diff --git a/dom/workers/ScriptLoader.cpp b/dom/workers/ScriptLoader.cpp
index bdc4efed23..758c1660e7 100644
--- a/dom/workers/ScriptLoader.cpp
+++ b/dom/workers/ScriptLoader.cpp
@@ -1202,7 +1202,7 @@ private:
       // load group's appId and browser element flag.
       MOZ_ASSERT(NS_LoadGroupMatchesPrincipal(channelLoadGroup, channelPrincipal));
 
-      mWorkerPrivate->SetPrincipal(channelPrincipal, channelLoadGroup);
+      mWorkerPrivate->SetPrincipalOnMainThread(channelPrincipal, channelLoadGroup);
 
       // We did inherit CSP in bug 1223647. If we do not already have a CSP, we
       // should get it from the HTTP headers on the worker script.
@@ -1290,7 +1290,14 @@ private:
 #endif
 
       mWorkerPrivate->InitChannelInfo(aChannelInfo);
-      mWorkerPrivate->SetPrincipal(responsePrincipal, loadGroup);
+
+      // Override the principal on the WorkerPrivate.  We just asserted that
+      // this is the same as our current WorkerPrivate principal, so this is
+      // almost a no-op.  We must do, it though, in order to avoid accidentally
+      // propagating the CSP object back to the ServiceWorkerRegistration
+      // principal.  If bug 965637 is fixed then this can be removed.
+      rv = mWorkerPrivate->SetPrincipalOnMainThread(responsePrincipal, loadGroup);
+      MOZ_DIAGNOSTIC_ASSERT(NS_SUCCEEDED(rv));
 
       rv = mWorkerPrivate->SetCSPFromHeaderValues(aCSPHeaderValue,
                                                   aCSPReportOnlyHeaderValue);
diff --git a/dom/workers/ServiceWorkerPrivate.cpp b/dom/workers/ServiceWorkerPrivate.cpp
index c66d98b0ea..f30ae67a19 100644
--- a/dom/workers/ServiceWorkerPrivate.cpp
+++ b/dom/workers/ServiceWorkerPrivate.cpp
@@ -1779,6 +1779,11 @@ ServiceWorkerPrivate::SpawnWorkerIfNeeded(WakeUpReason aWhy,
 
   WorkerPrivate::OverrideLoadInfoLoadGroup(info);
 
+  rv = info.SetPrincipalOnMainThread(info.mPrincipal, info.mLoadGroup);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
   AutoJSAPI jsapi;
   jsapi.Init();
   ErrorResult error;
diff --git a/dom/workers/WorkerPrivate.cpp b/dom/workers/WorkerPrivate.cpp
index 01a8368c81..7e96fc6ad3 100644
--- a/dom/workers/WorkerPrivate.cpp
+++ b/dom/workers/WorkerPrivate.cpp
@@ -3679,47 +3679,53 @@ WorkerPrivateParent<Derived>::SetBaseURI(nsIURI* aBaseURI)
   nsContentUtils::GetUTFOrigin(aBaseURI, mLocationInfo.mOrigin);
 }
 
-template <class Derived>
-void
-WorkerPrivateParent<Derived>::SetPrincipal(nsIPrincipal* aPrincipal,
-                                           nsILoadGroup* aLoadGroup)
+nsresult
+WorkerLoadInfo::SetPrincipalOnMainThread(nsIPrincipal* aPrincipal,
+                                         nsILoadGroup* aLoadGroup)
 {
   AssertIsOnMainThread();
   MOZ_ASSERT(NS_LoadGroupMatchesPrincipal(aLoadGroup, aPrincipal));
-  MOZ_ASSERT(!mLoadInfo.mPrincipalInfo);
 
-  mLoadInfo.mPrincipal = aPrincipal;
-  mLoadInfo.mPrincipalIsSystem = nsContentUtils::IsSystemPrincipal(aPrincipal);
+  mPrincipal = aPrincipal;
+  mPrincipalIsSystem = nsContentUtils::IsSystemPrincipal(aPrincipal);
 
-  aPrincipal->GetCsp(getter_AddRefs(mLoadInfo.mCSP));
+  nsresult rv = aPrincipal->GetCsp(getter_AddRefs(mCSP));
+  NS_ENSURE_SUCCESS(rv, rv);
 
-  if (mLoadInfo.mCSP) {
-    mLoadInfo.mCSP->GetAllowsEval(&mLoadInfo.mReportCSPViolations,
-                                  &mLoadInfo.mEvalAllowed);
+  if (mCSP) {
+    mCSP->GetAllowsEval(&mReportCSPViolations, &mEvalAllowed);
     // Set ReferrerPolicy
     bool hasReferrerPolicy = false;
-    uint32_t rp = mozilla::net::RP_Default;
+    uint32_t rp = mozilla::net::RP_Unset;
 
-    nsresult rv = mLoadInfo.mCSP->GetReferrerPolicy(&rp, &hasReferrerPolicy);
-    NS_ENSURE_SUCCESS_VOID(rv);
+    rv = mCSP->GetReferrerPolicy(&rp, &hasReferrerPolicy);
+    NS_ENSURE_SUCCESS(rv, rv);
 
     if (hasReferrerPolicy) {
-      mLoadInfo.mReferrerPolicy = static_cast<net::ReferrerPolicy>(rp);
+      mReferrerPolicy = static_cast<net::ReferrerPolicy>(rp);
     }
   } else {
-    mLoadInfo.mEvalAllowed = true;
-    mLoadInfo.mReportCSPViolations = false;
+    mEvalAllowed = true;
+    mReportCSPViolations = false;
   }
 
-  mLoadInfo.mLoadGroup = aLoadGroup;
+  mLoadGroup = aLoadGroup;
 
-  mLoadInfo.mPrincipalInfo = new PrincipalInfo();
-  mLoadInfo.mOriginAttributes = nsContentUtils::GetOriginAttributes(aLoadGroup);
+  mPrincipalInfo = new PrincipalInfo();
+  mOriginAttributes = nsContentUtils::GetOriginAttributes(aLoadGroup);
 
-  nsContentUtils::GetUTFOrigin(aPrincipal, mLoadInfo.mOrigin);
+  rv = PrincipalToPrincipalInfo(aPrincipal, mPrincipalInfo);
+  NS_ENSURE_SUCCESS(rv, rv);
 
-  MOZ_ALWAYS_SUCCEEDS(
-    PrincipalToPrincipalInfo(aPrincipal, mLoadInfo.mPrincipalInfo));
+  return NS_OK;
+}
+
+template <class Derived>
+nsresult
+WorkerPrivateParent<Derived>::SetPrincipalOnMainThread(nsIPrincipal* aPrincipal,
+                                                       nsILoadGroup* aLoadGroup)
+{
+  return mLoadInfo.SetPrincipalOnMainThread(aPrincipal, aLoadGroup);
 }
 
 template <class Derived>
diff --git a/dom/workers/WorkerPrivate.h b/dom/workers/WorkerPrivate.h
index b82e994028..1c9cb10ded 100644
--- a/dom/workers/WorkerPrivate.h
+++ b/dom/workers/WorkerPrivate.h
@@ -656,8 +656,8 @@ public:
       return mLoadInfo.mPrincipal;
   }
 
-  void
-  SetPrincipal(nsIPrincipal* aPrincipal, nsILoadGroup* aLoadGroup);
+  nsresult
+  SetPrincipalOnMainThread(nsIPrincipal* aPrincipal, nsILoadGroup* aLoadGroup);
 
   bool
   UsesSystemPrincipal() const
diff --git a/dom/workers/Workers.h b/dom/workers/Workers.h
index 6b0a0158d2..541dfed6aa 100644
--- a/dom/workers/Workers.h
+++ b/dom/workers/Workers.h
@@ -268,6 +268,8 @@ struct WorkerLoadInfo
   ~WorkerLoadInfo();
 
   void StealFrom(WorkerLoadInfo& aOther);
+
+  nsresult SetPrincipalOnMainThread(nsIPrincipal* aPrincipal, nsILoadGroup* aLoadGroup);
 };
 
 // All of these are implemented in RuntimeService.cpp