diff --git a/libxml.h b/libxml.h
index 6065d822..ee3d57c7 100644
--- a/libxml.h
+++ b/libxml.h
@@ -57,6 +57,13 @@
   #define ATTRIBUTE_DESTRUCTOR __attribute__((destructor))
 #endif
 
+#if (defined(__clang__) && __clang_major__ >= 18) || \
+    (defined(__GNUC__) && __GNUC__ >= 15)
+  #define ATTRIBUTE_COUNTED_BY(c) __attribute__((__counted_by__(c)))
+#else
+  #define ATTRIBUTE_COUNTED_BY(c)
+#endif
+
 #if defined(__clang__) || \
     (defined(__GNUC__) && (__GNUC__ >= 8) && !defined(__EDG__))
   #define ATTRIBUTE_NO_SANITIZE(arg) __attribute__((no_sanitize(arg)))
diff --git a/parser.c b/parser.c
index 945e80b0..9c5a02e0 100644
--- a/parser.c
+++ b/parser.c
@@ -993,7 +993,7 @@ struct _xmlDefAttrs {
     int maxAttrs;       /* the size of the array */
 #if __STDC_VERSION__ >= 199901L
     /* Using a C99 flexible array member avoids UBSan errors. */
-    xmlDefAttr attrs[]; /* array of localname/prefix/values/external */
+    xmlDefAttr attrs[] ATTRIBUTE_COUNTED_BY(maxAttrs);
 #else
     xmlDefAttr attrs[1];
 #endif