diff --git a/changes-entries/pr66034.txt b/changes-entries/pr66034.txt
new file mode 100644
index 00000000000..125cdc3f8e5
--- /dev/null
+++ b/changes-entries/pr66034.txt
@@ -0,0 +1,2 @@
+  *) mod_substitute: Fix an integer overflow that can happen in very special
+     setups.  PR 66034 [Ruediger Pluem]
diff --git a/modules/filters/mod_substitute.c b/modules/filters/mod_substitute.c
index 0b3dc806a4a..769d45b48cb 100644
--- a/modules/filters/mod_substitute.c
+++ b/modules/filters/mod_substitute.c
@@ -275,7 +275,7 @@ static apr_status_t do_pattmatch(ap_filter_t *f, apr_bucket *inb,
                     }
                 }
                 else if (script->regexp) {
-                    int left = bytes;
+                    apr_size_t left = bytes;
                     const char *pos = buff;
                     char *repl;
                     apr_size_t space_left = cfg->max_line_length;