fix asyncio SSL transports not sending the fatal TLS alert to the

peer when the TLS handshake fails

Author: kumaraditya303

Lib/asyncio/sslproto.py

@@ -588,6 +588,12 @@ def _on_handshake_complete(self, handshake_exc):
                 msg = 'SSL handshake failed on verifying the certificate'
             else:
                 msg = 'SSL handshake failed'
+            # gh-98078: When the handshake fails, OpenSSL leaves the fatal
+            # TLS alert (for example "bad certificate" or "protocol
+            # version") in the outgoing BIO.  Send it to the peer before
+            # closing the transport so that it knows why the handshake
+            # failed.
+            self._process_outgoing()
             self._fatal_error(exc, msg)
             self._wakeup_waiter(exc)
             return

Lib/test/test_asyncio/test_sslproto.py

@@ -753,14 +753,16 @@ def test_create_connection_ssl_failed_certificate(self):
         sslctx = test_utils.simple_server_sslcontext()
         client_sslctx = test_utils.simple_client_sslcontext(
             disable_verify=False)
+        server_err = None
 
         def server(sock):
+            nonlocal server_err
             try:
                 sock.start_tls(
                     sslctx,
                     server_side=True)
-            except ssl.SSLError:
-                pass
+            except ssl.SSLError as exc:
+                server_err = exc
             except OSError:
                 pass
             finally:
@@ -780,6 +782,62 @@ async def client(addr):
             with self.assertRaises(ssl.SSLCertVerificationError):
                 self.loop.run_until_complete(client(srv.addr))
 
+        # gh-98078: the client must send a fatal TLS alert to the
+        # server instead of just closing the connection, so that the
+        # server knows why the handshake failed.
+        self.assertIsInstance(server_err, ssl.SSLError)
+        self.assertIn('ALERT_UNKNOWN_CA', server_err.reason or '')
+
+    def test_create_server_ssl_failed_handshake_sends_alert(self):
+        # gh-98078: when the handshake fails, the server must send the
+        # fatal TLS alert generated by OpenSSL to the client before
+        # closing the connection, so that the client knows why the
+        # handshake failed (here: no TLS version in common).
+        if not ssl.HAS_TLSv1_3 or not ssl.HAS_TLSv1_2:
+            self.skipTest('needs TLSv1.2 and TLSv1.3 support')
+
+        self.loop.set_exception_handler(lambda loop, ctx: None)
+
+        server_context = test_utils.simple_server_sslcontext()
+        server_context.minimum_version = ssl.TLSVersion.TLSv1_3
+        client_context = test_utils.simple_client_sslcontext()
+        client_context.maximum_version = ssl.TLSVersion.TLSv1_2
+
+        client_done = self.loop.create_future()
+        client_err = None
+
+        def client(sock, addr):
+            nonlocal client_err
+            try:
+                sock.settimeout(self.TIMEOUT)
+                sock.connect(addr)
+                try:
+                    sock.start_tls(client_context)
+                except OSError as exc:
+                    client_err = exc
+                finally:
+                    sock.close()
+            finally:
+                self.loop.call_soon_threadsafe(
+                    client_done.set_result, None)
+
+        async def run_main():
+            server = await self.loop.create_server(
+                asyncio.Protocol, '127.0.0.1', 0, ssl=server_context)
+            addr = server.sockets[0].getsockname()
+            try:
+                with self.tcp_client(lambda sock: client(sock, addr),
+                                     timeout=self.TIMEOUT):
+                    await asyncio.wait_for(client_done, self.TIMEOUT)
+            finally:
+                server.close()
+                await server.wait_closed()
+
+        self.loop.run_until_complete(run_main())
+
+        self.assertIsInstance(client_err, ssl.SSLError)
+        self.assertIn('ALERT_PROTOCOL_VERSION', client_err.reason or '')
+
     def test_start_tls_client_corrupted_ssl(self):
         self.loop.set_exception_handler(lambda loop, ctx: None)
 

Misc/NEWS.d/next/Library/2026-07-12-10-00-00.gh-issue-98078.fA3lRt.rst

@@ -0,0 +1,4 @@
+Fix :mod:`asyncio` SSL transports not sending the fatal TLS alert to the
+peer when the TLS handshake fails. The peer can now tell why the handshake
+failed (for example, certificate verification failure or no TLS version in
+common) instead of seeing the connection abruptly closed.