Add constants for CSP keywords

This helps avoid potential errors introduced by incorrectly quoting CSP keywords.

Author: Rob Hudson

csp/constants.py

@@ -1,2 +1,12 @@
 HEADER = "Content-Security-Policy"
 HEADER_REPORT_ONLY = "Content-Security-Policy-Report-Only"
+
+NONE = "'none'"
+REPORT_SAMPLE = "'report-sample'"
+SELF = "'self'"
+STRICT_DYNAMIC = "'strict-dynamic'"
+UNSAFE_ALLOW_REDIRECTS = "'unsafe-allow-redirects'"
+UNSAFE_EVAL = "'unsafe-eval'"
+UNSAFE_HASHES = "'unsafe-hashes'"
+UNSAFE_INLINE = "'unsafe-inline'"
+WASM_UNSAFE_EVAL = "'wasm-unsafe-eval'"

csp/tests/test_middleware.py

@@ -6,7 +6,7 @@
 from django.test import RequestFactory
 from django.test.utils import override_settings
 
-from csp.constants import HEADER, HEADER_REPORT_ONLY
+from csp.constants import HEADER, HEADER_REPORT_ONLY, SELF
 from csp.middleware import CSPMiddleware
 from csp.tests.utils import response
 
@@ -23,7 +23,7 @@ def test_add_header():
 
 @override_settings(
     CONTENT_SECURITY_POLICY={"DIRECTIVES": {"default-src": ["example.com"]}},
-    CONTENT_SECURITY_POLICY_REPORT_ONLY={"DIRECTIVES": {"default-src": ["'self'"]}},
+    CONTENT_SECURITY_POLICY_REPORT_ONLY={"DIRECTIVES": {"default-src": [SELF]}},
 )
 def test_both_headers():
     request = rf.get("/")
@@ -51,7 +51,7 @@ def text_exclude():
 
 @override_settings(
     CONTENT_SECURITY_POLICY=None,
-    CONTENT_SECURITY_POLICY_REPORT_ONLY={"DIRECTIVES": {"default-src": ["'self'"]}},
+    CONTENT_SECURITY_POLICY_REPORT_ONLY={"DIRECTIVES": {"default-src": [SELF]}},
 )
 def test_report_only():
     request = rf.get("/")

csp/tests/test_utils.py

@@ -1,6 +1,7 @@
 from django.test.utils import override_settings
 from django.utils.functional import lazy
 
+from csp.constants import NONE, SELF
 from csp.utils import build_policy, default_config, DEFAULT_DIRECTIVES
 
 
@@ -182,7 +183,7 @@ def test_replace_missing_setting():
 
 
 def test_config():
-    policy = build_policy(config={"default-src": ["'none'"], "img-src": ["'self'"]})
+    policy = build_policy(config={"default-src": [NONE], "img-src": [SELF]})
     policy_eq("default-src 'none'; img-src 'self'", policy)
 
 

csp/utils.py

@@ -6,12 +6,13 @@
 from django.conf import settings
 from django.utils.encoding import force_str
 
+from csp.constants import SELF
 
 DEFAULT_DIRECTIVES = {
     # Fetch Directives
     "child-src": None,
     "connect-src": None,
-    "default-src": ["'self'"],
+    "default-src": [SELF],
     "script-src": None,
     "script-src-attr": None,
     "script-src-elem": None,

docs/configuration.rst

@@ -43,31 +43,54 @@ a more slightly strict policy and is used to test the policy without breaking th
 
 .. code-block:: python
 
+    from csp.constants import NONE, SELF
+
     CONTENT_SECURITY_POLICY = {
         "EXCLUDE_URL_PREFIXES": ["/excluded-path/"],
         "DIRECTIVES": {
-            "default-src": ["'self'", "cdn.example.net"],
-            "frame-ancestors": ["'self'"],
-            "form-action": ["'self'"],
+            "default-src": [SELF, "cdn.example.net"],
+            "frame-ancestors": [SELF],
+            "form-action": [SELF],
             "report-uri": "/csp-report/",
         },
     }
 
     CONTENT_SECURITY_POLICY_REPORT_ONLY = {
         "EXCLUDE_URL_PREFIXES": ["/excluded-path/"],
         "DIRECTIVES": {
-            "default-src": ["'none'"],
-            "connect-src": ["'self'"],
-            "img-src": ["'self'"],
-            "form-action": ["'self'"],
-            "frame-ancestors": ["'self'"],
-            "script-src": ["'self'"],
-            "style-src": ["'self'"],
+            "default-src": [NONE],
+            "connect-src": [SELF],
+            "img-src": [SELF],
+            "form-action": [SELF],
+            "frame-ancestors": [SELF],
+            "script-src": [SELF],
+            "style-src": [SELF],
             "upgrade-insecure-requests": True,
             "report-uri": "/csp-report/",
         },
     }
 
+.. note::
+
+    In the above example, the constant ``NONE`` is converted to the CSP keyword ``"'none'"`` and
+    is distinct from Python's ``None`` value. The CSP keyword ``'none'`` is a special value that
+    signifies that you do not want any sources for this directive. The ``None`` value is a
+    Python keyword that represents the absence of a value and when used as the value of a directive,
+    it will remove the directive from the policy.
+    
+    This is useful when using the ``@csp_replace`` decorator to effectively clear a directive from
+    the base configuration as defined in the settings. For example, if the Django settings the
+    ``frame-ancestors`` directive is set to a list of sources and you want to remove the
+    ``frame-ancestors`` directive from the policy for this view:
+
+    .. code-block:: python
+
+        from csp.decorators import csp_replace
+
+
+        @csp_replace({"frame-ancestors": None})
+        def my_view(request): ...
+
 
 Policy Settings
 ===============
@@ -86,8 +109,6 @@ policy.
        Scripting flaw on, e.g., ``excluded-page/`` can therefore be leveraged to access everything
        on the same origin.
 
-       # TODO: I can't find any documentation on the above warning.
-
 ``REPORT_PERCENTAGE``
     Percentage of requests that should see the ``report-uri`` directive.
     Use this to throttle the number of CSP violation reports made to your
@@ -101,8 +122,11 @@ policy.
 
     .. note::
        The "special" source values of ``'self'``, ``'unsafe-inline'``, ``'unsafe-eval'``,
-       ``'none'`` and hash-source (``'sha256-...'``) must be quoted!
-       e.g.: ``"default-src": ["'self'"]``. Without quotes they will not work as intended.
+       ``'strict-dynamic'``, ``'none'``, etc. must be quoted!  e.g.: ``"default-src": ["'self'"]``.
+       Without quotes they will not work as intended.
+       
+       Consider using the ``csp.constants`` module to get these values to help avoiding quoting
+       errors or typos, e.g., ``from csp.constants import SELF, STRICT_DYNAMIC``.
 
     .. note::
        Deprecated features of CSP in general have been moved to the bottom of this list.

docs/decorators.rst

@@ -7,29 +7,35 @@ Modifying the Policy with Decorators
 Content Security Policies should be restricted and paranoid by default.  You may, on some views,
 need to expand or change the policy. django-csp includes four decorators to help.
 
+All decorators take an optional keyword argument, ``REPORT_ONLY``, which defaults to ``False``. If
+set to ``True``, the decorator will update the report-only policy instead of the enforced policy.
 
 ``@csp_exempt``
 ===============
 
 Using the ``@csp_exempt`` decorator disables the CSP header on a given
 view.
 
-::
+.. code-block:: python
 
     from csp.decorators import csp_exempt
 
+
     # Will not have a CSP header.
     @csp_exempt()
     def myview(request):
         return render(...)
 
+
     # Will not have a CSP report-only header.
     @csp_exempt(REPORT_ONLY=True)
     def myview(request):
         return render(...)
 
 You can manually set this on a per-response basis by setting the ``_csp_exempt``
-or ``_csp_exempt_ro`` attribute on the response to ``True``::
+or ``_csp_exempt_ro`` attribute on the response to ``True``:
+
+.. code-block:: python
 
     # Also will not have a CSP header.
     def myview(request):
@@ -55,15 +61,17 @@ decorator excpects a single dictionary argument, where the keys are the directiv
 are either strings, lists or tuples. An optional argument, ``REPORT_ONLY``, can be set to ``True``
 to update the report-only policy instead of the enforced policy.
 
-::
+.. code-block:: python
 
     from csp.decorators import csp_update
 
+
     # Will append imgsrv.com to the list of values for `img-src` in the enforced policy.
     @csp_update({"img-src": "imgsrv.com"})
     def myview(request):
         return render(...)
 
+
     # Will append cdn-img.com to the list of values for `img-src` in the report-only policy.
     @csp_update({"img-src": "cdn-img.com"}, REPORT_ONLY=True)
     def myview(request):
@@ -77,41 +85,66 @@ The ``@csp_replace`` decorator allows you to **replace** a source list specified
 there is no setting, the value passed to the decorator will be used verbatim. (See the note under
 ``@csp_update``.) If the specified value is None, the corresponding key will not be included.
 
-The arguments and values are the same as ``@csp_update``::
+The arguments and values are the same as ``@csp_update``:
+
+.. code-block:: python
 
     from csp.decorators import csp_replace
 
+
     # Will allow images only from imgsrv2.com in the enforced policy.
     @csp_replace({"img-src": "imgsrv2.com"})
     def myview(request):
         return render(...)
 
+
     # Will allow images only from cdn-img2.com in the report-only policy.
     @csp_replace({"img-src": "imgsrv2.com"})
     def myview(request):
         return render(...)
 
+The ``csp_replace`` decorator can also be used to remove a directive from the policy by setting the
+value to ``None``. For example, if the ``frame-ancestors`` directive is set in the Django settings
+and you want to remove the ``frame-ancestors`` directive from the policy for this view:
+
+.. code-block:: python
+
+    from csp.decorators import csp_replace
+
+
+    @csp_replace({"frame-ancestors": None})
+    def myview(request):
+        return render(...)
+
 
 ``@csp``
 ========
 
 If you need to set the entire policy on a view, ignoring all the settings, you can use the ``@csp``
-decorator. This, and the other decorators, can be stacked to update both policies if both are in
-use, as shown below. The arguments and values are as above::
+decorator. This can be stacked to update both the enforced policy and the report-only policy if both
+are in use, as shown below.
 
+.. code-block:: python
+
+    from csp.constants import SELF, UNSAFE_INLINE
     from csp.decorators import csp
 
-    @csp({
-        "default_src": ["'self'"],
-        "img-src": ["imgsrv.com"],
-        "script-src": ["scriptsrv.com", "googleanalytics.com", "'unsafe-inline'"]}
-    })
-    @csp({
-        "default_src": ["'self'"],
-        "img-src": ["imgsrv.com"],
-        "script-src": ["scriptsrv.com", "googleanalytics.com"]},
-        "frame-src": ["'self'"],
-        REPORT_ONLY=True
-    })
+
+    @csp(
+        {
+            "default_src": [SELF],
+            "img-src": ["imgsrv.com"],
+            "script-src": ["scriptsrv.com", "googleanalytics.com", UNSAFE_INLINE],
+        }
+    )
+    @csp(
+        {
+            "default_src": [SELF],
+            "img-src": ["imgsrv.com"],
+            "script-src": ["scriptsrv.com", "googleanalytics.com"],
+            "frame-src": [SELF],
+        },
+        REPORT_ONLY=True,
+    )
     def myview(request):
         return render(...)