Why Does Pre-flight Request Not Return CORS Headers?

I believe I’ve read all the CORS-related posts here and I’m not seeing an answer that helps me resolve this issue.

I am seeing a “CORS header ‘Access-Control-Allow-Origin’ missing” error for a request to Okta’s authorize endpoint despite having configured Okta with appropriate Trusted Origins.

Frontend = NuxtJS (VueJS) running in node at localhost:3000
Backend = Phoenix running at localhost:4000

I have listed localhost:3000 and localhost:4000 as Trusted Origins for CORS and Redirect in Okta.

I did run the test in the “Enable CORS” section of the Okta doc and it worked fine.

I did try this in Firefox, Chrome and Safari with the same results in each.

The process is:

  1. The Javascript application issues GET /api/login and the backend responds with a 302 redirect to /auth/okta. Works fine.

  2. The Javascript application then requests GET /auth/okta as directed and the backend (by way of Ueberauth Okta responds with a 302 redirect to the Okta /authorize endpoint (works fine):

    https ://dev-7777777.okta.com/oauth2/v1/authorize?client_id=xyz&nonce=zyx&redirect_uri=http%3A%2F%2Flocalhost%3A4000%2Fauth%2Fokta%2Fcallback&response_type=code&scope=openid+email+profile&state=state-07F40B06C81E687853980FB5

  3. The Javascript application then issues a pre-flight (HTTP OPTIONS) request to Okta:

    OPTIONS /oauth2/v1/authorize?client_id=xyz&nonce=zyx&redirect_uri=http%3A%2F%2Flocalhost%3A4000%2Fauth%2Fokta%2Fcallback&response_type=code&scope=openid+email+profile&state=state-07F40B06C81E687853980FB5 HTTP/2
    Host: dev-777777.okta.com
    User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:82.0) Gecko/20100101 Firefox/82.0
    Accept: /
    Accept-Language: en-US,en;q=0.5
    Accept-Encoding: gzip, deflate, br
    Access-Control-Request-Method: GET
    Access-Control-Request-Headers: access-control-allow-origin
    Referer: http://localhost:3000/login
    Origin: http://localhost:3000
    Connection: keep-alive

to which Okta responds:

HTTP/2 200 OK
date: Mon, 09 Nov 2020 17:03:44 GMT
content-length: 0
server: nginx
public-key-pins-report-only: pin-sha256="r5EfzZxQVvQpKo3AgYRaT7X2bDO/kj3ACwmxfdT2zt8="; pin-sha256="MaqlcUgk2mvY/RFSGeSwBRkI+rZ6/dxe/DuQfBT/vnQ="; pin-sha256="72G5IEvDEWn+EThf3qjR7/bQSWaS2ZSLqolhnO6iyJI="; pin-sha256="rrV6CLCCvqnk89gWibYT0JO6fNQ8cCit7GGoiVTjCOg="; max-age=60; report-uri="https://okta.report-uri.com/r/default/hpkp/reportOnly"
x-okta-request-id: X6l2cMtA@8GfUasz6CloCAAAAyM
x-xss-protection: 0
p3p: CP="HONK"
x-rate-limit-limit: 10000
x-rate-limit-remaining: 9999
x-rate-limit-reset: 1604941484
content-security-policy-report-only: default-src 'self' *.oktacdn.com dev-7777777.okta.com; connect-src 'self' *.oktacdn.com *.mixpanel.com *.mapbox.com app.pendo.io data.pendo.io pendo-static-5634101834153984.storage.googleapis.com dev-4252537.okta.com dev-7777777-admin.okta.com https://oinmanager.okta.com data:; script-src 'unsafe-inline' 'unsafe-eval' 'self' *.oktacdn.com; style-src 'unsafe-inline' 'self' *.oktacdn.com app.pendo.io cdn.pendo.io pendo-static-5634101834153984.storage.googleapis.com; frame-src 'self' login.okta.com dev-7777777.okta.com dev-4252537-admin.okta.com; img-src 'self' *.oktacdn.com dev-7777777.okta.com *.tiles.mapbox.com *.mapbox.com app.pendo.io data.pendo.io cdn.pendo.io pendo-static-5634101834153984.storage.googleapis.com data: blob:; font-src data: 'self' *.oktacdn.com fonts.gstatic.com; report-uri https://okta.report-uri.com/r/d/csp/reportOnly; report-to csp-report
report-to: {"group":"csp-report","max_age":31536000,"endpoints":[{"url":"https://okta.report-uri.com/r/d/csp/reportOnly"}],"include_subdomains":true}
cache-control: no-cache, no-store
pragma: no-cache
expires: 0
x-frame-options: SAMEORIGIN
strict-transport-security: max-age=315360000; includeSubDomains
x-robots-tag: none
set-cookie: sid=""; Expires=Thu, 01-Jan-1970 00:00:10 GMT; Path=/
set-cookie: JSESSIONID=0DC93A2E22DE1B291C1A7F5F569F2810; Path=/; Secure; HttpOnly
set-cookie: DT=DI0VsaaIxY3QseA9wHcJRySiA;Version=1;Path=/;Max-Age=63072000;Secure;Expires=Wed, 09 Nov 2022 17:03:44 GMT;SameSite=None
set-cookie: sid=""; Expires=Thu, 01-Jan-1970 00:00:10 GMT; Path=/
X-Firefox-Spdy: h2

(Note that there are no CORS headers in the response.)

  1. And then I get the dreaded error:

    Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https ://dev-7777777.okta.com/oauth2/v1/authorize?client_id=xyz&nonce=zyx&redirect_uri=http%3A%2F%2Flocalhost%3A4000%2Fauth%2Fokta%2Fcallback&response_type=code&scope=openid+email+profile&state=state-07F40B06C81E687853980FB5. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).

which is, I believe, because there are no CORS headers in the pre-flight response so the browser denies the Javascript application (whose origin is localhost:3000) from requesting /authorize from Okta’s domain.

Why would the pre-flight request not return needed CORS-related response headers?

Thank you for any help.

According to the docs, you need to redirect to the /authorize endpoint

Note: When making requests to the /authorize endpoint, the browser (user agent) should be redirected to the endpoint. You can’t use AJAX with this endpoint.

Ah…thank you!! I read that but it didn’t sink in. I was initiating the login flow from Javascript so the redirect was being managed by Javascript. Simply flipping to a href rather than a function call made it work as expected.

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