Fix it.

Author: ZeroIntensity

Lib/test/test_interpreters/test_api.py

@@ -1649,6 +1649,10 @@ def test_set___main___attrs(self):
             self.assertIs(after2, None)
             self.assertEqual(after3.type.__name__, 'AssertionError')
 
+            with self.assertRaises(ValueError):
+                # GH-127165: Embedded NULL characters broke the lookup
+                _interpreters.set___main___attrs(interpid, {"\x00": 1})
+
         with self.subTest('from C-API'):
             with self.interpreter_from_capi() as interpid:
                 with self.assertRaisesRegex(InterpreterError, 'unrecognized'):

Python/crossinterp.c

@@ -342,6 +342,12 @@ _copy_string_obj_raw(PyObject *strobj, Py_ssize_t *p_size)
         return NULL;
     }
 
+    if (size != (Py_ssize_t) strlen(str))
+    {
+        PyErr_SetString(PyExc_ValueError, "found embedded NULL character");
+        return NULL;
+    }
+
     char *copied = PyMem_RawMalloc(size+1);
     if (copied == NULL) {
         PyErr_NoMemory();