[3.11] gh-156293: ssl: do not call the servername callback through a released SSLContext

The servername (SNI) callback located its SSLContext through a borrowed
pointer registered with OpenSSL, which can outlive the SSLContext object.
Look the context up from the SSL object instead, and unregister the
callback when the context is deallocated.

Backport of 87665c9,
adapted to this branch's servername callback code.

Author: gpshead

Lib/test/test_ssl.py

@@ -2038,6 +2038,86 @@ def test_unwrap(self):
         c_in.write(s_out.read())
         client.unwrap()
 
+    def test_sni_callback_context_released_and_callback_raises(self):
+        # Variant of the test below without a HelloRetryRequest: the callback
+        # switches the connection to another context, drops the last
+        # references to the context that carries it, and raises.  The C
+        # callback must not touch that context after the Python callback
+        # returned.
+        client_ctx, server_ctx, hostname = testing_context()
+        leaf_ctx = server_ctx
+
+        def sni_cb(sslobj, server_name, ctx):
+            sslobj.context = leaf_ctx
+            del ctx
+            raise LookupError("no certificate for " + repr(server_name))
+
+        def make_server():
+            dispatch_ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
+            dispatch_ctx.load_cert_chain(SIGNED_CERTFILE)
+            dispatch_ctx.sni_callback = sni_cb
+            s_in, s_out = ssl.MemoryBIO(), ssl.MemoryBIO()
+            server = dispatch_ctx.wrap_bio(s_in, s_out, server_side=True)
+            return server, s_in, s_out
+
+        server, s_in, s_out = make_server()
+        c_in, c_out = ssl.MemoryBIO(), ssl.MemoryBIO()
+        client = client_ctx.wrap_bio(c_in, c_out, server_hostname=hostname)
+        with self.assertRaises(ssl.SSLWantReadError):
+            client.do_handshake()
+        s_in.write(c_out.read())
+        with support.catch_unraisable_exception() as cm:
+            with self.assertRaises(ssl.SSLError):
+                server.do_handshake()
+            self.assertIsInstance(cm.unraisable.exc_value, LookupError)
+        self.assertIs(server.context, leaf_ctx)
+
+    def test_sni_callback_context_released_before_second_client_hello(self):
+        # The SSLContext carrying sni_callback may be released by the
+        # application once the callback has switched the connection over to
+        # another context.  If the server then sends a HelloRetryRequest, the
+        # second ClientHello makes OpenSSL consult the original SSL_CTX's
+        # servername callback again; that must not use the deallocated
+        # SSLContext object.
+        client_ctx, leaf_ctx, hostname = testing_context()
+        calls = []
+
+        def make_server():
+            dispatch_ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
+            dispatch_ctx.load_cert_chain(SIGNED_CERTFILE)
+            # Force a HelloRetryRequest: the client offers an X25519 key
+            # share first, the server only accepts P-384.
+            dispatch_ctx.set_ecdh_curve("secp384r1")
+            def sni_cb(sslobj, server_name, ctx):
+                calls.append(server_name)
+                sslobj.context = leaf_ctx
+            dispatch_ctx.sni_callback = sni_cb
+            s_in, s_out = ssl.MemoryBIO(), ssl.MemoryBIO()
+            server = dispatch_ctx.wrap_bio(s_in, s_out, server_side=True)
+            return server, s_in, s_out, weakref.ref(dispatch_ctx)
+
+        # After this only the C-level SSL object references dispatch_ctx.
+        server, s_in, s_out, dispatch_ref = make_server()
+        c_in, c_out = ssl.MemoryBIO(), ssl.MemoryBIO()
+        client = client_ctx.wrap_bio(c_in, c_out, server_hostname=hostname)
+        for _ in range(10):
+            for obj, out, peer_in in ((client, c_out, s_in),
+                                      (server, s_out, c_in)):
+                try:
+                    obj.do_handshake()
+                except ssl.SSLWantReadError:
+                    pass
+                if out.pending:
+                    peer_in.write(out.read())
+        client.do_handshake()
+        server.do_handshake()
+        support.gc_collect()
+        self.assertIsNone(dispatch_ref())
+        self.assertGreaterEqual(len(calls), 1)
+        self.assertEqual(calls[0], hostname)
+        self.assertIs(server.context, leaf_ctx)
+        self.assertIsNotNone(client.cipher())
+
 class SimpleBackgroundTests(unittest.TestCase):
     """Tests that connect to a simple server running in the background"""
 

Misc/NEWS.d/next/Security/2026-08-10-12-00-00.gh-issue-156293.sNIcbk.rst

@@ -0,0 +1,7 @@
+Fix a crash in :mod:`ssl` when an :attr:`~ssl.SSLContext.sni_callback`
+switches a connection to another :class:`~ssl.SSLContext` and the context
+that carries the callback is no longer referenced by the application.
+Servers that keep their ``sni_callback`` context alive (the usual case when
+it wraps the listening socket or is stored on the server object) were not
+affected.
+This addresses `CVE-2026-19445 <https://www.cve.org/CVERecord?id=CVE-2026-19445>`_.

Modules/_ssl.c

@@ -3256,6 +3256,9 @@ context_dealloc(PySSLContext *self)
     /* bpo-31095: UnTrack is needed before calling any callbacks */
     PyObject_GC_UnTrack(self);
     context_clear(self);
+    /* The SSL_CTX may outlive this object as the session_ctx of sockets that
+       were switched to another context; leave no Python callback behind. */
+    SSL_CTX_set_tlsext_servername_callback(self->ctx, NULL);
     SSL_CTX_free(self->ctx);
     PyMem_FREE(self->alpn_protocols);
     Py_TYPE(self)->tp_free(self);
@@ -4410,27 +4413,37 @@ _ssl__SSLContext_set_ecdh_curve(PySSLContext *self, PyObject *name)
 }
 
 static int
-_servername_callback(SSL *s, int *al, void *args)
+_servername_callback(SSL *s, int *al, void *Py_UNUSED(args))
 {
     int ret;
-    PySSLContext *sslctx = (PySSLContext *) args;
+    PySSLContext *sslctx;
     PySSLSocket *ssl;
     PyObject *result;
     /* The high-level ssl.SSLSocket object */
     PyObject *ssl_socket;
+    PyObject *sni_cb;
     const char *servername = SSL_get_servername(s, TLSEXT_NAMETYPE_host_name);
     PyGILState_STATE gstate = PyGILState_Ensure();
 
-    if (sslctx->set_sni_cb == NULL) {
-        /* remove race condition in this the call back while if removing the
-         * callback is in progress */
+    /* Do not use the SSL_CTX's servername arg to find the context: it is a
+       borrowed pointer to whichever _SSLContext installed the callback, and
+       that object may already be gone while OpenSSL still reaches this
+       callback through the connection's session_ctx (e.g. on the second
+       ClientHello after a HelloRetryRequest, once sni_callback has switched
+       the socket to another context).  The socket's current context is
+       always alive; hold strong references to it and to the callback while
+       they are used here. */
+    ssl = SSL_get_app_data(s);
+    assert(ssl != NULL);
+    sslctx = (PySSLContext *)Py_NewRef(ssl->ctx);
+    assert(Py_IS_TYPE(ssl, get_state_ctx(sslctx)->PySSLSocket_Type));
+    sni_cb = Py_XNewRef(sslctx->set_sni_cb);
+    if (sni_cb == NULL) {
+        Py_DECREF(sslctx);
         PyGILState_Release(gstate);
         return SSL_TLSEXT_ERR_OK;
     }
 
-    ssl = SSL_get_app_data(s);
-    assert(Py_IS_TYPE(ssl, get_state_ctx(sslctx)->PySSLSocket_Type));
-
     /* The servername callback expects an argument that represents the current
      * SSL connection and that has a .context attribute that can be changed to
      * identify the requested hostname. Since the official API is the Python
@@ -4451,7 +4464,7 @@ _servername_callback(SSL *s, int *al, void *args)
         goto error;
 
     if (servername == NULL) {
-        result = PyObject_CallFunctionObjArgs(sslctx->set_sni_cb, ssl_socket,
+        result = PyObject_CallFunctionObjArgs(sni_cb, ssl_socket,
                                               Py_None, sslctx, NULL);
     }
     else {
@@ -4474,14 +4487,14 @@ _servername_callback(SSL *s, int *al, void *args)
         }
         Py_DECREF(servername_bytes);
         result = PyObject_CallFunctionObjArgs(
-            sslctx->set_sni_cb, ssl_socket, servername_str,
+            sni_cb, ssl_socket, servername_str,
             sslctx, NULL);
         Py_DECREF(servername_str);
     }
     Py_DECREF(ssl_socket);
 
     if (result == NULL) {
-        PyErr_WriteUnraisable(sslctx->set_sni_cb);
+        PyErr_WriteUnraisable(sni_cb);
         *al = SSL_AD_HANDSHAKE_FAILURE;
         ret = SSL_TLSEXT_ERR_ALERT_FATAL;
     }
@@ -4502,11 +4515,15 @@ _servername_callback(SSL *s, int *al, void *args)
         Py_DECREF(result);
     }
 
+    Py_DECREF(sni_cb);
+    Py_DECREF(sslctx);
     PyGILState_Release(gstate);
     return ret;
 
 error:
     Py_DECREF(ssl_socket);
+    Py_DECREF(sni_cb);
+    Py_DECREF(sslctx);
     *al = SSL_AD_INTERNAL_ERROR;
     ret = SSL_TLSEXT_ERR_ALERT_FATAL;
     PyGILState_Release(gstate);
@@ -4546,7 +4563,6 @@ set_sni_callback(PySSLContext *self, PyObject *arg, void *c)
         Py_INCREF(arg);
         self->set_sni_cb = arg;
         SSL_CTX_set_tlsext_servername_callback(self->ctx, _servername_callback);
-        SSL_CTX_set_tlsext_servername_arg(self->ctx, self);
     }
     return 0;
 }