Force content disposition to attachment for specific content types

In this way we avoid HTML, XML, SVG and other files that can be rendered
by the browser to be served inline by default. Depending on the origin
from where these files are served, this might lead to XSS
vulnerabilities, and in the best case, to more realistic phishing
attacks and open redirects.

We force it rather than falling back to it when other disposition is not
provided. Otherwise it would be possible for someone to force inline
just by passing `disposition=inline` in the URL.

The list of content types to be served as attachments is configurable.

2 files changed, 12 insertions, 0 deletions

diff --git a/activestorage/lib/active_storage.rb b/activestorage/lib/active_storage.rb
index 01fd0f4415..3bd8ef664f 100644
--- a/activestorage/lib/active_storage.rb
+++ b/activestorage/lib/active_storage.rb
@@ -43,4 +43,5 @@ module ActiveStorage
   mattr_accessor :analyzers, default: []
   mattr_accessor :paths, default: {}
   mattr_accessor :variable_content_types, default: []
+  mattr_accessor :content_types_to_serve_as_binary, default: []
 end
diff --git a/activestorage/lib/active_storage/engine.rb b/activestorage/lib/active_storage/engine.rb
index d5c71670ff..1f15ac2d2d 100644
--- a/activestorage/lib/active_storage/engine.rb
+++ b/activestorage/lib/active_storage/engine.rb
@@ -18,6 +18,16 @@ module ActiveStorage
     config.active_storage.analyzers = [ ActiveStorage::Analyzer::ImageAnalyzer, ActiveStorage::Analyzer::VideoAnalyzer ]
     config.active_storage.paths = ActiveSupport::OrderedOptions.new
     config.active_storage.variable_content_types = [ "image/png", "image/gif", "image/jpg", "image/jpeg", "image/vnd.adobe.photoshop" ]
+    config.active_storage.content_types_to_serve_as_binary = [
+      "text/html",
+      "text/javascript",
+      "image/svg+xml",
+      "application/postscript",
+      "application/x-shockwave-flash",
+      "text/xml",
+      "application/xml",
+      "application/xhtml+xml"
+    ]
 
     config.eager_load_namespaces << ActiveStorage
 
@@ -29,6 +39,7 @@ module ActiveStorage
         ActiveStorage.analyzers  = app.config.active_storage.analyzers || []
         ActiveStorage.paths      = app.config.active_storage.paths || {}
         ActiveStorage.variable_content_types = app.config.active_storage.variable_content_types || []
+        ActiveStorage.content_types_to_serve_as_binary = app.config.active_storage.content_types_to_serve_as_binary || []
       end
     end