[3.10] gh-145986: Avoid unbound C recursion in conv_content_model in pyexpat.c (CVE 2026-4224) (GH-145987)

Fix C stack overflow (CVE-2026-4224) when an Expat parser
with a registered `ElementDeclHandler` parses inline DTD
containing deeply nested content model.

---------
(cherry picked from commit eb0e8be)
(cherry picked from commit e5caf45)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: StanFromIreland

Lib/test/test_pyexpat.py

@@ -16,7 +16,7 @@
 from xml.parsers import expat
 from xml.parsers.expat import errors
 
-from test.support import import_helper
+from test.support import import_helper, infinite_recursion
 from test.support import sortdict
 
 
@@ -648,6 +648,24 @@ def test_change_size_2(self):
         parser.Parse(xml2, True)
         self.assertEqual(self.n, 4)
 
+class ElementDeclHandlerTest(unittest.TestCase):
+    def test_deeply_nested_content_model(self):
+        # This should raise a RecursionError and not crash.
+        # See https://github.com/python/cpython/issues/145986.
+        N = 500_000
+        data = (
+            b'<!DOCTYPE root [\n<!ELEMENT root '
+            + b'(a, ' * N + b'a' + b')' * N
+            + b'>\n]>\n<root/>\n'
+        )
+
+        parser = expat.ParserCreate()
+        parser.ElementDeclHandler = lambda _1, _2: None
+        with infinite_recursion():
+            with self.assertRaises(RecursionError):
+                parser.Parse(data)
+
+
 class MalformedInputTest(unittest.TestCase):
     def test1(self):
         xml = b"\0\r\n"

Misc/NEWS.d/next/Security/2026-03-14-17-31-39.gh-issue-145986.ifSSr8.rst

@@ -0,0 +1,4 @@
+:mod:`xml.parsers.expat`: Fixed a crash caused by unbounded C recursion when
+converting deeply nested XML content models with
+:meth:`~xml.parsers.expat.xmlparser.ElementDeclHandler`.
+This addresses :cve:`2026-4224`.

Modules/pyexpat.c

@@ -574,6 +574,10 @@ static PyObject *
 conv_content_model(XML_Content * const model,
                    PyObject *(*conv_string)(const XML_Char *))
 {
+    if (Py_EnterRecursiveCall(" in conv_content_model")) {
+        return NULL;
+    }
+
     PyObject *result = NULL;
     PyObject *children = PyTuple_New(model->numchildren);
     int i;
@@ -585,14 +589,16 @@ conv_content_model(XML_Content * const model,
                                                  conv_string);
             if (child == NULL) {
                 Py_XDECREF(children);
-                return NULL;
+                goto done;
             }
             PyTuple_SET_ITEM(children, i, child);
         }
         result = Py_BuildValue("(iiO&N)",
                                model->type, model->quant,
                                conv_string,model->name, children);
     }
+done:
+    Py_LeaveRecursiveCall();
     return result;
 }