python: require local protection to be absent

for CSRF to be likely

python/ql/lib/semmle/python/Concepts.qll

@@ -106,7 +106,8 @@ module FileSystemWriteAccess {
 }
 
 /**
- * A data-flow node that may set or unset Cross-site request forgery protection.
+ * A data-flow node that may set or unset Cross-site request forgery protection
+ * in a global manner.
  *
  * Extend this class to refine existing API models. If you want to model new APIs,
  * extend `CSRFProtectionSetting::Range` instead.
@@ -122,7 +123,8 @@ class CSRFProtectionSetting extends DataFlow::Node instanceof CSRFProtectionSett
 /** Provides a class for modeling new CSRF protection setting APIs. */
 module CSRFProtectionSetting {
   /**
-   * A data-flow node that may set or unset Cross-site request forgery protection.
+   * A data-flow node that may set or unset Cross-site request forgery protection
+   * in a global manner.
    *
    * Extend this class to model new APIs. If you want to refine existing API models,
    * extend `CSRFProtectionSetting` instead.
@@ -136,6 +138,39 @@ module CSRFProtectionSetting {
   }
 }
 
+/**
+ * A data-flow node that provides Cross-site request forgery protection
+ * for a specific part of an application.
+ *
+ * Extend this class to refine existing API models. If you want to model new APIs,
+ * extend `CSRFProtection::Range` instead.
+ */
+class CSRFProtection extends DataFlow::Node instanceof CSRFProtection::Range {
+  /**
+   * Gets a `Function` representing the protected interaction
+   * (probably a request handler).
+   */
+  Function getProtected() { result = super.getProtected() }
+}
+
+/** Provides a class for modeling new CSRF protection setting APIs. */
+module CSRFProtection {
+  /**
+   * A data-flow node that provides Cross-site request forgery protection
+   * for a specific part of an application.
+   *
+   * Extend this class to model new APIs. If you want to refine existing API models,
+   * extend `CSRFProtection` instead.
+   */
+  abstract class Range extends DataFlow::Node {
+    /**
+     * Gets a `Function` representing the protected interaction
+     * (probably a request handler).
+     */
+    abstract Function getProtected();
+  }
+}
+
 /** Provides classes for modeling path-related APIs. */
 module Path {
   /**

python/ql/lib/semmle/python/frameworks/Django.qll

@@ -2346,3 +2346,20 @@ module PrivateDjango {
     }
   }
 }
+
+private class DjangoCSRFDecorator extends CSRFProtection::Range {
+  Function function;
+
+  DjangoCSRFDecorator() {
+    this =
+      API::moduleImport("django")
+          .getMember("views")
+          .getMember("decorators")
+          .getMember("csrf")
+          .getMember("csrf_protect")
+          .getAUse() and
+    this.asExpr() = function.getADecorator()
+  }
+
+  override Function getProtected() { result = function }
+}

python/ql/src/Security/CWE-352/CSRFProtectionDisabled.ql

@@ -15,5 +15,7 @@ import python
 import semmle.python.Concepts
 
 from CSRFProtectionSetting s
-where s.getVerificationSetting() = false
+where
+  s.getVerificationSetting() = false and
+  not exists(CSRFProtection p)
 select s, "Potential CSRF vulnerability due to forgery protection being disabled or weakened."

python/ql/test/library-tests/frameworks/django-v2-v3/response_test.py

@@ -1,5 +1,6 @@
 from django.http.response import HttpResponse, HttpResponseRedirect, HttpResponsePermanentRedirect, JsonResponse, HttpResponseNotFound
 from django.views.generic import RedirectView
+from django.views.decorators.csrf import csrf_protect
 import django.shortcuts
 import json
 
@@ -117,6 +118,7 @@ class CustomJsonResponse(JsonResponse):
     def __init__(self, banner, content, *args, **kwargs):
         super().__init__(content, *args, content_type="text/html", **kwargs)
 
+@csrf_protect
 def safe__custom_json_response(request):
     return CustomJsonResponse("ACME Responses", {"foo": request.GET.get("foo")})  # $HttpResponse mimetype=application/json MISSING: responseBody=Dict SPURIOUS: responseBody="ACME Responses"