Issue #2854817 by Liam Morland, longwave, alexpott, JoshaHubbers,...

Issue #2854817 by Liam Morland, longwave, alexpott, JoshaHubbers, effulgentsia, alex-b, Mile23, thalles, mr.baileys, Wim Leers, xjm: Duplicate X-Content-Type-Options headers both with the value nosniff

.htaccess

@@ -173,7 +173,13 @@ AddEncoding gzip svgz
 
 # Various header fixes.
 <IfModule mod_headers.c>
-  # Disable content sniffing, since it's an attack vector.
+  # Disable content sniffing for all responses, since it's an attack vector.
+  # This header is also set in FinishResponseSubscriber, which depending on
+  # Apache configuration might get placed in the 'onsuccess' table. To prevent
+  # header duplication, unset that one prior to setting in the 'always' table.
+  # See "To circumvent this limitation..." in
+  # https://httpd.apache.org/docs/current/mod/mod_headers.html.
+  Header onsuccess unset X-Content-Type-Options
   Header always set X-Content-Type-Options nosniff
   # Disable Proxy header, since it's an attack vector.
   RequestHeader unset Proxy

core/assets/scaffold/files/htaccess

@@ -173,7 +173,13 @@ AddEncoding gzip svgz
 
 # Various header fixes.
 <IfModule mod_headers.c>
-  # Disable content sniffing, since it's an attack vector.
+  # Disable content sniffing for all responses, since it's an attack vector.
+  # This header is also set in FinishResponseSubscriber, which depending on
+  # Apache configuration might get placed in the 'onsuccess' table. To prevent
+  # header duplication, unset that one prior to setting in the 'always' table.
+  # See "To circumvent this limitation..." in
+  # https://httpd.apache.org/docs/current/mod/mod_headers.html.
+  Header onsuccess unset X-Content-Type-Options
   Header always set X-Content-Type-Options nosniff
   # Disable Proxy header, since it's an attack vector.
   RequestHeader unset Proxy