gh-160171: Fix ssl get_unverified_chain() on resumed server sessions (GH-160172)

Include the client's certificate when the session was resumed from a
ticket, and return [] instead of raising AttributeError when the client
sent no certificate.

ANT-2026-2RMA0IPV

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

Author: gpshead

Doc/library/ssl.rst

@@ -1296,13 +1296,21 @@ SSL sockets also have the following additional methods and attributes:
    If certificate verification was disabled method acts the same as
    :meth:`~SSLSocket.get_unverified_chain`.
 
+   The verified chain is not stored with a session, so the list is empty
+   when the session is resumed (see :attr:`~SSLSocket.session_reused`).
+
    .. versionadded:: 3.13
 
 .. method:: SSLSocket.get_unverified_chain()
 
    Returns raw certificate chain provided by the other
    end of the SSL channel as a list of DER-encoded bytes.
 
+   No certificates are exchanged when a session is resumed (see
+   :attr:`~SSLSocket.session_reused`); the list then holds what was stored
+   with the session, which for a session resumed from a ticket on the
+   server side is the peer's own certificate only.
+
    .. versionadded:: 3.13
 
 .. method:: SSLSocket.cipher()

Lib/test/test_ssl.py

@@ -3017,11 +3017,19 @@ def run(self):
                         cert = self.sslconn.getpeercert()
                         self.write(repr(cert).encode("us-ascii") + b"\n")
                     elif stripped == b'VERIFIEDCHAIN':
-                        certs = self.sslconn._sslobj.get_verified_chain()
-                        self.write(len(certs).to_bytes(1, "big") + b"\n")
+                        try:
+                            certs = self.sslconn.get_verified_chain()
+                        except Exception as e:
+                            self.write(repr(e).encode("us-ascii") + b"\n")
+                        else:
+                            self.write(len(certs).to_bytes(1, "big") + b"\n")
                     elif stripped == b'UNVERIFIEDCHAIN':
-                        certs = self.sslconn._sslobj.get_unverified_chain()
-                        self.write(len(certs).to_bytes(1, "big") + b"\n")
+                        try:
+                            certs = self.sslconn.get_unverified_chain()
+                        except Exception as e:
+                            self.write(repr(e).encode("us-ascii") + b"\n")
+                        else:
+                            self.write(len(certs).to_bytes(1, "big") + b"\n")
                     else:
                         if (support.verbose and
                             self.server.connectionchatty):
@@ -5701,6 +5709,66 @@ def test_internal_chain_server(self):
                 res = s.recv(1024)
                 self.assertEqual(res, b'\x02\n')
 
+    def test_internal_chain_server_resumed_session(self):
+        # A session resumed from a ticket stores the client's certificate
+        # but not the chain it was sent with.
+        for version in ssl.TLSVersion.TLSv1_2, ssl.TLSVersion.TLSv1_3:
+            if not has_tls_version(version):
+                continue
+            with self.subTest(version=version.name):
+                self.check_chain_server_resumed_session(version)
+
+    def check_chain_server_resumed_session(self, version):
+        client_context, server_context, hostname = testing_context()
+        client_context.load_cert_chain(SIGNED_CERTFILE)
+        server_context.verify_mode = ssl.CERT_REQUIRED
+        client_context.minimum_version = version
+        client_context.maximum_version = version
+
+        server = ThreadedEchoServer(context=server_context, chatty=False)
+        with server:
+            with client_context.wrap_socket(
+                socket.socket(),
+                server_hostname=hostname
+            ) as s:
+                s.connect((HOST, server.port))
+                s.write(b'UNVERIFIEDCHAIN\n')
+                res = s.recv(1024)
+                self.assertEqual(res, b'\x02\n')
+                # TLS 1.3 sends the ticket after the handshake, so only
+                # get the session once data was received.
+                session = s.session
+                self.assertTrue(session.has_ticket)
+            with client_context.wrap_socket(
+                socket.socket(),
+                server_hostname=hostname,
+                session=session
+            ) as s:
+                s.connect((HOST, server.port))
+                self.assertTrue(s.session_reused)
+                s.write(b'UNVERIFIEDCHAIN\n')
+                res = s.recv(1024)
+                self.assertEqual(res, b'\x01\n')
+                # The verified chain is not stored with the session at all.
+                s.write(b'VERIFIEDCHAIN\n')
+                res = s.recv(1024)
+                self.assertEqual(res, b'\x00\n')
+
+    def test_internal_chain_server_no_client_cert(self):
+        client_context, server_context, hostname = testing_context()
+        server_context.verify_mode = ssl.CERT_OPTIONAL
+
+        server = ThreadedEchoServer(context=server_context, chatty=False)
+        with server:
+            with client_context.wrap_socket(
+                socket.socket(),
+                server_hostname=hostname
+            ) as s:
+                s.connect((HOST, server.port))
+                s.write(b'UNVERIFIEDCHAIN\n')
+                res = s.recv(1024)
+                self.assertEqual(res, b'\x00\n')
+
 
 class TestSSLDebug(unittest.TestCase):
 

Misc/NEWS.d/next/Library/2026-10-22-00-33-42.gh-issue-160171.11l4om.rst

@@ -0,0 +1,4 @@
+On the server side, :meth:`ssl.SSLSocket.get_unverified_chain` now returns
+the client's certificate when the session was resumed from a session ticket,
+instead of an empty list, and returns an empty list instead of raising
+:exc:`AttributeError` when the client sent no certificate.

Modules/_ssl.c

@@ -1991,31 +1991,37 @@ _ssl__SSLSocket_get_unverified_chain_impl(PySSLSocket *self)
 /*[clinic end generated code: output=5acdae414e13f913 input=079f8ff5c205cb3b]*/
 {
     PyObject *retval;
+    X509 *peer = NULL;
     /* borrowed reference */
-    /* TODO: include SSL_get_peer_certificate() for server-side sockets */
     STACK_OF(X509) *chain = SSL_get_peer_cert_chain(self->ssl);
-    if (chain == NULL) {
+    /* OpenSSL does not include the peer cert in the chain for server side
+     * connections, so the chain alone does not tell whether the client sent
+     * a certificate: the chain is empty when it did not, and NULL when the
+     * session was resumed from a ticket (which stores the peer cert only). */
+    if (self->socket_type == PY_SSL_SERVER) {
+        peer = SSL_get_peer_certificate(self->ssl);
+        if (peer == NULL) {
+            Py_RETURN_NONE;
+        }
+    }
+    else if (chain == NULL) {
         Py_RETURN_NONE;
     }
-    retval = _PySSL_CertificateFromX509Stack(self->ctx->state, chain, 1);
+    retval = chain == NULL
+        ? PyList_New(0)
+        : _PySSL_CertificateFromX509Stack(self->ctx->state, chain, 1);
     if (retval == NULL) {
+        X509_free(peer);
         return NULL;
     }
-    /* OpenSSL does not include peer cert for server side connections */
-    if (self->socket_type == PY_SSL_SERVER) {
-        PyObject *peerobj = NULL;
-        X509 *peer = SSL_get_peer_certificate(self->ssl);
-
-        if (peer == NULL) {
-            peerobj = Py_NewRef(Py_None);
-        } else {
-            /* consume X509 reference on success */
-            peerobj = _PySSL_CertificateFromX509(self->ctx->state, peer, 0);
-            if (peerobj == NULL) {
-                X509_free(peer);
-                Py_DECREF(retval);
-                return NULL;
-            }
+    if (peer != NULL) {
+        /* consume X509 reference on success */
+        PyObject *peerobj = _PySSL_CertificateFromX509(self->ctx->state,
+                                                       peer, 0);
+        if (peerobj == NULL) {
+            X509_free(peer);
+            Py_DECREF(retval);
+            return NULL;
         }
         int res = PyList_Insert(retval, 0, peerobj);
         Py_DECREF(peerobj);