--- a/dom/devicestorage/nsDeviceStorage.cpp
+++ b/dom/devicestorage/nsDeviceStorage.cpp
@@ -799,16 +799,26 @@ DeviceStorageFile::NormalizeFilePath() {
 #endif
 }
 
 void
 DeviceStorageFile::AppendRelativePath(const nsAString& aPath) {
   if (!mFile) {
     return;
   }
+  if (!IsSafePath(aPath)) {
+    // All of the APIs (in the child) do checks to verify that the path is
+    // valid and return PERMISSION_DENIED if a non-safe path is entered.
+    // This check is done in the parent and prevents a compromised
+    // child from bypassing the check. It shouldn't be possible for this
+    // code path to be taken with a non-compromised child.
+    NS_WARNING("Unsafe path detected - ignoring");
+    NS_WARNING(NS_LossyConvertUTF16toASCII(aPath).get());
+    return;
+  }
 #if defined(XP_WIN)
   // replace forward slashes with backslashes,
   // since nsLocalFileWin chokes on them
   nsString temp;
   temp.Assign(aPath);
 
   PRUnichar* cur = temp.BeginWriting();
   PRUnichar* end = temp.EndWriting();