Make Content-Security-Policy disallow 'object-src' by default

Closes #190

h5bp/security/content-security-policy.conf

@@ -41,10 +41,17 @@
 #      The `frame-ancestors` directive helps avoid "Clickjacking" attacks and
 #      is similar to the `X-Frame-Options` header.
 #
-#      Browsers that support the CSP header will ignore `X-Frame-Options` if
+#      Browsers that support the CSP header should ignore `X-Frame-Options` if
 #      `frame-ancestors` is also specified.
 #
-#  (5) Forces the browser to treat all the resources that are served over HTTP
+#  (5) Elements controlled by `object-src` are perhaps coincidentally
+#      considered legacy HTML elements and are not receiving new standardized
+#      features (such as the security attributes `sandbox` or `allow` for
+#      `<iframe>`).
+#      Therefore it is recommended to restrict this fetch-directive (e.g.,
+#      explicitly set `object-src 'none'` if possible).
+#
+#  (6) Forces the browser to treat all the resources that are served over HTTP
 #      as if they were loaded securely over HTTPS by setting the
 #      `upgrade-insecure-requests` directive.
 #
@@ -53,7 +60,7 @@
 #      loaded over HTTPS you must include the `Strict-Transport-Security`
 #      header.
 #
-#  (6) The `Content-Security-Policy` header is included in all responses
+#  (7) The `Content-Security-Policy` header is included in all responses
 #      that are able to execute scripting. This includes the commonly used
 #      file types: HTML, XML and PDF documents. Although Javascript files
 #      can not execute script in a "browsing context", they are still included
@@ -75,6 +82,6 @@
 # https://content-security-policy.com/
 
 <IfModule mod_headers.c>
-    #                                   (1)                 (2)              (3)                 (4)                     (5)                         (6)
-    Header always set Content-Security-Policy "default-src 'self'; base-uri 'none'; form-action 'self'; frame-ancestors 'none'; upgrade-insecure-requests" "expr=%{CONTENT_TYPE} =~ m#text\/(html|javascript)|application\/pdf|xml#i"
+    #                                          (1)                 (2)              (3)                 (4)                     (5)                (6)                                                 (7)
+    Header always set Content-Security-Policy "default-src 'self'; base-uri 'none'; form-action 'self'; frame-ancestors 'none'; object-src 'none'; upgrade-insecure-requests" "expr=%{CONTENT_TYPE} =~ m#text\/(html|javascript)|application\/pdf|xml#i"
 </IfModule>