middleware: Implement middleware for common headers such as CSP

- This allows overriding these headers in individual pages easily instead of
relaxing global policy.

- Drop the obsolete CSP directive "block-all-mixed-content" and avoid a console
warning in Firefox.

Tests:

- Load a page and notice in the browser developer tools that the three headers
referrer-policy, content-security-policy, and x-content-type-options are set as
before.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

data/etc/apache2/conf-available/freedombox.conf

@@ -47,48 +47,6 @@
     RedirectMatch "^/$" "/plinth"
 </IfFile>
 
-##
-## Disable sending Referer (sic) header from FreedomBox web interface to
-## external websites. This improves privacy by not disclosing FreedomBox
-## domains/URLs to external domains. Apps such as blogs which want to popularize
-## themselves with referrer header may still do so.
-##
-## A strict Content Security Policy.
-## - @fonts are allowed only from FreedomBox itself.
-## - <frame>/<iframe> sources are disabled.
-## - <img> sources are allowed only from FreedomBox itself.
-## - Manifest file is not allowed as there is none yet.
-## - <audio>, <video>, <track> tags are not allowed yet.
-## - <object>, <embed>, <applet> tags are not allowed yet.
-## - Allow JS from FreedomBox itself (no inline and attribute scripts).
-## - Allow inline CSS and CSS files from Freedombox itself.
-## - Web worker sources are allowed only from FreedomBox itself (for JSXC).
-## - All other fetch sources including Ajax are not allowed from FreedomBox
-##   itself.
-## - <base> tag is not allowed.
-## - No plugins types are alllowed since object-src is 'none'.
-## - Form action should be to FreedomBox itself.
-## - This interface may be not embedded in <frame>, <iframe>, etc. tags.
-## - When serving HTTPS, don't allow HTTP assets.
-##
-## Enable strict sandboxing enabled with some exceptions:
-## - Allow running Javascript.
-## - Allow popups as sometimes we use <a target=_blank>
-## - Allow popups to have different sandbox requirements as we launch apps' web
-##   clients.
-## - Allow forms to support configuration forms.
-## - Allow policies to treat same origin differently from other origins
-## - Allow downloads such as backup tarballs.
-##
-## Disable browser guessing of MIME types. FreedoBox already sets good content
-## types for all the common file types.
-##
-<LocationMatch "^/(plinth|freedombox)">
-    Header set Referrer-Policy 'same-origin'
-    Header set Content-Security-Policy "font-src 'self'; frame-src 'none'; img-src 'self' data:; manifest-src 'none'; media-src 'none'; object-src 'none'; script-src 'self'; style-src 'self'; worker-src 'self'; default-src 'self'; base-uri 'none'; sandbox allow-scripts allow-popups allow-popups-to-escape-sandbox allow-forms allow-same-origin allow-downloads; form-action 'self'; frame-ancestors 'none'; block-all-mixed-content;"
-    Header set X-Content-Type-Options 'nosniff'
-</LocationMatch>
-
 ##
 ## On all sites, provide FreedomBox on a default path: /plinth
 ##

plinth/middleware.py

@@ -188,3 +188,93 @@ class CommonErrorMiddleware(MiddlewareMixin):
         breadcrumbs = views.get_breadcrumbs(request)
         parent_index = 1 if len(breadcrumbs) > 1 else 0
         return list(breadcrumbs.keys())[parent_index]
+
+
+class CSPDict(dict):
+    """A dictionary to store Content Security Policy.
+
+    And return a full value of the HTTP header.
+    """
+
+    def get_header_value(self) -> str:
+        """Return the string header value for the policy stored."""
+        return ' '.join([f'{key} {value};' for key, value in self.items()])
+
+
+CONTENT_SECURITY_POLICY = CSPDict({
+    # @fonts are allowed only from FreedomBox itself.
+    'font-src': "'self'",
+    # <frame>/<iframe> sources are disabled.
+    'frame-src': "'none'",
+    # <img> sources are allowed only from FreedomBox itself. Allow
+    # data: URLs for SVGs in CSS.
+    'img-src': "'self' data:",
+    # Manifest file is not allowed as there is none yet.
+    'manifest-src': "'none'",
+    # <audio>, <video>, <track> tags are not allowed yet.
+    'media-src': "'none'",
+    # <object>, <embed>, <applet> tags are not allowed yet. No plugins
+    # types are alllowed since object-src is 'none'.
+    'object-src': "'none'",
+    # Allow JS from FreedomBox itself (no inline and attribute
+    # scripts).
+    'script-src': "'self'",
+    # Allow inline CSS and CSS files from Freedombox itself.
+    'style-src': "'self'",
+    # Web worker sources are allowed only from FreedomBox itself (for
+    # JSXC).
+    'worker-src': "'self'",
+    # All other fetch sources including Ajax are not allowed from
+    # FreedomBox itself.
+    'default-src': "'self'",
+    # <base> tag is not allowed.
+    'base-uri': "'none'",
+    # Enable strict sandboxing enabled with some exceptions:
+    # - Allow running Javascript.
+    # - Allow popups as sometimes we use <a target=_blank>
+    # - Allow popups to have different sandbox requirements as we
+    #   launch apps' web clients.
+    # - Allow forms to support configuration forms.
+    # - Allow policies to treat same origin differently from other
+    # - origins
+    # - Allow downloads such as backup tarballs.
+    'sandbox': 'allow-scripts allow-popups '
+               'allow-popups-to-escape-sandbox allow-forms '
+               'allow-same-origin allow-downloads',
+    # Form action should be to FreedomBox itself.
+    'form-action': "'self'",
+    # This interface may be not embedded in <frame>, <iframe>, etc.
+    # tags.
+    'frame-ancestors': "'none'",
+})
+
+
+class CommonHeadersMiddleware:
+
+    def __init__(self, get_response):
+        """Initialize the middleware object."""
+        self.get_response = get_response
+
+    def __call__(self, request):
+        """Add common security middleware."""
+        # Disable sending Referer (sic) header from FreedomBox web interface to
+        # external websites. This improves privacy by not disclosing FreedomBox
+        # domains/URLs to external domains. Apps such as blogs which want to
+        # popularize themselves with referrer header may still do so.
+        response = self.get_response(request)
+        if not response.get('Referrer-Policy'):
+            response['Referrer-Policy'] = 'same-origin'
+
+        # Disable browser guessing of MIME types. FreedoBox already sets good
+        # content types for all the common file types.
+        if not response.get('X-Content-Type-Options'):
+            response['X-Content-Type-Options'] = 'nosniff'
+
+        csp = ' '.join([
+            f'{key} {value};'
+            for key, value in CONTENT_SECURITY_POLICY.items()
+        ])
+        if not response.get('Content-Security-Policy'):
+            response['Content-Security-Policy'] = csp
+
+        return response

plinth/settings.py

@@ -149,6 +149,7 @@ LOGIN_REDIRECT_URL = 'index'
 MESSAGE_TAGS: dict = {}
 
 MIDDLEWARE = (
+    'plinth.middleware.CommonHeadersMiddleware',
     'django.middleware.security.SecurityMiddleware',
     'django.contrib.sessions.middleware.SessionMiddleware',
     'django.middleware.locale.LocaleMiddleware',