origin requests, the most common way a developer would come into contact with this policy. Likewise, one of the most common ways a developer would come into contact with CSP is because it is blocking cross-origin requests, e.g., in a greasemonkey script.

[Jody Sowald]

Is it a correct simplification to say that Content-Security-Policy prevents calls to external resources and Cross-Origin-Resource-Sharing prevents calls from external sources? to provide an example. For abc.com to show def.net in an iframe, abc.com must not block def.net with its CSP settings and def.net must not block abc.com with its CORS settings.

[paradite]

@JodySowald that would seem fitting.