diff --git a/caps/BasePrincipal.cpp b/caps/BasePrincipal.cpp
index 78b09bb249..c59ca5e37c 100644
--- a/caps/BasePrincipal.cpp
+++ b/caps/BasePrincipal.cpp
@@ -343,7 +343,8 @@ OriginAttributes::IsFirstPartyEnabled()
   return sFirstPartyIsolation;
 }
 
-BasePrincipal::BasePrincipal()
+BasePrincipal::BasePrincipal(PrincipalKind aKind)
+  : mKind(aKind)
 {}
 
 BasePrincipal::~BasePrincipal()
diff --git a/caps/BasePrincipal.h b/caps/BasePrincipal.h
index dd60d33746..1fa95b786e 100644
--- a/caps/BasePrincipal.h
+++ b/caps/BasePrincipal.h
@@ -251,7 +251,14 @@ public:
 class BasePrincipal : public nsJSPrincipals
 {
 public:
-  BasePrincipal();
+  enum PrincipalKind {
+    eNullPrincipal,
+    eCodebasePrincipal,
+    eExpandedPrincipal,
+    eSystemPrincipal
+  };
+
+  explicit BasePrincipal(PrincipalKind aKind);
 
   enum DocumentDomainConsideration { DontConsiderDocumentDomain, ConsiderDocumentDomain};
   bool Subsumes(nsIPrincipal* aOther, DocumentDomainConsideration aConsideration);
@@ -301,17 +308,19 @@ public:
   uint32_t PrivateBrowsingId() const { return mOriginAttributes.mPrivateBrowsingId; }
   bool IsInIsolatedMozBrowserElement() const { return mOriginAttributes.mInIsolatedMozBrowser; }
 
-  enum PrincipalKind {
-    eNullPrincipal,
-    eCodebasePrincipal,
-    eExpandedPrincipal,
-    eSystemPrincipal
-  };
-
-  virtual PrincipalKind Kind() = 0;
+  PrincipalKind Kind() const { return mKind; }
 
   already_AddRefed<BasePrincipal> CloneStrippingUserContextIdAndFirstPartyDomain();
 
+  /**
+   * Returns true if this principal's CSP should override a document's CSP for
+   * loads that it triggers. Currently true only for expanded principals which
+   * subsume the document principal.
+   */
+  bool OverridesCSP(nsIPrincipal* aDocumentPrincipal) {
+    return mKind == eExpandedPrincipal && Subsumes(aDocumentPrincipal, DontConsiderDocumentDomain);
+  }
+
 protected:
   virtual ~BasePrincipal();
 
@@ -333,6 +342,7 @@ protected:
   nsCOMPtr<nsIContentSecurityPolicy> mCSP;
   nsCOMPtr<nsIContentSecurityPolicy> mPreloadCSP;
   PrincipalOriginAttributes mOriginAttributes;
+  PrincipalKind mKind;
 };
 
 } // namespace mozilla
diff --git a/caps/nsNullPrincipal.h b/caps/nsNullPrincipal.h
index b294747730..01800cf4ce 100644
--- a/caps/nsNullPrincipal.h
+++ b/caps/nsNullPrincipal.h
@@ -36,7 +36,9 @@ public:
   // This should only be used by deserialization, and the factory constructor.
   // Other consumers should use the Create and CreateWithInheritedAttributes
   // methods.
-  nsNullPrincipal() {}
+  nsNullPrincipal()
+    : BasePrincipal(eNullPrincipal)
+  {}
 
   NS_DECL_NSISERIALIZABLE
 
@@ -60,8 +62,6 @@ public:
 
   virtual nsresult GetScriptLocation(nsACString &aStr) override;
 
-  PrincipalKind Kind() override { return eNullPrincipal; }
-
  protected:
   virtual ~nsNullPrincipal() {}
 
diff --git a/caps/nsPrincipal.cpp b/caps/nsPrincipal.cpp
index 169ee0cb8c..00ac01ecbe 100644
--- a/caps/nsPrincipal.cpp
+++ b/caps/nsPrincipal.cpp
@@ -64,10 +64,11 @@ nsPrincipal::InitializeStatics()
 }
 
 nsPrincipal::nsPrincipal()
-  : mCodebaseImmutable(false)
+  : BasePrincipal(eCodebasePrincipal)
+  , mCodebaseImmutable(false)
   , mDomainImmutable(false)
   , mInitialized(false)
-{ }
+{}
 
 nsPrincipal::~nsPrincipal()
 { 
@@ -514,6 +515,7 @@ struct OriginComparator
 
 nsExpandedPrincipal::nsExpandedPrincipal(nsTArray<nsCOMPtr<nsIPrincipal>> &aWhiteList,
                                          const PrincipalOriginAttributes& aAttrs)
+  : BasePrincipal(eExpandedPrincipal)
 {
   // We force the principals to be sorted by origin so that nsExpandedPrincipal
   // origins can have a canonical form.
diff --git a/caps/nsPrincipal.h b/caps/nsPrincipal.h
index c122952c70..7acbf2520f 100644
--- a/caps/nsPrincipal.h
+++ b/caps/nsPrincipal.h
@@ -46,8 +46,6 @@ public:
    */
   static void InitializeStatics();
 
-  PrincipalKind Kind() override { return eCodebasePrincipal; }
-
   nsCOMPtr<nsIURI> mDomain;
   nsCOMPtr<nsIURI> mCodebase;
   // If mCodebaseImmutable is true, mCodebase is non-null and immutable
@@ -82,8 +80,6 @@ public:
   virtual nsresult GetScriptLocation(nsACString &aStr) override;
   nsresult GetOriginInternal(nsACString& aOrigin) override;
 
-  PrincipalKind Kind() override { return eExpandedPrincipal; }
-
 protected:
   virtual ~nsExpandedPrincipal();
 
diff --git a/caps/nsSystemPrincipal.h b/caps/nsSystemPrincipal.h
index 5faec50cfd..35c64d8cbd 100644
--- a/caps/nsSystemPrincipal.h
+++ b/caps/nsSystemPrincipal.h
@@ -36,7 +36,9 @@ public:
   NS_IMETHOD GetBaseDomain(nsACString& aBaseDomain) override;
   nsresult GetOriginInternal(nsACString& aOrigin) override;
 
-  nsSystemPrincipal() {}
+  nsSystemPrincipal()
+    : BasePrincipal(eSystemPrincipal)
+  {}
 
   virtual nsresult GetScriptLocation(nsACString &aStr) override;
 
@@ -52,8 +54,6 @@ protected:
   {
     return true;
   }
-
-  PrincipalKind Kind() override { return eSystemPrincipal; }
 };
 
 #endif // nsSystemPrincipal_h__
diff --git a/dom/security/nsCSPService.cpp b/dom/security/nsCSPService.cpp
index ec32889acf..eab7f48478 100644
--- a/dom/security/nsCSPService.cpp
+++ b/dom/security/nsCSPService.cpp
@@ -140,12 +140,18 @@ CSPService::ShouldLoad(uint32_t aContentType,
     return NS_OK;
   }
 
-  // query the principal of the document; if no document is passed, then
-  // fall back to using the requestPrincipal (e.g. service workers do not
-  // pass a document).
+  // Find a principal to retrieve the CSP from. If we don't have a context node
+  // (because, for instance, the load originates in a service worker), or the
+  // requesting principal's CSP overrides our document CSP, use the request
+  // principal. Otherwise, use the document principal.
   nsCOMPtr<nsINode> node(do_QueryInterface(aRequestContext));
-  nsCOMPtr<nsIPrincipal> principal = node ? node->NodePrincipal()
-                                          : aRequestPrincipal;
+  nsCOMPtr<nsIPrincipal> principal;
+  if (!node || (aRequestPrincipal &&
+                BasePrincipal::Cast(aRequestPrincipal)->OverridesCSP(node->NodePrincipal()))) {
+    principal = aRequestPrincipal;
+  } else  {
+    principal = node->NodePrincipal();
+  }
   if (!principal) {
     // if we can't query a principal, then there is nothing to do.
     return NS_OK;