crypto/tls: reject trailing messages after client/server hello

For TLS 1.3, after procesesing the server/client hello, if there isn't a
CCS message, reject the trailing messages which were appended to the
hello messages. This prevents an on-path attacker from injecting
plaintext messages into the handshake.

Additionally, check that we don't have any buffered messages before we
switch the read traffic secret regardless, since any buffered messages
would have been under an old key which is no longer appropriate.

We also invert the ordering of setting the read/write secrets so that if
we fail when changing the read secret we send the alert using the
correct write secret.

Fixes #76443
Fixes CVE-2025-61730

Change-Id: If6ba8ad16f48d5cd5db5574824062ad4244a5b52
Reviewed-on: https://go-review.googlesource.com/c/go/+/724120
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Coia Prant <coiaprant@gmail.com>

src/crypto/tls/conn.go

@@ -224,6 +224,9 @@ func (hc *halfConn) changeCipherSpec() error {
 	return nil
 }
 
+// setTrafficSecret sets the traffic secret for the given encryption level. setTrafficSecret
+// should not be called directly, but rather through the Conn setWriteTrafficSecret and
+// setReadTrafficSecret wrapper methods.
 func (hc *halfConn) setTrafficSecret(suite *cipherSuiteTLS13, level QUICEncryptionLevel, secret []byte) {
 	hc.trafficSecret = secret
 	hc.level = level
@@ -1339,9 +1342,6 @@ func (c *Conn) handleKeyUpdate(keyUpdate *keyUpdateMsg) error {
 		return c.in.setErrorLocked(c.sendAlert(alertInternalError))
 	}
 
-	newSecret := cipherSuite.nextTrafficSecret(c.in.trafficSecret)
-	c.in.setTrafficSecret(cipherSuite, QUICEncryptionLevelInitial, newSecret)
-
 	if keyUpdate.updateRequested {
 		c.out.Lock()
 		defer c.out.Unlock()
@@ -1359,7 +1359,12 @@ func (c *Conn) handleKeyUpdate(keyUpdate *keyUpdateMsg) error {
 		}
 
 		newSecret := cipherSuite.nextTrafficSecret(c.out.trafficSecret)
-		c.out.setTrafficSecret(cipherSuite, QUICEncryptionLevelInitial, newSecret)
+		c.setWriteTrafficSecret(cipherSuite, QUICEncryptionLevelInitial, newSecret)
+	}
+
+	newSecret := cipherSuite.nextTrafficSecret(c.in.trafficSecret)
+	if err := c.setReadTrafficSecret(cipherSuite, QUICEncryptionLevelInitial, newSecret); err != nil {
+		return err
 	}
 
 	return nil
@@ -1576,7 +1581,9 @@ func (c *Conn) handshakeContext(ctx context.Context) (ret error) {
 			// Provide the 1-RTT read secret now that the handshake is complete.
 			// The QUIC layer MUST NOT decrypt 1-RTT packets prior to completing
 			// the handshake (RFC 9001, Section 5.7).
-			c.quicSetReadSecret(QUICEncryptionLevelApplication, c.cipherSuite, c.in.trafficSecret)
+			if err := c.quicSetReadSecret(QUICEncryptionLevelApplication, c.cipherSuite, c.in.trafficSecret); err != nil {
+				return err
+			}
 		} else {
 			c.out.Lock()
 			a, ok := errors.AsType[alert](c.out.err)
@@ -1672,3 +1679,25 @@ func (c *Conn) VerifyHostname(host string) error {
 	}
 	return c.peerCertificates[0].VerifyHostname(host)
 }
+
+// setReadTrafficSecret sets the read traffic secret for the given encryption level. If
+// being called at the same time as setWriteTrafficSecret, the caller must ensure the call
+// to setWriteTrafficSecret happens first so any alerts are sent at the write level.
+func (c *Conn) setReadTrafficSecret(suite *cipherSuiteTLS13, level QUICEncryptionLevel, secret []byte) error {
+	// Ensure that there are no buffered handshake messages before changing the
+	// read keys, since that can cause messages to be parsed that were encrypted
+	// using old keys which are no longer appropriate.
+	if c.hand.Len() != 0 {
+		c.sendAlert(alertUnexpectedMessage)
+		return errors.New("tls: handshake buffer not empty before setting read traffic secret")
+	}
+	c.in.setTrafficSecret(suite, level, secret)
+	return nil
+}
+
+// setWriteTrafficSecret sets the write traffic secret for the given encryption level. If
+// being called at the same time as setReadTrafficSecret, the caller must ensure the call
+// to setWriteTrafficSecret happens first so any alerts are sent at the write level.
+func (c *Conn) setWriteTrafficSecret(suite *cipherSuiteTLS13, level QUICEncryptionLevel, secret []byte) {
+	c.out.setTrafficSecret(suite, level, secret)
+}

src/crypto/tls/handshake_client_tls13.go

@@ -490,16 +490,17 @@ func (hs *clientHandshakeStateTLS13) establishHandshakeKeys() error {
 	handshakeSecret := earlySecret.HandshakeSecret(sharedKey)
 
 	clientSecret := handshakeSecret.ClientHandshakeTrafficSecret(hs.transcript)
-	c.out.setTrafficSecret(hs.suite, QUICEncryptionLevelHandshake, clientSecret)
+	c.setWriteTrafficSecret(hs.suite, QUICEncryptionLevelHandshake, clientSecret)
 	serverSecret := handshakeSecret.ServerHandshakeTrafficSecret(hs.transcript)
-	c.in.setTrafficSecret(hs.suite, QUICEncryptionLevelHandshake, serverSecret)
+	if err := c.setReadTrafficSecret(hs.suite, QUICEncryptionLevelHandshake, serverSecret); err != nil {
+		return err
+	}
 
 	if c.quic != nil {
-		if c.hand.Len() != 0 {
-			c.sendAlert(alertUnexpectedMessage)
-		}
 		c.quicSetWriteSecret(QUICEncryptionLevelHandshake, hs.suite.id, clientSecret)
-		c.quicSetReadSecret(QUICEncryptionLevelHandshake, hs.suite.id, serverSecret)
+		if err := c.quicSetReadSecret(QUICEncryptionLevelHandshake, hs.suite.id, serverSecret); err != nil {
+			return err
+		}
 	}
 
 	err = c.config.writeKeyLog(keyLogLabelClientHandshake, hs.hello.random, clientSecret)
@@ -710,7 +711,9 @@ func (hs *clientHandshakeStateTLS13) readServerFinished() error {
 
 	hs.trafficSecret = hs.masterSecret.ClientApplicationTrafficSecret(hs.transcript)
 	serverSecret := hs.masterSecret.ServerApplicationTrafficSecret(hs.transcript)
-	c.in.setTrafficSecret(hs.suite, QUICEncryptionLevelApplication, serverSecret)
+	if err := c.setReadTrafficSecret(hs.suite, QUICEncryptionLevelApplication, serverSecret); err != nil {
+		return err
+	}
 
 	err = c.config.writeKeyLog(keyLogLabelClientTraffic, hs.hello.random, hs.trafficSecret)
 	if err != nil {
@@ -813,16 +816,13 @@ func (hs *clientHandshakeStateTLS13) sendClientFinished() error {
 		return err
 	}
 
-	c.out.setTrafficSecret(hs.suite, QUICEncryptionLevelApplication, hs.trafficSecret)
+	c.setWriteTrafficSecret(hs.suite, QUICEncryptionLevelApplication, hs.trafficSecret)
 
 	if !c.config.SessionTicketsDisabled && c.config.ClientSessionCache != nil {
 		c.resumptionSecret = hs.masterSecret.ResumptionMasterSecret(hs.transcript)
 	}
 
 	if c.quic != nil {
-		if c.hand.Len() != 0 {
-			c.sendAlert(alertUnexpectedMessage)
-		}
 		c.quicSetWriteSecret(QUICEncryptionLevelApplication, hs.suite.id, hs.trafficSecret)
 	}
 

src/crypto/tls/handshake_server_tls13.go

@@ -410,7 +410,9 @@ func (hs *serverHandshakeStateTLS13) checkForResumption() error {
 				return err
 			}
 			earlyTrafficSecret := hs.earlySecret.ClientEarlyTrafficSecret(transcript)
-			c.quicSetReadSecret(QUICEncryptionLevelEarly, hs.suite.id, earlyTrafficSecret)
+			if err := c.quicSetReadSecret(QUICEncryptionLevelEarly, hs.suite.id, earlyTrafficSecret); err != nil {
+				return err
+			}
 		}
 
 		c.didResume = true
@@ -514,6 +516,14 @@ func (hs *serverHandshakeStateTLS13) sendDummyChangeCipherSpec() error {
 func (hs *serverHandshakeStateTLS13) doHelloRetryRequest(selectedGroup CurveID) (*keyShare, error) {
 	c := hs.c
 
+	// Make sure the client didn't send extra handshake messages alongside
+	// their initial client_hello. If they sent two client_hello messages,
+	// we will consume the second before they respond to the server_hello.
+	if c.hand.Len() != 0 {
+		c.sendAlert(alertUnexpectedMessage)
+		return nil, errors.New("tls: handshake buffer not empty before HelloRetryRequest")
+	}
+
 	// The first ClientHello gets double-hashed into the transcript upon a
 	// HelloRetryRequest. See RFC 8446, Section 4.4.1.
 	if err := transcriptMsg(hs.clientHello, hs.transcript); err != nil {
@@ -733,17 +743,18 @@ func (hs *serverHandshakeStateTLS13) sendServerParameters() error {
 	}
 	hs.handshakeSecret = earlySecret.HandshakeSecret(hs.sharedKey)
 
-	clientSecret := hs.handshakeSecret.ClientHandshakeTrafficSecret(hs.transcript)
-	c.in.setTrafficSecret(hs.suite, QUICEncryptionLevelHandshake, clientSecret)
 	serverSecret := hs.handshakeSecret.ServerHandshakeTrafficSecret(hs.transcript)
-	c.out.setTrafficSecret(hs.suite, QUICEncryptionLevelHandshake, serverSecret)
+	c.setWriteTrafficSecret(hs.suite, QUICEncryptionLevelHandshake, serverSecret)
+	clientSecret := hs.handshakeSecret.ClientHandshakeTrafficSecret(hs.transcript)
+	if err := c.setReadTrafficSecret(hs.suite, QUICEncryptionLevelHandshake, clientSecret); err != nil {
+		return err
+	}
 
 	if c.quic != nil {
-		if c.hand.Len() != 0 {
-			c.sendAlert(alertUnexpectedMessage)
-		}
 		c.quicSetWriteSecret(QUICEncryptionLevelHandshake, hs.suite.id, serverSecret)
-		c.quicSetReadSecret(QUICEncryptionLevelHandshake, hs.suite.id, clientSecret)
+		if err := c.quicSetReadSecret(QUICEncryptionLevelHandshake, hs.suite.id, clientSecret); err != nil {
+			return err
+		}
 	}
 
 	err := c.config.writeKeyLog(keyLogLabelClientHandshake, hs.clientHello.random, clientSecret)
@@ -887,13 +898,9 @@ func (hs *serverHandshakeStateTLS13) sendServerFinished() error {
 
 	hs.trafficSecret = hs.masterSecret.ClientApplicationTrafficSecret(hs.transcript)
 	serverSecret := hs.masterSecret.ServerApplicationTrafficSecret(hs.transcript)
-	c.out.setTrafficSecret(hs.suite, QUICEncryptionLevelApplication, serverSecret)
+	c.setWriteTrafficSecret(hs.suite, QUICEncryptionLevelApplication, serverSecret)
 
 	if c.quic != nil {
-		if c.hand.Len() != 0 {
-			// TODO: Handle this in setTrafficSecret?
-			c.sendAlert(alertUnexpectedMessage)
-		}
 		c.quicSetWriteSecret(QUICEncryptionLevelApplication, hs.suite.id, serverSecret)
 	}
 
@@ -1123,7 +1130,9 @@ func (hs *serverHandshakeStateTLS13) readClientFinished() error {
 		return errors.New("tls: invalid client finished hash")
 	}
 
-	c.in.setTrafficSecret(hs.suite, QUICEncryptionLevelApplication, hs.trafficSecret)
+	if err := c.setReadTrafficSecret(hs.suite, QUICEncryptionLevelApplication, hs.trafficSecret); err != nil {
+		return err
+	}
 
 	return nil
 }

src/crypto/tls/handshake_test.go

@@ -7,6 +7,7 @@ package tls
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"crypto/ed25519"
 	"crypto/x509"
 	"encoding/hex"
@@ -638,3 +639,142 @@ var clientEd25519KeyPEM = testingKey(`
 -----BEGIN TESTING KEY-----
 MC4CAQAwBQYDK2VwBCIEINifzf07d9qx3d44e0FSbV4mC/xQxT644RRbpgNpin7I
 -----END TESTING KEY-----`)
+
+func TestServerHelloTrailingMessage(t *testing.T) {
+	// In TLS 1.3 the change cipher spec message is optional. If a CCS message
+	// is not sent, after reading the ServerHello, the read traffic secret is
+	// set, and all following messages must be encrypted. If the server sends
+	// additional unencrypted messages in a record with the ServerHello, the
+	// client must either fail or ignore the additional messages.
+
+	c, s := localPipe(t)
+	go func() {
+		ctx := context.Background()
+		srv := Server(s, testConfig)
+		clientHello, _, err := srv.readClientHello(ctx)
+		if err != nil {
+			testFatal(t, err)
+		}
+
+		hs := serverHandshakeStateTLS13{
+			c:           srv,
+			ctx:         ctx,
+			clientHello: clientHello,
+		}
+		if err := hs.processClientHello(); err != nil {
+			testFatal(t, err)
+		}
+		if err := transcriptMsg(hs.clientHello, hs.transcript); err != nil {
+			testFatal(t, err)
+		}
+
+		record, err := concatHandshakeMessages(hs.hello, &encryptedExtensionsMsg{alpnProtocol: "h2"})
+		if err != nil {
+			testFatal(t, err)
+		}
+
+		if _, err := s.Write(record); err != nil {
+			testFatal(t, err)
+		}
+		srv.Close()
+	}()
+
+	cli := Client(c, testConfig)
+	expectedErr := "tls: handshake buffer not empty before setting read traffic secret"
+	if err := cli.Handshake(); err == nil {
+		t.Fatal("expected error from incomplete handshake, got nil")
+	} else if err.Error() != expectedErr {
+		t.Fatalf("expected error %q, got %q", expectedErr, err.Error())
+	}
+}
+
+func TestClientHelloTrailingMessage(t *testing.T) {
+	// Same as TestServerHelloTrailingMessage but for the client side.
+
+	c, s := localPipe(t)
+	go func() {
+		cli := Client(c, testConfig)
+
+		hello, _, _, err := cli.makeClientHello()
+		if err != nil {
+			testFatal(t, err)
+		}
+
+		record, err := concatHandshakeMessages(hello, &certificateMsgTLS13{})
+		if err != nil {
+			testFatal(t, err)
+		}
+
+		if _, err := c.Write(record); err != nil {
+			testFatal(t, err)
+		}
+		cli.Close()
+	}()
+
+	srv := Server(s, testConfig)
+	expectedErr := "tls: handshake buffer not empty before setting read traffic secret"
+	if err := srv.Handshake(); err == nil {
+		t.Fatal("expected error from incomplete handshake, got nil")
+	} else if err.Error() != expectedErr {
+		t.Fatalf("expected error %q, got %q", expectedErr, err.Error())
+	}
+}
+
+func TestDoubleClientHelloHRR(t *testing.T) {
+	// If a client sends two ClientHello messages in a single record, and the
+	// server sends a HRR after reading the first ClientHello, the server must
+	// either fail or ignore the trailing ClientHello.
+
+	c, s := localPipe(t)
+
+	go func() {
+		cli := Client(c, testConfig)
+
+		hello, _, _, err := cli.makeClientHello()
+		if err != nil {
+			testFatal(t, err)
+		}
+		hello.keyShares = nil
+
+		record, err := concatHandshakeMessages(hello, hello)
+		if err != nil {
+			testFatal(t, err)
+		}
+
+		if _, err := c.Write(record); err != nil {
+			testFatal(t, err)
+		}
+		cli.Close()
+	}()
+
+	srv := Server(s, testConfig)
+	expectedErr := "tls: handshake buffer not empty before HelloRetryRequest"
+	if err := srv.Handshake(); err == nil {
+		t.Fatal("expected error from incomplete handshake, got nil")
+	} else if err.Error() != expectedErr {
+		t.Fatalf("expected error %q, got %q", expectedErr, err.Error())
+	}
+}
+
+// concatHandshakeMessages marshals and concatenates the given handshake
+// messages into a single record.
+func concatHandshakeMessages(msgs ...handshakeMessage) ([]byte, error) {
+	var marshalled []byte
+	for _, msg := range msgs {
+		data, err := msg.marshal()
+		if err != nil {
+			return nil, err
+		}
+		marshalled = append(marshalled, data...)
+	}
+	m := len(marshalled)
+	outBuf := make([]byte, recordHeaderLen)
+	outBuf[0] = byte(recordTypeHandshake)
+	vers := VersionTLS12
+	outBuf[1] = byte(vers >> 8)
+	outBuf[2] = byte(vers)
+	outBuf[3] = byte(m >> 8)
+	outBuf[4] = byte(m)
+	outBuf = append(outBuf, marshalled...)
+	return outBuf, nil
+}

src/crypto/tls/quic.go

@@ -402,13 +402,22 @@ func (c *Conn) quicReadHandshakeBytes(n int) error {
 	return nil
 }
 
-func (c *Conn) quicSetReadSecret(level QUICEncryptionLevel, suite uint16, secret []byte) {
+func (c *Conn) quicSetReadSecret(level QUICEncryptionLevel, suite uint16, secret []byte) error {
+	// Ensure that there are no buffered handshake messages before changing the
+	// read keys, since that can cause messages to be parsed that were encrypted
+	// using old keys which are no longer appropriate.
+	// TODO(roland): we should merge this check with the similar one in setReadTrafficSecret.
+	if c.hand.Len() != 0 {
+		c.sendAlert(alertUnexpectedMessage)
+		return errors.New("tls: handshake buffer not empty before setting read traffic secret")
+	}
 	c.quic.events = append(c.quic.events, QUICEvent{
 		Kind:  QUICSetReadSecret,
 		Level: level,
 		Suite: suite,
 		Data:  secret,
 	})
+	return nil
 }
 
 func (c *Conn) quicSetWriteSecret(level QUICEncryptionLevel, suite uint16, secret []byte) {