XMLHttpRequest cannot load XXX, no 'Access-Control-Allow-Origin' header-PHP Chinese Network Q&A

tl;dr; About same-origin policy

I have a Grunt process that starts an instance of express.js server. This was all working perfectly fine until just now, but now it starts serving a blank page and displays the following in the error log in the developer console of Chrome (latest version):
XMLHttpRequest cannot load https://www.example.com/ There is no 'Access-Control-Allow-Origin' header on the requested resource. Therefore, access from 'http://localhost:4300' is not allowed.
What's preventing me from accessing this page?
app.options('/url...', function(req, res, next){ res.header('Access-Control-Allow-Origin', "*"); res.header('Access-Control-Allow-Methods', 'POST'); res.header("Access-Control-Allow-Headers", "accept, content-type"); res.header("Access-Control-Max-Age", "1728000"); return res.sendStatus(200); });
About the Same Origin Policy
This is theSame Origin Policy. It is a security feature implemented by browsers.
Your particular case is showing how it is implemented for XMLHttpRequest (and you'll get identical results if you were to use fetch), but it also applies to other things (such as images loaded onto aor documents loaded into an
</code>), just with slightly different implementations.</p>
          <p>The standard scenario that demonstrates the need for the SOP can be demonstrated with<a href="https://en.wikipedia.org/wiki/Alice_and_Bob" rel="nofollow noreferrer">three characters</a>:</p>
          <ul>
           <li>Alice is a person with a web browser</li>
           <li>Bob runs a website (<code>https://www.example.com/</code>in your example)</li>
           <li>Mallory runs a website (<code>http://localhost:4300</code>in your example)</li>
          </ul>
          <p>Alice is logged into Bob's site and has some confidential data there. Perhaps it is a company intranet (accessible only to browsers on the LAN), or her online banking (accessible only with a cookie you get after entering a username and password).</p>
          <p>Alice visits Mallory's website which has some JavaScript that causes Alice's browser to make an HTTP request to Bob's website (from her IP address with her cookies, etc). This could be as simple as using<code>XMLHttpRequest</code>and reading the<code>responseText</code>.</p>
          <p>The browser's Same Origin Policy prevents that JavaScript from reading the data returned by Bob's website (which Bob and Alice don't want Mallory to access). (Note that you can, for example, display an image using an<code><img></code>element across origins because the content of the image is not exposed to JavaScript (or Mallory) … unless you throw canvas into the mix in which case you<em>will</em>generate a same-origin violation error).</p>
          <hr>
          <h1>Why the Same Origin Policy applies when you don't think it should</h1>
          <p>For any given URL it is possible that the SOP is not needed. A couple of common scenarios where this is the case are:</p>
          <ul>
           <li>Alice, Bob, and Mallory are the same person.</li>
           <li>Bob is providing entirely public information</li>
          </ul>
          <p>… but the browser has no way of knowing if either of the above is true, so trust is not automatic and the SOP is applied. Permission has to be granted explicitly before the browser will give the data it has received from Bob to some other website.</p>
          <hr>
          <h1>Why the Same Origin Policy applies to JavaScript in a web page but little else</h1>
          <h2>Outside the web page</h2>
          <p>Browser extensions<code>*</code>, the Network tab in browser developer tools, and applications like Postman are installed software. They aren't passing data from one website to the JavaScript belonging to a different website<em>just because you visited that different website</em>. Installing software usually takes a more conscious choice.</p>
          <p>There isn't a third party (Mallory) who is considered a risk.</p>
          <p><code>*</code>Browser extensions do need to be written carefully to avoid cross-origin issues.<a href="https://www.chromium.org/Home/chromium-security/extension-content-script-fetches" rel="nofollow noreferrer">See the Chrome documentation for example</a>.</p>
          <h2>Inside the webpage</h2>
          <p>Most of the time, there isn't a great deal of information leakage when just showing something on a webpage.</p>
          <p>If you use an<code><img></code>element to load an image, then it gets shown on the page, but very little information is exposed to Mallory. JavaScript can't read the image (unless you use a<code>crossOrigin</code>attribute to explicitly enable request permission with CORS) and then copy it to her server.</p>
          <p>That said, some information does leak so, to<a href="https://github.com/whatwg/html/issues/1888" rel="nofollow noreferrer">quote Domenic Denicola (of Google)</a>:</p>
          <p>This is why you need CORS permission to load fonts across origins.</p>
          <hr>
          <h1>Why you can display data on the page without reading it with JS</h1>
          <p>There are a number of circumstances where Mallory's site can cause a browser to fetch data from a third party and display it (e.g. by adding an<code><img></code>element to display an image). It isn't possible for Mallory's JavaScript to read the data in that resource though, only Alice's browser and Bob's server can do that, so it is still secure.</p>
          <hr>
          <h1>CORS</h1>
          <p>The<code>Access-Control-Allow-Origin</code>HTTP<strong>response</strong>header referred to in the error message is part of the<a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS" rel="nofollow noreferrer">CORS</a>standard which allows Bob to explicitly grant permission to Mallory's site to access the data via Alice's browser.</p>
          <p>A basic implementation would just include:</p>
          <pre class="brush:php;toolbar:false;">Access-Control-Allow-Origin: *</pre>
          <p>… in the response headers to permit any website to read the data.</p>
          <pre class="brush:php;toolbar:false;">Access-Control-Allow-Origin: http://example.com</pre>
          <p>… would allow only a specific site to access it, and Bob can dynamically generate that based on the<code>Origin</code><em>request</em>header to permit multiple, but not all, sites to access it.</p>
          <p>The specifics of how Bob sets that response header depend on Bob's HTTP server and/or server-side programming language. Users of Node.js/Express.js should use the<a href="https://www.npmjs.com/package/cors" rel="nofollow noreferrer">well-documented CORS middleware</a>. Users of other platforms should take a look at this<a href="https://enable-cors.org/server.html" rel="nofollow noreferrer">collection of guides for various common configurations</a>that might help.</p>
          <p>NB: Some requests are complex and send a<a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS" rel="nofollow noreferrer">preflight</a>OPTIONS request that the server will have to respond to before the browser will send the GET/POST/PUT/Whatever request that the JS wants to make. Implementations of CORS that only add<code>Access-Control-Allow-Origin</code>to specific URLs often get tripped up by this.</p>
          <hr>
          <p>Obviously granting permission via CORS is something Bob would only do only if either:</p>
          <ul>
           <li>The data was not private<em>or</em></li>
           <li>Mallory was trusted</li>
          </ul>
          <hr>
          <h2>How do I add these headers?</h2>
          <p>It depends on your server-side environment.</p>
          <p>If you can, use a library designed to handle CORS as they will present you with simple options instead of having to deal with everything manually.</p>
          <p><a href="https://enable-cors.org/server.html" rel="nofollow noreferrer">Enable-Cors.org</a>has a list of documentation for specific platforms and frameworks that you might find useful.</p>
          <h2>But I'm not Bob!</h2>
          <p>There is no standard mechanism for<em>Mallory</em>to add this header because it has to come from Bob's website, which she does not control.</p>
          <p>If Bob is running a public API then there might be a mechanism to turn on CORS (perhaps by formatting the request in a certain way, or a config option after logging into a Developer Portal site for Bob's site). This will have to be a mechanism implemented by Bob though. Mallory could read the documentation on Bob's site to see if something is available, or she could talk to Bob and ask him to implement CORS.</p>
          <hr>
          <h1>Error messages which mention "Response for preflight"</h1>
          <p>Some cross-origin requests are<a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS" rel="nofollow noreferrer">preflighted</a>.</p>
          <p>This happens when (roughly speaking) you try to make a cross-origin request that:</p>
          <ul>
           <li>Includes credentials like cookies</li>
           <li>Couldn't be generated with a regular HTML form (e.g. sets a Content-Type that you couldn't use in a form's<code>enctype</code>or some other request header(s)).</li>
          </ul>
          <h2>If you are correctly doing something that needs a preflight</h2>
          <p>In these cases then<strong>the rest of this answer still applies</strong>but you also need to make sure that the server can listen for the preflight request (which will be<code>OPTIONS</code>(and not<code>GET</code>,<code>POST</code>, or whatever you were trying to send) and respond to it with the right<code>Access-Control-Allow-Origin</code>header but also<code>Access-Control-Allow-Methods</code>and<code>Access-Control-Allow-Headers</code>to allow your specific HTTP methods or headers.</p>
          <h2>If you are triggering a preflight by mistake</h2>
          <p>Sometimes people make mistakes when trying to construct Ajax requests, and sometimes these trigger the need for a preflight. If the API is designed to allow cross-origin requests but doesn't require anything that would need a preflight, then this can break access.</p>
          <p>Common mistakes that trigger this include:</p>
          <ul>
           <li>trying to put<code>Access-Control-Allow-Origin</code>and other CORS response headers on the request. These don't belong on the request, don't do anything helpful (what would be the point of a permissions system where you could grant yourself permission?), and must appear only on the response.</li>
           <li>trying to put a<code>Content-Type: application/json</code>header on a GET request that has no request body the content of which to describe (typically when the author confuses<code>Content-Type</code>and<code>Accept</code>).</li>
          </ul>
          <p>In either of these cases, removing the extra request header will often be enough to avoid the need for a preflight (which will solve the problem when communicating with APIs that support simple requests but not preflighted requests).</p>
          <hr>
          <h1>Opaque responses (<code>no-cors</code>mode)</h1>
          <p>Sometimes you need to make an HTTP request, but you don't need to read the response. e.g. if you are posting a log message to the server for recording.</p>
          <p>If you are using<a href="https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch" rel="nofollow noreferrer">the<code>fetch</code>API</a>(rather than<code>XMLHttpRequest</code>), then you can configure it to not try to use CORS.</p>
          <p>Note that<strong>this won't let you do anything that you require CORS to do</strong>.<strong>You will not be able to read the response</strong>. You will not be able to make a request that requires a preflight.</p>
          <p>It will let you make a simple request, not see the response, and not fill the Developer Console with error messages.</p>
          <p>How to do it is explained by the Chrome error message given when you make a request using<code>fetch</code>and don't get permission to view the response with CORS:</p>
          <p>Thus:</p>
          <pre class="brush:php;toolbar:false;">fetch("http://example.com", { mode: "no-cors" });</pre>
          <hr>
          <h1>Alternatives to CORS</h1>
          <h2>JSONP</h2>
          <p>Bob could also provide the data using a hack like<a href="https://stackoverflow.com/questions/2067472/what-is-jsonp-all-about">JSONP</a>which is how people did cross-origin Ajax before CORS came along.</p>
          <p>It works by presenting the data in the form of a JavaScript program that injects the data into Mallory's page.</p>
          <p>It requires that Mallory trust Bob not to provide malicious code.</p>
          <p>Note the common theme: The site providing the data has to tell the browser that it is OK for a third-party site to access the data it is sending to the browser.</p>
          <p>Since JSONP works by appending a<code><script></code>element to load the data in the form of a JavaScript program that calls a function already in the page, attempting to use the JSONP technique on a URL that returns JSON will fail — typically with a CORB error — because JSON is not JavaScript.</p>
          <h2>Move the two resources to a single Origin</h2>
          <p>If the HTML document the JS runs in and the URL being requested are on the same origin (sharing the same scheme, hostname, and port) then the Same Origin Policy grants permission by default. CORS is not needed.</p>
          <h2>A Proxy</h2>
          <p>Mallory<em>could</em>use server-side code to fetch the data (which she could then pass from her server to Alice's browser through HTTP as usual).</p>
          <p>It will either:</p>
          <ul>
           <li>add CORS headers</li>
           <li>convert the response to JSONP</li>
           <li>exist on the same origin as the HTML document</li>
          </ul>
          <p>That server-side code could be written & hosted by a third party (such as CORS Anywhere). Note the privacy implications of this: The third party can monitor who proxies what across their servers.</p>
          <p>Bob wouldn't need to grant any permissions for that to happen.</p>
          <p>There are no security implications here since that is just between Mallory and Bob. There is no way for Bob to think that Mallory is Alice and to provide Mallory with data that should be kept confidential between Alice and Bob.</p>
          <p>Consequently, Mallory can only use this technique to read<em>public</em>data.</p>
          <p>Do note, however, that taking content from someone else's website and displaying it on your own might be a violation of<em>copyright</em>and open you up to legal action.</p>
          <h2>Writing something other than a web app</h2>
          <p>As noted in the section "Why the Same Origin Policy only applies to JavaScript in a web page", you can avoid the SOP by not writing JavaScript in a webpage.</p>
          <p>That doesn't mean you can't continue to use JavaScript and HTML, but you could distribute it using some other mechanism, such as Node-WebKit or PhoneGap.</p>
          <h1>Browser extensions</h1>
          <p>It is possible for a browser extension to inject the CORS headers in the response before the Same Origin Policy is applied.</p>
          <p>These can be useful for development but are not practical for a production site (asking every user of your site to install a browser extension that disables a security feature of their browser is unreasonable).</p>
          <p>They also tend to work only with simple requests (failing when handling preflight OPTIONS requests).</p>
          <p>Having a proper development environment with a local development<em>server</em>is usually a better approach.</p>
          <hr>
          <h1>Other security risks</h1>
          <p>Note that SOP / CORS do not mitigate<a href="https://en.wikipedia.org/wiki/Cross-site_scripting" rel="nofollow noreferrer">XSS</a>,<a href="https://en.wikipedia.org/wiki/Cross-site_request_forgery" rel="nofollow noreferrer">CSRF</a>, or<a href="https://en.wikipedia.org/wiki/SQL_injection" rel="nofollow noreferrer">SQL Injection</a>attacks which need to be handled independently.</p>
          <hr>
          <h1>Summary</h1>
          <ul>
           <li>There is nothing you can do in<em>your</em>client-side code that will enable CORS access to someone<em>else's</em>server.</li>
           <li>If you control the server the request is being made to: Add CORS permissions to it.</li>
           <li>If you are friendly with the person who controls it: Get them to add CORS permissions to it.</li>
           <li>If it is a public service then (keeping in mind that<strong>most third-party APIs are designed to be interacted with only by your server-side code and do not support CORS</strong>but for those that are):
            <ul>
             <li>Read their API documentation to see what they say about accessing it with client-side JavaScript:
              <ul>
               <li>They might tell you to use specific URLs</li>
               <li>They might support JSONP instead of CORS</li>
               <li>They might not support cross-origin access from client-side code at all (this might be a deliberate decision on security grounds, especially if you have to pass a personalized API Key in each request).</li>
              </ul></li>
             <li>Make sure you aren't triggering a preflight request you don't need. The API might grant permission for simple requests but not preflighted requests.</li>
            </ul></li>
           <li>If none of the above apply: Get the browser to talk to<em>your</em>server instead, and then have your server fetch the data from the other server and pass it on. (There are also third-party hosted services that attach CORS headers to publically accessible resources that you could use).</li>
          </ul>
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