Set Origin header to "null" for cross origin redirects.

net/url_request/url_request.cc

Index: net/url_request/url_request.cc
diff --git a/net/url_request/url_request.cc b/net/url_request/url_request.cc
index f84fca48064eea65ad733b1bd9ef0d89f229f2c0..9e4816bce6ad8bc5c8649562e8598fe4df2eb56a 100644
--- a/net/url_request/url_request.cc
+++ b/net/url_request/url_request.cc
@@ -37,6 +37,8 @@
 #include "net/url_request/url_request_job_manager.h"
 #include "net/url_request/url_request_netlog_params.h"
 #include "net/url_request/url_request_redirect_job.h"
+#include "url/gurl.h"
+#include "url/origin.h"
 
 using base::Time;
 using std::string;
@@ -942,6 +944,26 @@ int URLRequest::Redirect(const RedirectInfo& redirect_info) {
     method_ = redirect_info.new_method;
   }
 
+  // Cross-origin redirects should not result in an Origin header value that is
+  // equal to the original request's Origin header. This is necessary to prevent
+  // a reflection of POST requests to bypass CSRF protections. If the header was
+  // not set to "null", a POST request from origin A to a malicious origin M
+  // could be redirected by M back to A.
+  //
+  // RFC 6454, The Web Origin Concept, suggests that a User Agent may add a
+  // *list* of origins to the Origin header on redirect, but this is not what
+  // Firefox or IE do in practice. Additionally, it could be argued that the
+  // "privacy-sensitive contexts" that require a "null" Origin header described
+  // in section 7.3 (https://tools.ietf.org/html/rfc6454#section-7.3) apply
+  // here.

[davidben, 2015/03/24 23:47:38]

This should cite CORS or Fetch (see my comment on previous patch set).

[jww, 2015/03/27 22:16:15]

See my response to your other comment (tl;dr "Done")

+  //
+  // See also https://crbug.com/465517.
+  if (redirect_info.new_url.GetOrigin() != url().GetOrigin() &&
+      extra_request_headers_.HasHeader(HttpRequestHeaders::kOrigin)) {
+    extra_request_headers_.SetHeader(HttpRequestHeaders::kOrigin,
+                                     url::Origin().string());
+  }
+
   referrer_ = redirect_info.new_referrer;
   first_party_for_cookies_ = redirect_info.new_first_party_for_cookies;