diff --git a/b2g/app/b2g.js b/b2g/app/b2g.js
index cd9f03b41427..70c80b17cef4 100644
--- a/b2g/app/b2g.js
+++ b/b2g/app/b2g.js
@@ -860,6 +860,15 @@ pref("media.webspeech.synth.enabled", true);
 pref("dom.mozDownloads.enabled", true);
 pref("dom.downloads.max_retention_days", 7);
 
+// External Helper Application Handling
+//
+// All external helper application handling can require the docshell to be
+// active before allowing the external helper app service to handle content.
+//
+// To prevent SD card DoS attacks via downloads we disable background handling.
+//
+pref("security.exthelperapp.disable_background_handling", true);
+
 // Inactivity time in milliseconds after which we shut down the OS.File worker.
 pref("osfile.reset_worker_delay", 5000);
 
diff --git a/uriloader/base/nsURILoader.cpp b/uriloader/base/nsURILoader.cpp
index ece2607e5c93..e1545f272d51 100644
--- a/uriloader/base/nsURILoader.cpp
+++ b/uriloader/base/nsURILoader.cpp
@@ -49,6 +49,7 @@
 
 #include "nsDocLoader.h"
 #include "mozilla/Attributes.h"
+#include "mozilla/Preferences.h"
 
 #ifdef PR_LOGGING
 PRLogModuleInfo* nsURILoader::mLog = nullptr;
@@ -58,6 +59,9 @@ PRLogModuleInfo* nsURILoader::mLog = nullptr;
 #define LOG_ERROR(args) PR_LOG(nsURILoader::mLog, PR_LOG_ERROR, args)
 #define LOG_ENABLED() PR_LOG_TEST(nsURILoader::mLog, PR_LOG_DEBUG)
 
+#define NS_PREF_DISABLE_BACKGROUND_HANDLING \
+    "security.exthelperapp.disable_background_handling"
+
 /**
  * The nsDocumentOpenInfo contains the state required when a single
  * document is being opened in order to discover the content type...
@@ -519,6 +523,35 @@ nsresult nsDocumentOpenInfo::DispatchContent(nsIRequest *request, nsISupports *
   // All attempts to dispatch this content have failed.  Just pass it off to
   // the helper app service.
   //
+
+  //
+  // Optionally, we may want to disable background handling by the external
+  // helper application service.
+  //
+  if (mozilla::Preferences::GetBool(NS_PREF_DISABLE_BACKGROUND_HANDLING,
+                                    false)) {
+    // First, we will ensure that the parent docshell is in an active
+    // state as we will disallow all external application handling unless it is
+    // in the foreground.
+    nsCOMPtr<nsIDocShell> docShell(do_GetInterface(m_originalContext));
+    if (!docShell) {
+      // If we can't perform our security check we definitely don't want to go
+      // any further!
+      LOG(("Failed to get DocShell to ensure it is active before anding off to "
+           "helper app service. Aborting."));
+      return NS_ERROR_FAILURE;
+    }
+
+    // Ensure the DocShell is active before continuing.
+    bool isActive = false;
+    docShell->GetIsActive(&isActive);
+    if (!isActive) {
+      LOG(("  Check for active DocShell returned false. Aborting hand off to "
+           "helper app service."));
+      return NS_ERROR_DOM_SECURITY_ERR;
+    }
+  }
+
   nsCOMPtr<nsIExternalHelperAppService> helperAppService =
     do_GetService(NS_EXTERNALHELPERAPPSERVICE_CONTRACTID, &rv);
   if (helperAppService) {