diff --git a/expat/lib/xmlparse.c b/expat/lib/xmlparse.c
--- a/expat/lib/xmlparse.c
+++ b/expat/lib/xmlparse.c
@@ -3487,9 +3487,15 @@ doContentInternal(XML_Parser parser, int startTagLevel, const ENCODING *enc,
       } else if (! entity) {
         if (parser->m_skippedEntityHandler)
           parser->m_skippedEntityHandler(parser->m_handlerArg, name, 0);
+/* BEGIN MOZILLA CHANGE (Bug 35984 - Undeclared entities are ignored when external DTD not found) */
+#if 0
         else if (parser->m_defaultHandler)
           reportDefault(parser, enc, s, next);
         break;
+#else
+        return XML_ERROR_UNDEFINED_ENTITY;
+#endif
+/* END MOZILLA CHANGE */
       }
       if (entity->open)
         return XML_ERROR_RECURSIVE_ENTITY_REF;
@@ -6785,7 +6791,13 @@ appendAttributeValue(XML_Parser parser, const ENCODING *enc, XML_Bool isCdata,
         if ((pool == &parser->m_tempPool) && parser->m_defaultHandler)
           reportDefault(parser, enc, ptr, next);
         */
+/* BEGIN MOZILLA CHANGE (Bug 35984 - Undeclared entities are ignored when external DTD not found) */
+#if 0
         break;
+#else
+        return XML_ERROR_UNDEFINED_ENTITY;
+#endif
+/* END MOZILLA CHANGE */
       }
       if (entity->open) {
         if (enc == parser->m_encoding) {