Handle missing complex field start characters

3 files changed, 39 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 8b88dd9..b2ef148 100644
--- a/NEWS
+++ b/NEWS
@@ -7,6 +7,9 @@
   Note that it is still strongly recommended to process untrusted documents in
   a separate thread with a timeout to avoid potential similar issues.
 
+* Handle complex field separator and end characters without corresponding start
+  characters.
+
 # 1.12.1
 
 * Fix: on Windows, when an image's content type includes a backslash in the
diff --git a/mammoth/docx/body_xml.py b/mammoth/docx/body_xml.py
index 2a1ffaf..4abf7f3 100644
--- a/mammoth/docx/body_xml.py
+++ b/mammoth/docx/body_xml.py
@@ -203,6 +203,11 @@ def _create_reader(numbering, content_types, relationships, styles, docx_file, f
             del current_instr_text[:]
 
         elif fld_char_type == "end":
+            if len(complex_field_stack) == 0:
+                return _empty_result_with_message(results.warning(
+                    "ignoring complex field end character without corresponding start character",
+                ))
+
             complex_field = complex_field_stack.pop()
             if isinstance(complex_field, complex_fields.Begin):
                 complex_field = parse_current_instr_text(complex_field)
@@ -211,6 +216,11 @@ def _create_reader(numbering, content_types, relationships, styles, docx_file, f
                 return _success(documents.checkbox(checked=complex_field.checked))
 
         elif fld_char_type == "separate":
+            if len(complex_field_stack) == 0:
+                return _empty_result_with_message(results.warning(
+                    "ignoring complex field separator character without corresponding start character",
+                ))
+
             complex_field_separate = complex_field_stack.pop()
             complex_field = parse_current_instr_text(complex_field_separate)
             complex_field_stack.append(complex_field)
diff --git a/tests/docx/body_xml_tests.py b/tests/docx/body_xml_tests.py
index 664fb5a..b524392 100644
--- a/tests/docx/body_xml_tests.py
+++ b/tests/docx/body_xml_tests.py
@@ -681,6 +681,32 @@ class ComplexFieldTests(object):
             is_empty_run,
         )))
 
+    def test_separator_character_without_corresponding_start_character_is_ignored(self):
+        element = xml_element("w:p", {}, [
+            self._SEPARATE_COMPLEX_FIELD,
+        ])
+
+        result = _read_document_xml_element(element)
+
+        assert_that(result.value, is_paragraph(children=is_sequence(
+            is_empty_run,
+        )))
+        expected_warning = results.warning("ignoring complex field separator character without corresponding start character")
+        assert_equal([expected_warning], result.messages)
+
+    def test_end_character_without_corresponding_start_character_is_ignored(self):
+        element = xml_element("w:p", {}, [
+            self._END_COMPLEX_FIELD,
+        ])
+
+        result = _read_document_xml_element(element)
+
+        assert_that(result.value, is_paragraph(children=is_sequence(
+            is_empty_run,
+        )))
+        expected_warning = results.warning("ignoring complex field end character without corresponding start character")
+        assert_equal([expected_warning], result.messages)
+
 
 class CheckboxTests:
     def test_complex_field_checkbox_without_separate_is_read(self):