diff --git a/msgpack/_packer.pyx b/msgpack/_packer.pyx
index c44c6366..899f0a68 100644
--- a/msgpack/_packer.pyx
+++ b/msgpack/_packer.pyx
@@ -157,6 +157,7 @@ cdef class Packer:
         cdef const char* rawval
         cdef Py_ssize_t L
         cdef Py_buffer view
+        cdef object odata
         cdef bint strict = self.strict_types
 
         if o is None:
@@ -213,8 +214,12 @@ cdef class Packer:
                 self._pack(v, nest_limit)
         elif type(o) is ExtType if strict else isinstance(o, ExtType):
             # This should be before Tuple because ExtType is namedtuple.
-            rawval = o.data
-            L = len(o.data)
+            # Size the body from the bytes object itself. len() can be
+            # overridden on a bytes subclass so it disagrees with rawval,
+            # which would make msgpack_pack_raw_body read past the buffer.
+            odata = o.data
+            rawval = odata
+            L = Py_SIZE(odata)
             if L > ITEM_LIMIT:
                 raise ValueError("EXT data is too large")
             msgpack_pack_ext(&self.pk, <long>o.code, L)
@@ -287,11 +292,18 @@ cdef class Packer:
 
     @cython.critical_section
     def pack_ext_type(self, typecode, data):
+        cdef const char *rawval
+        cdef Py_ssize_t L
         self._check_exports()
-        if len(data) > ITEM_LIMIT:
+        # Take the body length from the bytes object, not len(): a bytes
+        # subclass can override __len__ so it no longer matches rawval and
+        # msgpack_pack_raw_body reads past the buffer.
+        rawval = data
+        L = Py_SIZE(data)
+        if L > ITEM_LIMIT:
             raise ValueError("ext data too large")
-        msgpack_pack_ext(&self.pk, typecode, len(data))
-        msgpack_pack_raw_body(&self.pk, data, len(data))
+        msgpack_pack_ext(&self.pk, typecode, L)
+        msgpack_pack_raw_body(&self.pk, rawval, L)
         if self.autoreset:
             buf = PyBytes_FromStringAndSize(self.pk.buf, self.pk.length)
             self.pk.length = 0
diff --git a/test/test_extension.py b/test/test_extension.py
index b9d8aa68..e68ddba6 100644
--- a/test/test_extension.py
+++ b/test/test_extension.py
@@ -57,6 +57,29 @@ def test_pack_ext_type_autoreset():
     assert packer.bytes() == b""
 
 
+@pytest.mark.skipif(
+    msgpack.Packer.__module__ == "msgpack.fallback",
+    reason="out-of-bounds read is specific to the C extension",
+)
+def test_pack_ext_type_len_override():
+    # A bytes subclass whose __len__ over-reports its size must not make the
+    # packer copy more than the real buffer.
+    class BadLen(bytes):
+        def __len__(self):
+            return 4096
+
+    data = BadLen(b"AB")  # real length 2
+    assert bytes.__len__(data) == 2
+
+    packed = msgpack.packb(ExtType(0x42, data))
+    assert packed == b"\xd5\x42AB"  # fixext 2, sized by the real buffer
+    assert msgpack.unpackb(packed) == ExtType(0x42, b"AB")
+
+    packer = msgpack.Packer(autoreset=False)
+    packer.pack_ext_type(0x42, data)
+    assert packer.bytes() == b"\xd5\x42AB"
+
+
 def test_unpack_ext_type():
     def check(b, expected):
         assert msgpack.unpackb(b) == expected