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nginx configuration for CORS (Cross-Origin Resource Sharing), with an origin whitelist, and HTTP Basic Access authentication allowed
#
# A CORS (Cross-Origin Resouce Sharing) config for nginx
#
# == Purpose
#
# This nginx configuration enables CORS requests in the following way:
# - enables CORS just for origins on a whitelist specified by a regular expression
# - CORS preflight request (OPTIONS) are responded immediately
# - Access-Control-Allow-Credentials=true for GET and POST requests
# - Access-Control-Max-Age=20days, to minimize repetitive OPTIONS requests
# - various superluous settings to accommodate nonconformant browsers
#
# == Comment on echoing Access-Control-Allow-Origin
#
# How do you allow CORS requests only from certain domains? The last
# published W3C candidate recommendation states that the
# Access-Control-Allow-Origin header can include a list of origins.
# (See: http://www.w3.org/TR/2013/CR-cors-20130129/#access-control-allow-origin-response-header )
# However, browsers do not support this well and it likely will be
# dropped from the spec (see, http://www.rfc-editor.org/errata_search.php?rfc=6454&eid=3249 ).
#
# The usual workaround is for the server to keep a whitelist of
# acceptable origins on the server (as a regular expression), match
# the request's Origin header against the list, and echo it back
#
# (Yes you can use '*' to accept all origins but this is too open and
# prevents using 'Access-Control-Allow-Credentials: true', which is
# needed for HTTP Basic Access authentication.)
#
# == Comment on spec
#
# Comments below are all based on my reading of the CORS spec as of
# 2013-Jan-29 ( http://www.w3.org/TR/2013/CR-cors-20130129/ ), the
# XMLHttpRequest spec (
# http://www.w3.org/TR/2012/WD-XMLHttpRequest-20121206/ ), and
# experimentation with latest versions of Firefox, Chrome, Safari at
# that point in time.
#
# == Changelog
#
# based on https://gist.github.com/alexjs/4165271
#
location / {
# if the request included an Origin: header with an origin on the whitelist,
# then it is some kind of CORS request.
# specifically, this example allow CORS requests from
# scheme : http or https
# authority : any authority ending in "mckinsey.com"
# port : nothing, or :<any_number>
if ($http_origin ~* (https?://.*\.mckinsey\.com(:[0-9]+)?)) {
set $cors "true";
}
# Nginx doesn't support nested If statements, so we use string
# concatenation to create a flag for compound conditions
# OPTIONS indicates a CORS pre-flight request
if ($request_method = 'OPTIONS') {
set $cors "${cors}options";
}
# non-OPTIONS indicates a normal CORS request
if ($request_method = 'GET') {
set $cors "${cors}get";
}
if ($request_method = 'POST') {
set $cors "${cors}post";
}
# if it's a GET or POST, set the standard CORS responses header
if ($cors = "trueget") {
# Tells the browser this origin may make cross-origin requests
# (Here, we echo the requesting origin, which matched the whitelist.)
add_header 'Access-Control-Allow-Origin' "$http_origin";
# Tells the browser it may show the response, when XmlHttpRequest.withCredentials=true.
add_header 'Access-Control-Allow-Credentials' 'true';
# # Tell the browser which response headers the JS can see, besides the "simple response headers"
# add_header 'Access-Control-Expose-Headers' 'myresponseheader';
}
if ($cors = "truepost") {
# Tells the browser this origin may make cross-origin requests
# (Here, we echo the requesting origin, which matched the whitelist.)
add_header 'Access-Control-Allow-Origin' "$http_origin";
# Tells the browser it may show the response, when XmlHttpRequest.withCredentials=true.
add_header 'Access-Control-Allow-Credentials' 'true';
# # Tell the browser which response headers the JS can see
# add_header 'Access-Control-Expose-Headers' 'myresponseheader';
}
# if it's OPTIONS, for a CORS preflight request, then respond immediately with no response body
if ($cors = "trueoptions") {
# Tells the browser this origin may make cross-origin requests
# (Here, we echo the requesting origin, which matched the whitelist.)
add_header 'Access-Control-Allow-Origin' "$http_origin";
# in a preflight response, tells browser the subsequent actual request can include user credentials (e.g., cookies)
add_header 'Access-Control-Allow-Credentials' 'true';
#
# Return special preflight info
#
# Tell browser to cache this pre-flight info for 20 days
add_header 'Access-Control-Max-Age' 1728000;
# Tell browser we respond to GET,POST,OPTIONS in normal CORS requests.
#
# Not officially needed but still included to help non-conforming browsers.
#
# OPTIONS should not be needed here, since the field is used
# to indicate methods allowed for "actual request" not the
# preflight request.
#
# GET,POST also should not be needed, since the "simple
# methods" GET,POST,HEAD are included by default.
#
# We should only need this header for non-simple requests
# methods (e.g., DELETE), or custom request methods (e.g., XMODIFY)
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
# Tell browser we accept these headers in the actual request
#
# A dynamic, wide-open config would just echo back all the headers
# listed in the preflight request's
# Access-Control-Request-Headers.
#
# A dynamic, restrictive config, would just echo back the
# subset of Access-Control-Request-Headers headers which are
# allowed for this resource.
#
# This static, fairly open config just returns a hardcoded set of
# headers that covers many cases, including some headers that
# are officially unnecessary but actually needed to support
# non-conforming browsers
#
# Comment on some particular headers below:
#
# Authorization -- practically and officially needed to support
# requests using HTTP Basic Access authentication. Browser JS
# can use HTTP BA authentication with an XmlHttpRequest object
# req by calling
#
# req.withCredentials=true, and
# req.setRequestHeader('Authorization','Basic ' + window.btoa(theusername + ':' + thepassword))
#
# Counterintuitively, the username and password fields on
# XmlHttpRequest#open cannot be used to set the authorization
# field automatically for CORS requests.
#
# Content-Type -- this is a "simple header" only when it's
# value is either application/x-www-form-urlencoded,
# multipart/form-data, or text/plain; and in that case it does
# not officially need to be included. But, if your browser
# code sets the content type as application/json, for example,
# then that makes the header non-simple, and then your server
# must declare that it allows the Content-Type header.
#
# Accept,Accept-Language,Content-Language -- these are the
# "simple headers" and they are officially never
# required. Practically, possibly required.
#
# Origin -- logically, should not need to be explicitly
# required, since it's implicitly required by all of
# CORS. officially, it is unclear if it is required or
# forbidden! practically, probably required by existing
# browsers (Gecko does not request it but WebKit does, so
# WebKit might choke if it's not returned back).
#
# User-Agent,DNT -- officially, should not be required, as
# they cannot be set as "author request headers". practically,
# may be required.
#
# My Comment:
#
# The specs are contradictory, or else just confusing to me,
# in how they describe certain headers as required by CORS but
# forbidden by XmlHttpRequest. The CORS spec says the browser
# is supposed to set Access-Control-Request-Headers to include
# only "author request headers" (section 7.1.5). And then the
# server is supposed to use Access-Control-Allow-Headers to
# echo back the subset of those which is allowed, telling the
# browser that it should not continue and perform the actual
# request if it includes additional headers (section 7.1.5,
# step 8). So this implies the browser client code must take
# care to include all necessary headers as author request
# headers.
#
# However, the spec for XmlHttpRequest#setRequestHeader
# (section 4.6.2) provides a long list of headers which the
# the browser client code is forbidden to set, including for
# instance Origin, DNT (do not track), User-Agent, etc.. This
# is understandable: these are all headers that we want the
# browser itself to control, so that malicious browser client
# code cannot spoof them and for instance pretend to be from a
# different origin, etc..
#
# But if XmlHttpRequest forbids the browser client code from
# setting these (as per the XmlHttpRequest spec), then they
# are not author request headers. And if they are not author
# request headers, then the browser should not include them in
# the preflight request's Access-Control-Request-Headers. And
# if they are not included in Access-Control-Request-Headers,
# then they should not be echoed by
# Access-Control-Allow-Headers. And if they are not echoed by
# Access-Control-Allow-Headers, then the browser should not
# continue and execute actual request. So this seems to imply
# that the CORS and XmlHttpRequest specs forbid certain
# widely-used fields in CORS requests, including the Origin
# field, which they also require for CORS requests.
#
# The bottom line: it seems there are headers needed for the
# web and CORS to work, which at the moment you should
# hard-code into Access-Control-Allow-Headers, although
# official specs imply this should not be necessary.
#
add_header 'Access-Control-Allow-Headers' 'Authorization,Content-Type,Accept,Origin,User-Agent,DNT,Cache-Control,X-Mx-ReqToken,Keep-Alive,X-Requested-With,If-Modified-Since';
# build entire response to the preflight request
# no body in this response
add_header 'Content-Length' 0;
# (should not be necessary, but included for non-conforming browsers)
add_header 'Content-Type' 'text/plain charset=UTF-8';
# indicate successful return with no content
return 204;
}
}
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