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The short version of this issue is we are seeing the typical CORS error (x has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.) however we are absolutely sending the specified headers. The requests are fine to begin with however after n (pattern undetermined) amount of time SOME (no real pattern to this other than it's a random 1 or 2 assets referenced in the html file) requests will suddenly start failing. On a hard refresh or with disabling cache, the issue is resolved.
We're wondering how caching may affect CORS in this case? Or if the issue lies elsewhere?
What we see is the asset is loaded fine in the first instance.
Here's a cURL representation of what the browser (chrome, not tested elsewhere) sends to the server (cloudfront in front of s3):
curl -I 'https://assets-frontend.kalohq.ink/style.allapps.add899080acbbeed5bb6a7301d234b65.css' -H 'Referer: https://lystable.kalohq.ink/projects/2180?edit=true' -H 'Origin: https://lystable.kalohq.ink' -H 'DPR: 2' -H 'User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gec And the headers in response to this look like:
HTTP/1.1 200 OK Content-Type: text/css Content-Length: 5632 Connection: keep-alive Date: Wed, 28 Jun 2017 09:23:04 GMT Access-Control-Allow-Origin: * Access-Control-Allow-Methods: GET Access-Control-Max-Age: 3000 Last-Modified: Wed, 28 Jun 2017 09:16:15 GMT ETag: "ece4babc2509d989254638493ff4c742" Cache-Control: max-age=31556926 Content-Encoding: gzip Accept-Ranges: bytes Server: AmazonS3 Vary: Origin,Access-Control-Request-Headers,Access-Control-Request-Method Age: 3384 X-Cache: Hit from cloudfront Via: 1.1 adc13b6f5827d04caa2efba65479257c.cloudfront.net (CloudFront) X-Amz-Cf-Id: PcC2qL04aC4DPtNuwCudckVNM3QGhz4jiDL10IDkjIBnCOK3hxoMoQ== After this you can browse the site for a while, refresh a few times and everything is fine and dandy.
But then you might refresh and suddenly you see the error in console:
Access to CSS stylesheet at 'https://assets-frontend.kalohq.ink/style.allapps.add899080acbbeed5bb6a7301d234b65.css' from origin 'https://kalohq.ink' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'https://kalohq.ink' is therefore not allowed access. At this point if you hard-refresh or disable the cache and reload the page everything goes back to working. This is why we're pointing at browser caching behaviour playing with CORS at this point.
The HTML file loading these assets is as follows:
<!doctype html><html lang="en"><head><meta charset="utf-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><title>Kalo</title><meta name="description" content="Kalo is used by the best teams on the planet to onboard, manage, and pay their freelancers. "><meta name="viewport" content="width=device-width,initial-scale=1"><meta http-equiv="Accept-CH" content="Width,DPR,Save-Data"><script>window.performance&&"function"==typeof window.performance.mark&&window.performance.mark("start load bootstrap"),console.log("Kalo v0.214.1 🎉")</script><script type="text/javascript" crossorigin="anonymous">window.webpackManifest={0:"moment-timezone-data.8189aab661847dea1b73.chunk.js",1:"1.7645e36f0742ed31139b.chunk.js",2:"2.bf0a1c9b400d715e3138.chunk.js",3:"3.d077b7a1cede6f6960e6.chunk.js",4:"4.0bbd51f182d8fa3f4951.chunk.js",5:"5.1dcf124ea7874546fc7a.chunk.js",6:"6.85ee04326ef5cfe2c084.chunk.js",7:"7.cf718eabaa3814fcb47c.chunk.js",8:"8.4c4c5b070e09afe037a1.chunk.js",9:"9.ba3b9a5f540f057fca46.chunk.js",10:"10.3c850061770df8801575.chunk.js",11:"11.df971dd9c4ab435fd421.chunk.js",12:"12.81905afa591a4796dcfc.chunk.js",13:"13.0f78c0c77d45cd79ac26.chunk.js",14:"14.f8f9f24d15e1cc4372a1.chunk.js",15:"15.6badd92530b5da668e98.chunk.js",16:"16.ef87b8dc2f87ca2d40a1.chunk.js",17:"17.bf842b852470057c4f0b.chunk.js",18:"18.f091321e6a0bbf16bf1f.chunk.js",19:"19.0297861a162b49308887.chunk.js",20:"20.7281da4b01eb4eb4bf1f.chunk.js",21:"21.781ca5137a9c76031df2.chunk.js",22:"22.c7dfd45fc0bd41c7618d.chunk.js",23:"23.8c4885794fd57453884a.chunk.js",24:"24.1447090b6f41a311414e.chunk.js",25:"25.021a38e680888fe2ac7e.chunk.js",26:"26.1afe06be0d6164d3409a.chunk.js",27:"27.dc70b696039ad4762a3b.chunk.js",28:"28.8c383709ce92ecae6b0c.chunk.js",29:"29.f594eb538f606ae17c50.chunk.js",30:"30.a2c1dfc70e0fac57b2a4.chunk.js",31:"31.2eaee95b85227b23ccd8.chunk.js",32:"32.528e99c8151fef966483.chunk.js",33:"33.c3b7530ab92bc1280136.chunk.js",34:"34.1eb5635dc498ad450839.chunk.js",35:"35.e71c1e7bc6092ff2a35f.chunk.js",36:"36.0d174c67ddb177944140.chunk.js",37:"37.af1c6ed4cde9120da636.chunk.js",38:"38.fb0dd22a16e7b597ef93.chunk.js",39:"39.c17f705a3438de3dc997.chunk.js",40:"40.d509fa240e2adf2888aa.chunk.js",41:"41.37d2f0e0e06a3c7d816b.chunk.js",42:"42.4febbf78adc3084afec3.chunk.js",43:"43.7aa48b320fcf69adb0a3.chunk.js",44:"44.5e6da9391c7412910447.chunk.js",45:"45.a17d5b7c5e534f260841.chunk.js",46:"46.a1d3a7790959ac892ed0.chunk.js",47:"47.241627b0e5da4ce35606.chunk.js",48:"48.84f9532a64f5a3beb20c.chunk.js",49:"49.f8527afe7cade8fc293a.chunk.js",50:"50.776b466f9019479de8fc.chunk.js",51:"51.ca34827c84d4bcc82079.chunk.js",52:"52.517f4f6c63395646cdd7.chunk.js",53:"53.e3a2103e4151cd13300f.chunk.js",54:"athena.5e6c5b01662cea2c8b1a.chunk.js",55:"hera.b69b80db056ad9c9389f.chunk.js",56:"hermes.29bb236b97c128e8b6ee.chunk.js",57:"iris.834233a6fb064bf576a9.chunk.js",58:"hephaestus.7ac71b3274dda739ba1f.chunk.js",59:"59.ce1aefa687f2ef9c9908.chunk.js",60:"60.5070b818882287dfc402.chunk.js",61:"61.19d5149d0a2bd9ef3c1e.chunk.js",62:"62.d7831f900b939591822e.chunk.js"}</script><link 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a=document,c=function(){c.c(arguments)};c.q=[],c.c=function(t){c.q.push(t)},e.Intercom=c,e.attachEvent?e.attachEvent("onload",t):e.addEventListener("load",t,!1)}}()</script><script type="text/javascript" src="https://assets-frontend.kalohq.ink/vendor.83e606c69fc5ae7aeb9b.js" crossorigin="anonymous"></script><script type="text/javascript" src="https://assets-frontend.kalohq.ink/allapps.commons.8395b1aa9666e3271c40.js" crossorigin="anonymous"></script><script type="text/javascript" src="https://assets-frontend.kalohq.ink/hermes.29bb236b97c128e8b6ee.js" crossorigin="anonymous"></script></body></html> Something to note here is that all script and link tags have crossorigin="anonymous". Also note the preload and prefetch tags.
The issue is mostly affecting stylesheets it seems BUT scripts have also been affected in the same manner. Again it's really odd that it seems to pick randomly which assets will break and when. Considering these two facts perhaps it is even based on the reference ordering in the document/load order.
A few final clarifications hopefully to help:
Any help or guidance with this issue is going to be hugely appreciated. It's pretty blocking at the moment!
Update:
So we have managed to get what appears to be a continuously working build out without any apparent issues. Hard to know for 100% without time due to seemingly sporadic/random nature of the issue. What we changed was the following:
access-control-max-age to -1 which disables this. We wouldn't expect this to have any effect because this should only (reading spec) affect preflight requests which don't occur for GET requests.We are now doing further testing to try and isolate one or a combo of these as the culprits. We can then dig further into what is happening there.
Note this solving the issue has now been proved incorrect. See Update 2.
Update 2:
We have had further reports and occurrences in-house of the issue after the previous rollout which we thought bypassed the issue. One affect the previous rollout did have was that the issue is now seen much less frequently. Again a hard refresh fixes everything.
The issue is identical to previously described still and so far we have not seen first-hand a failure to load JS since the first occurrence - always seems to be a CSS file failing now.
Update 3:
Some pretty important information I didn't mention originally is the change which happened around the time this issue started presenting itself.
Last Monday we released a bundle refactor, powered by webpack which meant assets became shared between deployments. For example if an output file allapps.commons.HASH123.css didn't change between release v1 and v2 then the idea is we could leverage browser caching.
What still happens however is that the script uploading these assets to S3 IS currently dumbly uploading and overriding the original file. We were under the assumption this change would be pretty harmless since the file is the same name and contents but perhaps this has some adverse effect?
Another effect of this release was that now there will be a lot more assets due to aggressive code splitting. One thing to note here though is that none of the async chunks seem to suffer from the same problem (they're using jsonp afterall) and the issue is only with those assets reference via <script> and <link> tags.
You can find the build artifacts of the release PRIOR to the breaking release here. And find the NEW build artifacts of the current active release showing infrequent issues here. You can also find our deploy scripts here
All resources can be found on google drive here.
Update 4:
This issue is still occurring and has now been reported on an async chunk which is loaded on-demand. Looking at the webpack runtime these scripts are loaded by adding a new script tag to the page, again with crossorigin="anonymous".
Update 5:
On each build we now use a unique salt (the release version) when hashing the file names. This means no assets are shared between builds. The issue has continued to persist after this release.
Update 6:
I've uploaded a .har file showing this issue occurring over a user session.
Search for the following string "url": "https://assets-frontend.kalohq.ink/style.allapps.add899080acbbeed5bb6a7301d234b65.css", and see the various requests made for this asset. You will see the first few are fine and have the headers you'd the expect. The last occurrence (line 32624) is the one which failed.
{ "startedDateTime": "2017-06-28T09:40:15.534Z", "time": 0, "request": { "method": "GET", "url": "https://assets-frontend.kalohq.ink/style.allapps.add899080acbbeed5bb6a7301d234b65.css", "httpVersion": "unknown", "headers": [ { "name": "Referer", "value": "https://kalohq.ink/account" }, { "name": "Origin", "value": "https://kalohq.ink" }, { "name": "DPR", "value": "2" }, { "name": "User-Agent", "value": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36" } ], "queryString": [], "cookies": [], "headersSize": -1, "bodySize": 0 }, "response": { "status": 0, "statusText": "", "httpVersion": "unknown", "headers": [], "cookies": [], "content": { "size": 0, "mimeType": "x-unknown" }, "redirectURL": "", "headersSize": -1, "bodySize": -1, "_transferSize": 0, "_error": "" }, "cache": {}, "timings": { "blocked": -1, "dns": -1, "connect": -1, "send": 0, "wait": 0, "receive": 0, "ssl": -1 }, "serverIPAddress": "", "pageref": "page_10" }, Update 7:
So last night we pushed a change which removed the usage of the crossorigin="anonymous" attribute everywhere. So far we have not seen the issue occur (still waiting given the nature of the issue) but are seeing some interesting and unexpected responses from the requests being made now. Would be great if we could get some clarification on what exactly is happening here. I don't believe we expected removing crossorigin="anonymous" to have such an effect or even understand why it was so broken before since our server is setup to send the correct headers AND the Vary header.
Request from cli to s3, with an Origin header, no cors response headers
curl -I 'https://s3.amazonaws.com/olympus.lystable.com/style.allapps.5ebcc4d28ec238a53f46d6c8e12900d1.css' -H 'Pragma: no-cache' -H 'Accept-Encoding: gzip, deflate, br' -H 'Accept-Language: en-GB,en-US;q=0.8,en;q=0.6' -H 'User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/59.0.3071.109 Chrome/59.0.3071.109 Safari/537.36' -H 'Accept: text/css,*/*;q=0.1' -H 'Referer: https://asos.kalohq.com/categories' -H 'Connection: keep-alive' -H 'DPR: 1' -H 'Cache-Control: no-cache' -H "Origin: https://kalohq.com" --compressed HTTP/1.1 200 OK x-amz-id-2: kxOvBrYsKyZ42wGgJu8iyRZ8q6j5DHDC6QoK1xn2e8FO1wIEEVkxQ0JvGQTmwrN/Njf8EOlmLrE= x-amz-request-id: DA8B5488D3A7EF73 Date: Thu, 13 Jul 2017 13:27:47 GMT Last-Modified: Thu, 13 Jul 2017 11:30:50 GMT ETag: "c765a0a215cb4c9a074f22c3863c1223" Cache-Control: max-age=31556926 Content-Encoding: gzip Accept-Ranges: bytes Content-Type: text/css Content-Length: 5887 Server: AmazonS3 Request a moment later from cli again to s3 with just origin header. Now suddenly gives all expected cors headers back...
curl -H "Origin: https://kalohq.com" -I https://assets-frontend.kalohq.com/style.allapps.5ebcc4d28ec238a53f46d6c8e12900d1.css HTTP/1.1 200 OK Content-Type: text/css Content-Length: 5887 Connection: keep-alive Date: Thu, 13 Jul 2017 13:33:09 GMT Access-Control-Allow-Origin: * Access-Control-Allow-Methods: GET Access-Control-Max-Age: -1 Last-Modified: Thu, 13 Jul 2017 11:30:50 GMT ETag: "c765a0a215cb4c9a074f22c3863c1223" Cache-Control: max-age=31556926 Content-Encoding: gzip Accept-Ranges: bytes Server: AmazonS3 Vary: Origin,Access-Control-Request-Headers,Access-Control-Request-Method Age: 69 X-Cache: Hit from cloudfront Via: 1.1 a19c66da9b402e0bee3fd29619661850.cloudfront.net (CloudFront) X-Amz-Cf-Id: 3wQ7Z6EaAcMscGirwsYVi1M_rvoc1fbI034QY4QZd6IqmlRzLRllEg== Update 8:
The removal of crossorigin="anonymous" tags have resolved the issue. Investigation into why this suddenly started being an issue with this release is ongoing since we had this attribute on script tags before.
All resources useful in this investigation can be found on google drive here.
410
If the server is under your control, add the origin of the requesting site to the set of domains permitted access by adding it to the Access-Control-Allow-Origin header's value. You can also configure a site to allow any site to access it by using the * wildcard. You should only use this for public APIs.
CORS caching for CDNsThis caches the response in public caches (e.g. CDNs) for 24 hours, which should be enough for most cases without risking cache invalidation becoming a problem. For initial testing, you might want to set the cache time shorter, and increase it once you're happy that everything is set up correctly.
Enable CORS Using IIS ManagerNavigate to the website you need to edit the response headers for. A dialog box will open. For name enter "Access-Control-Allow-Origin" and for Value enter an asterisk ( * ). Click Ok, you are done.
https://assets-frontend.kalohq.ink/style.allapps.add899080acbbeed5bb6a7301d234b65.css only returns CORS headers when an "Origin" header is present (which is sent with a CORS request, but not regular requests).
Here's what happens:
<link rel="stylesheet">). This caches due to the Cache-Control header.Access-Control-Allow-Origin header.The server is at fault here, it should use the Vary header to indicate its response changes depending on the Origin header (and others). It sends this header in response to CORS requests, but it should send it in response to non-CORS requests too.
Chrome is somewhat at fault here, as it should use the credentials mode of the request as part of the caching key, so a non-credentialed request (such as those sent by fetch()) shouldn't match items in the cache that were requested with credentials. I think there are other browsers that behave like Chrome here, but Firefox doesn't.
However, since you're using a CDN, you can't rely on browsers to get this right, as the caching may still happen at the CDN. Adding the correct Vary header is the right fix.
tl;dr: Add the following header to all of your responses that support CORS:
Vary: Origin, Access-Control-Request-Headers, Access-Control-Request-Method We experienced the same problem when migrating our JS to Webpack.
Our setup is similar:
When migrating to Webpack, we wanted to take advantage of JS sourcemaps for better error reporting to Airbrake. To allow errors to be catched properly, the crossorigin="anonymous" attribute had to be set on the script tags. The reason why is explained here: https://blog.sentry.io/2016/05/17/what-is-script-error.html
Part of the problem was that CORS response headers were sometimes returned, sometimes not, triggering a CORS errror in the browser. Cloudfront servers were caching the response with OR without the CORS headers, depending on the first client request making a Miss request.
So two possible outcomes:
This made the problem look like it was random, but it was just a matter of race condition (how the first client made the request) and different headers cached on different Cloudfront servers: timing and location dependent. Add to that the fact that browsers might cache these wrong headers...
So you need to properly configure Cloudront's distribution behavior to:
Here is the configuration that solved our problem.
S3 bucket / Permissions / CORS configuration:
<?xml version="1.0" encoding="UTF-8"?> <CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/"> <CORSRule> <AllowedOrigin>*</AllowedOrigin> <AllowedMethod>GET</AllowedMethod> <AllowedMethod>HEAD</AllowedMethod> <MaxAgeSeconds>300</MaxAgeSeconds> <AllowedHeader>*</AllowedHeader> </CORSRule> </CORSConfiguration> Cloudfront distribution / Edit Behavior:
We now experience a problem similar to yours, as we just migrated our CSS to Webpack. We are experiencing even more sporadic CORS errors for CSS files. We are trying to remove the crossorigin="anonymous" attribute on <link rel="stylesheet" /> tags since we don't need error tracking for CSS files.
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