Raise ValueError if value is not bytes for TIFF_BYTE or TIFF_ASCII tag

Tests/test_file_libtiff.py

@@ -377,6 +377,21 @@ def test_tag_type(
             assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
             assert reloaded.tag_v2[37000] == 100
 
+    @pytest.mark.parametrize("tagtype", (TiffTags.BYTE, TiffTags.ASCII))
+    def test_non_bytes(
+        self, tagtype: int, monkeypatch: pytest.MonkeyPatch, tmp_path: Path
+    ) -> None:
+        monkeypatch.setattr(TiffImagePlugin, "WRITE_LIBTIFF", True)
+
+        ifd = TiffImagePlugin.ImageFileDirectory_v2()
+        ifd[37000] = 100
+        ifd.tagtype[37000] = tagtype
+
+        out = tmp_path / "temp.tif"
+        im = Image.new("L", (1, 1))
+        with pytest.raises(ValueError, match="Incorrect tag value type"):
+            im.save(out, tiffinfo=ifd)
+
     def test_inknames_tag(
         self, monkeypatch: pytest.MonkeyPatch, tmp_path: Path
     ) -> None:

src/encode.c

@@ -849,6 +849,11 @@ PyImaging_LibTiffEncoderNew(PyObject *self, PyObject *args) {
 
         if (type == TIFF_BYTE || type == TIFF_UNDEFINED ||
             key_int == TIFFTAG_INKNAMES) {
+            if (!PyBytes_Check(value)) {
+                Py_DECREF(encoder);
+                PyErr_SetString(PyExc_ValueError, "Incorrect tag value type");
+                return NULL;
+            }
             status = ImagingLibTiffSetField(
                 &encoder->state,
                 (ttag_t)key_int,
@@ -1022,6 +1027,11 @@ PyImaging_LibTiffEncoderNew(PyObject *self, PyObject *args) {
                     &encoder->state, (ttag_t)key_int, (INT8)PyLong_AsLong(value)
                 );
             } else if (type == TIFF_ASCII) {
+                if (!PyBytes_Check(value)) {
+                    Py_DECREF(encoder);
+                    PyErr_SetString(PyExc_ValueError, "Incorrect tag value type");
+                    return NULL;
+                }
                 status = ImagingLibTiffSetField(
                     &encoder->state, (ttag_t)key_int, PyBytes_AsString(value)
                 );