Twitter API authorization fails CORS preflight in browser

Question

I'm trying to do the 3-legged authorization necessary to call the Twitter APIs in a browser. The process starts with getting a request token by POSTing a signed request to /oauth/request_token (this is also how sign in with Twitter begins).

My problem is that before the browser will POST to the Twitter API endpoint, it wants to preflight the request with an OPTIONS method. This preflight request is always returning status 400 (Bad Request).

Here's an example that you can cut and paste into a browser console that supports the Fetch API:

fetch('https://api.twitter.com/oauth/request_token', { method: 'POST', mode: 'cors', headers: new Headers({ authorization: 'xxx' }), body: 'oauth_callback=http%3A%2F%2Flocalhost%2F' });

On Chrome, the preflight request looks like this (Firefox is similar):

OPTIONS /oauth/request_token HTTP/1.1
accept:*/*
accept-encoding:gzip, deflate, sdch
accept-language:en-US,en;q=0.8
access-control-request-headers:authorization, content-type
access-control-request-method:POST
cache-control:no-cache
origin:null
pragma:no-cache
user-agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/48.0.2564.116 Safari/537.36

And the preflight response looks like this:

HTTP/1.1 400 Bad Request
content-length: 0
date: Tue, 08 Mar 2016 22:21:37 GMT
server: tsa_a
x-connection-hash: 529e3d8338caeb980077637d86db5df1

Note that problem is not that I didn't specify a real authorization header in the example above. The value of the authorization header is not used in the preflight request.

If I print out the components of my POST request to the console and assemble the pieces into a curl command (which doesn't preflight) then I can get a request token. But if I try to simulate the preflight request in curl, I haven't been able to get that to work:

$ curl -v -X OPTIONS -H "access-control-request-headers:authorization,content-type" -H "access-control-request-method:POST" -H "origin:http://example.com" https://api.twitter.com/oauth/request_token
*   Trying 199.59.148.20...
* Connected to api.twitter.com (199.59.148.20) port 443 (#0)
* ALPN, offering http/1.1
* Cipher selection: ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:@STRENGTH
* successfully set certificate verify locations:
*   CAfile: /opt/local/share/curl/curl-ca-bundle.crt
  CApath: none
* TLSv1.2 (OUT), TLS header, Certificate Status (22):
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
* TLSv1.2 (IN), TLS handshake, Server finished (14):
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
* TLSv1.2 (OUT), TLS change cipher, Client hello (1):
* TLSv1.2 (OUT), TLS handshake, Finished (20):
* TLSv1.2 (IN), TLS change cipher, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Finished (20):
* SSL connection using TLSv1.2 / ECDHE-ECDSA-AES128-GCM-SHA256
* ALPN, server accepted to use http/1.1
* Server certificate:
*    subject: C=US; ST=CA; L=San Francisco; O=Twitter, Inc.; OU=Twitter Security; CN=api.twitter.com
*    start date: Aug 11 00:00:00 2015 GMT
*    expire date: Aug 15 12:00:00 2016 GMT
*    subjectAltName: api.twitter.com matched
*    issuer: C=US; O=DigiCert Inc; OU=www.digicert.com; CN=DigiCert SHA2 High Assurance Server CA
*    SSL certificate verify ok.
> OPTIONS /oauth/request_token HTTP/1.1
> Host: api.twitter.com
> User-Agent: curl/7.47.1
> Accept: */*
> access-control-request-headers:authorization,content-type
> access-control-request-method:POST
> origin:http://example.com
> 
< HTTP/1.1 400 Bad Request
< content-length: 0
< date: Tue, 08 Mar 2016 23:06:44 GMT
< server: tsa_a
< x-connection-hash: 66174829ef6d3f5e5ec641ac080ad19c
< 
* Connection #0 to host api.twitter.com left intact

What am I missing that will let me do a successful CORS preflight to https://api.twitter.com/oauth/request_token?

Answer 1

So the unsatisfying resolution appears to be that the Twitter API does not support CORS. This seems a little astonishing to me, as it means that the API cannot be used from a browser.

That policy decision is probably related to their OAuth implementation, which is vulnerable to anyone with access to the calling platform. Maybe that was okay back in 2010, but most of the other major internet players have figured out how to do client-based authorization.