@@ -13,6 +13,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #23361: Fix possible overflow in Windows subprocess creation code.
+
 - logging.handlers.QueueListener now takes a respect_handler_level keyword
   argument which, if set to True, will pass messages to handlers taking handler
   levels into account.
 
@@ -670,13 +670,23 @@ getenvironment(PyObject* environment)
                 "environment can only contain strings");
             goto error;
         }
+        if (totalsize > PY_SSIZE_T_MAX - PyUnicode_GET_LENGTH(key) - 1) {
+            PyErr_SetString(PyExc_OverflowError, "environment too long");
+            goto error;
+        }
         totalsize += PyUnicode_GET_LENGTH(key) + 1;    /* +1 for '=' */
+        if (totalsize > PY_SSIZE_T_MAX - PyUnicode_GET_LENGTH(value) - 1) {
+            PyErr_SetString(PyExc_OverflowError, "environment too long");
+            goto error;
+        }
         totalsize += PyUnicode_GET_LENGTH(value) + 1;  /* +1 for '\0' */
     }
 
-    buffer = PyMem_Malloc(totalsize * sizeof(Py_UCS4));
-    if (! buffer)
+    buffer = PyMem_NEW(Py_UCS4, totalsize);
+    if (! buffer) {
+        PyErr_NoMemory();
         goto error;
+    }
     p = buffer;
     end = buffer + totalsize;