[3.12] gh-156293: Use-after-free for server-side SSLContext with sni_callback

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>

Author: sethmlarson

Doc/library/ssl.rst

@@ -1666,6 +1666,12 @@ to speed up repeated connections from the same clients.
    :class:`SSLContext` representing a certificate chain that matches the server
    name.
 
+   If the callback assigns a new context to :attr:`SSLSocket.context`, any
+   further ClientHello message on the same connection (for example after a
+   TLS 1.3 HelloRetryRequest) is dispatched to the new context's
+   *sni_callback*, if it has one; the original callback is not called again
+   for that connection.
+
    Due to the early negotiation phase of the TLS connection, only limited
    methods and attributes are usable like
    :meth:`SSLSocket.selected_alpn_protocol` and :attr:`SSLSocket.context`.
@@ -1689,6 +1695,11 @@ to speed up repeated connections from the same clients.
 
    .. versionadded:: 3.7
 
+   .. versionchanged:: next
+      After the callback assigns a new :attr:`SSLSocket.context`, later
+      ClientHello messages on the connection are dispatched to the new
+      context's *sni_callback*.
+
 .. attribute:: SSLContext.set_servername_callback(server_name_callback)
 
    This is a legacy API retained for backwards compatibility. When possible,

Lib/test/test_ssl.py

@@ -1818,6 +1818,86 @@ 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`.

Modules/_ssl.c

@@ -3184,6 +3184,9 @@ 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);
@@ -4346,27 +4349,37 @@ _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
@@ -4387,7 +4400,7 @@ _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 {
@@ -4410,14 +4423,14 @@ _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;
     }
@@ -4438,11 +4451,15 @@ _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);
@@ -4480,7 +4497,6 @@ set_sni_callback(PySSLContext *self, PyObject *arg, void *c)
         }
         self->set_sni_cb = Py_NewRef(arg);
         SSL_CTX_set_tlsext_servername_callback(self->ctx, _servername_callback);
-        SSL_CTX_set_tlsext_servername_arg(self->ctx, self);
     }
     return 0;
 }