Reject ext32 length 0xffffffff in the C unpacker

The trail length of an ext value includes the typecode byte, so
0xffffffff + 1 wrapped to zero in 32-bit arithmetic. Unpacker.skip()
then treated the 5-byte ext32 header as a complete object and parsed
the remaining bytes as the next object. unpack() was already rejected
by the max_ext_len check, but with a confusing "0 exceeds" message.

Raise ValueError before adding the typecode byte.

Reported by Oz Kaiser (@tatikoma).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: methane

msgpack/unpack_template.h

@@ -232,10 +232,15 @@ static inline int unpack_execute(bool construct, unpack_context* ctx, const char
                 again_fixed_trail_if_zero(ACS_EXT_VALUE,
                                           _msgpack_load16(uint16_t,n)+1,
                                           _ext_zero);
-            case CS_EXT_32:
-                again_fixed_trail_if_zero(ACS_EXT_VALUE,
-                                          _msgpack_load32(uint32_t,n)+1,
-                                          _ext_zero);
+            case CS_EXT_32: {
+                    uint32_t size = _msgpack_load32(uint32_t,n);
+                    // trail includes the typecode, so size+1 must not wrap around.
+                    if (size == UINT32_MAX) {
+                        PyErr_Format(PyExc_ValueError, "%u exceeds max ext length", size);
+                        goto _failed;
+                    }
+                    again_fixed_trail_if_zero(ACS_EXT_VALUE, size+1, _ext_zero);
+                }
             case CS_FLOAT: {
                     double f;
 #if PY_VERSION_HEX >= 0x030B00A7

test/test_limits.py

@@ -191,6 +191,15 @@ def test_max_ext_len():
         unpacker.unpack()
 
 
+@pytest.mark.parametrize("method", ["unpack", "skip"])
+def test_ext32_max_length(method):
+    # ext32 with length 0xffffffff followed by bytes that look like another object.
+    unpacker = Unpacker()
+    unpacker.feed(b"\xc9\xff\xff\xff\xff" + packb({"a": 1}))
+    with pytest.raises(UnpackValueError):
+        getattr(unpacker, method)()
+
+
 # PyPy fails following tests because of constant folding?
 # https://bugs.pypy.org/issue1721
 # @pytest.mark.skipif(True, reason="Requires very large memory.")