diff --git a/ext/uri/tests/gh20668_global_buffer_overflow.phpt b/ext/uri/tests/gh20668_global_buffer_overflow.phpt
new file mode 100644
index 0000000000000..ae6013500b01d
--- /dev/null
+++ b/ext/uri/tests/gh20668_global_buffer_overflow.phpt
@@ -0,0 +1,18 @@
+--TEST--
+GH-20668 lobal buffer overflow in url.c
+--FILE--
+<?php
+
+$str = 'file:///datafoo:test';
+$url = Uri\WhatWg\Url::parse($str);
+$host = $url->getAsciiHost();
+
+try {
+    $url->withHost($host);
+} catch (Throwable $e) {
+    echo $e::class, ": ", $e->getMessage(), PHP_EOL;
+}
+
+?>
+--EXPECT--
+Uri\WhatWg\InvalidUrlException: The specified host is malformed (DomainInvalidCodePoint)