From b257f320ce6044c658e3cf086b1ae4b4c6380515 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?B=C3=A9n=C3=A9dikt=20Tran?=
 <10796600+picnixz@users.noreply.github.com>
Date: Sat, 28 Mar 2026 14:35:52 +0100
Subject: [PATCH] gh-143376: validate usages of `_interpchannels.ChannelID` in
 `_channelid_shared`

---
 Lib/test/test_interpreters/test_channels.py   | 24 +++++++++++++++++++
 ...-03-28-14-34-18.gh-issue-143376.FvWTwi.rst |  2 ++
 Modules/_interpchannelsmodule.c               | 11 +++++++++
 3 files changed, 37 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-03-28-14-34-18.gh-issue-143376.FvWTwi.rst

diff --git a/Lib/test/test_interpreters/test_channels.py b/Lib/test/test_interpreters/test_channels.py
index 5437792b5a7014..0670522dc88b58 100644
--- a/Lib/test/test_interpreters/test_channels.py
+++ b/Lib/test/test_interpreters/test_channels.py
@@ -30,6 +30,30 @@ def test_highlevel_reloaded(self):
         # See gh-115490 (https://github.com/python/cpython/issues/115490).
         importlib.reload(channels)
 
+    def test_channel_id_types(self):
+        # See https://github.com/python/cpython/issues/143376.
+        class End:
+            __slots__ = ()
+            def __new__(cls, chan=None):
+                return object.__new__(cls)
+            @property
+            def _id(self):
+                return object()
+
+        class Send(End): pass
+        class Recv(End): pass
+
+        _ = importlib.reload(_channels)
+        _._register_end_types(Send, Recv)
+        chan = _._channel_id(_.create(), _resolve=True)
+        interp = interpreters.create()
+        # The TypeError exception arises from our internal check,
+        # but subinterpreters use a generic exception message instead
+        # while retaining the original exception type.
+        err = "does not support cross-interpreter data"
+        with self.assertRaisesRegex(TypeError, err):
+            interp.prepare_main(chan=Send(chan))
+
 
 class TestChannels(TestBase):
 
diff --git a/Misc/NEWS.d/next/Library/2026-03-28-14-34-18.gh-issue-143376.FvWTwi.rst b/Misc/NEWS.d/next/Library/2026-03-28-14-34-18.gh-issue-143376.FvWTwi.rst
new file mode 100644
index 00000000000000..981e0ede58fde8
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-03-28-14-34-18.gh-issue-143376.FvWTwi.rst
@@ -0,0 +1,2 @@
+:mod:`concurrent.interpreters`: fix a crash when internal cross-interpreter
+channel ends have incorrect channel IDs. Patch by Bénédikt Tran.
diff --git a/Modules/_interpchannelsmodule.c b/Modules/_interpchannelsmodule.c
index 3c356cb40d2bca..830f94c254e455 100644
--- a/Modules/_interpchannelsmodule.c
+++ b/Modules/_interpchannelsmodule.c
@@ -2633,6 +2633,17 @@ _channelid_from_xid(_PyXIData_t *data)
 static int
 _channelid_shared(PyThreadState *tstate, PyObject *obj, _PyXIData_t *data)
 {
+    module_state *st = _get_current_module_state();
+    if (st == NULL) {
+        return -1;
+    }
+    if (!PyObject_IsInstance(obj, (PyObject *)st->ChannelIDType)) {
+        PyErr_Format(PyExc_TypeError,
+                     "'_id' attribute must be an %N object, not %T",
+                     st->ChannelIDType, obj);
+        return -1;
+    }
+
     if (_PyXIData_InitWithSize(
             data, tstate->interp, sizeof(struct _channelid_xid), obj,
             _channelid_from_xid