diff --git a/include/libxml/parser.h b/include/libxml/parser.h
index 508f37e87..e941e3769 100644
--- a/include/libxml/parser.h
+++ b/include/libxml/parser.h
@@ -1727,6 +1727,9 @@ typedef enum {
      * This option enables DTD validation which requires to load
      * external DTDs and external entities (both general and
      * parameter entities) unless XML_PARSE_NO_XXE was set.
+     *
+     * DTD validation is vulnerable to algorithmic complexity
+     * attacks and should never be enabled with untrusted input.
      */
     XML_PARSE_DTDVALID = 1<<4,
     /**