Bug 40432: Prevent probing installed applications

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1711084

uriloader/exthandler/nsExternalHelperAppService.cpp

@@ -866,8 +866,33 @@ nsresult nsExternalHelperAppService::GetFileTokenForPath(
 //////////////////////////////////////////////////////////////////////////////////////////////////////
 // begin external protocol service default implementation...
 //////////////////////////////////////////////////////////////////////////////////////////////////////
+
+static const char kExternalProtocolPrefPrefix[] =
+    "network.protocol-handler.external.";
+static const char kExternalProtocolDefaultPref[] =
+    "network.protocol-handler.external-default";
+
 NS_IMETHODIMP nsExternalHelperAppService::ExternalProtocolHandlerExists(
     const char* aProtocolScheme, bool* aHandlerExists) {
+
+  // Replicate the same check performed in LoadURI.
+  // Deny load if the prefs say to do so
+  nsAutoCString externalPref(kExternalProtocolPrefPrefix);
+  externalPref += aProtocolScheme;
+  bool allowLoad = false;
+  *aHandlerExists = false;
+  if (NS_FAILED(Preferences::GetBool(externalPref.get(), &allowLoad))) {
+    // no scheme-specific value, check the default
+    if (NS_FAILED(
+            Preferences::GetBool(kExternalProtocolDefaultPref, &allowLoad))) {
+      return NS_OK;  // missing default pref
+    }
+  }
+
+  if (!allowLoad) {
+    return NS_OK;  // explicitly denied
+  }
+
   nsCOMPtr<nsIHandlerInfo> handlerInfo;
   nsresult rv = GetProtocolHandlerInfo(nsDependentCString(aProtocolScheme),
                                        getter_AddRefs(handlerInfo));
@@ -910,11 +935,6 @@ NS_IMETHODIMP nsExternalHelperAppService::IsExposedProtocol(
   return NS_OK;
 }
 
-static const char kExternalProtocolPrefPrefix[] =
-    "network.protocol-handler.external.";
-static const char kExternalProtocolDefaultPref[] =
-    "network.protocol-handler.external-default";
-
 // static
 nsresult nsExternalHelperAppService::EscapeURI(nsIURI* aURI, nsIURI** aResult) {
   MOZ_ASSERT(aURI);