crypto/tls: don't copy auto-rotated session ticket keys in Config.Clone

Once a tls.Config is used, it is not safe to mutate. We provide the
Clone method in order to allow users to copy and modify a Config that
is in use.

If Config.SessionTicketKey is not populated, and if
Config.SetSessionTicketKeys has not been called, we automatically
populate and rotate session ticket keys. Clone was previously copying
these keys into the new Config, meaning that two Configs could share
the same auto-rotated session ticket keys. This could allow sessions to
be resumed across different Configs, which may have completely different
configurations.

This change updates Clone to not copy the auto-rotated session ticket
keys.

Additionally, when resuming a session, check that not just that the leaf
certificate is unexpired, but that the entire certificate chain is still
unexpired.

Fixes #77113
Fixes CVE-2025-68121

Change-Id: I011df7329de83068d11b3f0c793763692d018a98
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3300
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/736709
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/crypto/tls/common.go

@@ -980,6 +980,10 @@ const maxSessionTicketLifetime = 7 * 24 * time.Hour
 
 // Clone returns a shallow clone of c or nil if c is nil. It is safe to clone a [Config] that is
 // being used concurrently by a TLS client or server.
+//
+// If Config.SessionTicketKey is unpopulated, and Config.SetSessionTicketKeys has not been
+// called, the clone will not share the same auto-rotated session ticket keys as the original
+// Config in order to prevent sessions from being resumed across Configs.
 func (c *Config) Clone() *Config {
 	if c == nil {
 		return nil
@@ -1020,7 +1024,8 @@ func (c *Config) Clone() *Config {
 		EncryptedClientHelloRejectionVerify: c.EncryptedClientHelloRejectionVerify,
 		EncryptedClientHelloKeys:            c.EncryptedClientHelloKeys,
 		sessionTicketKeys:                   c.sessionTicketKeys,
-		autoSessionTicketKeys:               c.autoSessionTicketKeys,
+		// We explicitly do not copy autoSessionTicketKeys, so that Configs do
+		// not share the same auto-rotated keys.
 	}
 }
 

src/crypto/tls/handshake_server.go

@@ -520,8 +520,13 @@ func (hs *serverHandshakeState) checkForResumption() error {
 	if sessionHasClientCerts && c.config.ClientAuth == NoClientCert {
 		return nil
 	}
-	if sessionHasClientCerts && c.config.time().After(sessionState.peerCertificates[0].NotAfter) {
-		return nil
+	if sessionHasClientCerts {
+		now := c.config.time()
+		for _, c := range sessionState.peerCertificates {
+			if now.After(c.NotAfter) {
+				return nil
+			}
+		}
 	}
 	if sessionHasClientCerts && c.config.ClientAuth >= VerifyClientCertIfGiven &&
 		len(sessionState.verifiedChains) == 0 {

src/crypto/tls/handshake_server_test.go

@@ -13,6 +13,7 @@ import (
 	"crypto/rand"
 	"crypto/tls/internal/fips140tls"
 	"crypto/x509"
+	"crypto/x509/pkix"
 	"encoding/pem"
 	"errors"
 	"fmt"
@@ -2153,3 +2154,103 @@ func TestHandshakeContextHierarchy(t *testing.T) {
 		t.Errorf("Unexpected client error: %v", err)
 	}
 }
+
+func TestHandshakeChainExpiryResumptionTLS12(t *testing.T) {
+	t.Run("TLS1.2", func(t *testing.T) {
+		testHandshakeChainExpiryResumption(t, VersionTLS12)
+	})
+	t.Run("TLS1.3", func(t *testing.T) {
+		testHandshakeChainExpiryResumption(t, VersionTLS13)
+	})
+}
+
+func testHandshakeChainExpiryResumption(t *testing.T, version uint16) {
+	now := time.Now()
+	createChain := func(leafNotAfter, rootNotAfter time.Time) (certDER []byte, root *x509.Certificate) {
+		tmpl := &x509.Certificate{
+			Subject:               pkix.Name{CommonName: "root"},
+			NotBefore:             rootNotAfter.Add(-time.Hour * 24),
+			NotAfter:              rootNotAfter,
+			IsCA:                  true,
+			BasicConstraintsValid: true,
+		}
+		rootDER, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, &testECDSAPrivateKey.PublicKey, testECDSAPrivateKey)
+		if err != nil {
+			t.Fatalf("CreateCertificate: %v", err)
+		}
+		root, err = x509.ParseCertificate(rootDER)
+		if err != nil {
+			t.Fatalf("ParseCertificate: %v", err)
+		}
+
+		tmpl = &x509.Certificate{
+			Subject:   pkix.Name{},
+			DNSNames:  []string{"expired-resume.example.com"},
+			NotBefore: leafNotAfter.Add(-time.Hour * 24),
+			NotAfter:  leafNotAfter,
+			KeyUsage:  x509.KeyUsageDigitalSignature,
+		}
+		certDER, err = x509.CreateCertificate(rand.Reader, tmpl, root, &testECDSAPrivateKey.PublicKey, testECDSAPrivateKey)
+		if err != nil {
+			t.Fatalf("CreateCertificate: %v", err)
+		}
+
+		return certDER, root
+	}
+
+	initialLeafDER, initialRoot := createChain(now.Add(time.Hour), now.Add(2*time.Hour))
+
+	serverConfig := testConfig.Clone()
+	serverConfig.MaxVersion = version
+	serverConfig.Certificates = []Certificate{{
+		Certificate: [][]byte{initialLeafDER},
+		PrivateKey:  testECDSAPrivateKey,
+	}}
+	serverConfig.ClientCAs = x509.NewCertPool()
+	serverConfig.ClientCAs.AddCert(initialRoot)
+	serverConfig.ClientAuth = RequireAndVerifyClientCert
+	serverConfig.Time = func() time.Time {
+		return now
+	}
+
+	clientConfig := testConfig.Clone()
+	clientConfig.MaxVersion = version
+	clientConfig.Certificates = []Certificate{{
+		Certificate: [][]byte{initialLeafDER},
+		PrivateKey:  testECDSAPrivateKey,
+	}}
+	clientConfig.RootCAs = x509.NewCertPool()
+	clientConfig.RootCAs.AddCert(initialRoot)
+	clientConfig.ServerName = "expired-resume.example.com"
+	clientConfig.ClientSessionCache = NewLRUClientSessionCache(32)
+
+	testResume := func(t *testing.T, sc, cc *Config, expectResume bool) {
+		t.Helper()
+		ss, cs, err := testHandshake(t, cc, sc)
+		if err != nil {
+			t.Fatalf("handshake: %v", err)
+		}
+		if cs.DidResume != expectResume {
+			t.Fatalf("DidResume = %v; want %v", cs.DidResume, expectResume)
+		}
+		if ss.DidResume != expectResume {
+			t.Fatalf("DidResume = %v; want %v", cs.DidResume, expectResume)
+		}
+	}
+
+	testResume(t, serverConfig, clientConfig, false)
+	testResume(t, serverConfig, clientConfig, true)
+
+	freshLeafDER, freshRoot := createChain(now.Add(2*time.Hour), now.Add(3*time.Hour))
+	clientConfig.Certificates = []Certificate{{
+		Certificate: [][]byte{freshLeafDER},
+		PrivateKey:  testECDSAPrivateKey,
+	}}
+	serverConfig.Time = func() time.Time {
+		return now.Add(1*time.Hour + 30*time.Minute)
+	}
+	serverConfig.ClientCAs = x509.NewCertPool()
+	serverConfig.ClientCAs.AddCert(freshRoot)
+
+	testResume(t, serverConfig, clientConfig, false)
+}

src/crypto/tls/handshake_server_tls13.go

@@ -314,6 +314,7 @@ func (hs *serverHandshakeStateTLS13) checkForResumption() error {
 		return nil
 	}
 
+pskIdentityLoop:
 	for i, identity := range hs.clientHello.pskIdentities {
 		if i >= maxClientPSKIdentities {
 			break
@@ -366,8 +367,13 @@ func (hs *serverHandshakeStateTLS13) checkForResumption() error {
 		if sessionHasClientCerts && c.config.ClientAuth == NoClientCert {
 			continue
 		}
-		if sessionHasClientCerts && c.config.time().After(sessionState.peerCertificates[0].NotAfter) {
-			continue
+		if sessionHasClientCerts {
+			now := c.config.time()
+			for _, c := range sessionState.peerCertificates {
+				if now.After(c.NotAfter) {
+					continue pskIdentityLoop
+				}
+			}
 		}
 		if sessionHasClientCerts && c.config.ClientAuth >= VerifyClientCertIfGiven &&
 			len(sessionState.verifiedChains) == 0 {

src/crypto/tls/tls_test.go

@@ -935,8 +935,8 @@ func TestCloneNonFuncFields(t *testing.T) {
 		}
 	}
 	// Set the unexported fields related to session ticket keys, which are copied with Clone().
-	c1.autoSessionTicketKeys = []ticketKey{c1.ticketKeyFromBytes(c1.SessionTicketKey)}
 	c1.sessionTicketKeys = []ticketKey{c1.ticketKeyFromBytes(c1.SessionTicketKey)}
+	// We explicitly don't copy autoSessionTicketKeys in Clone, so don't set it.
 
 	c2 := c1.Clone()
 	if !reflect.DeepEqual(&c1, c2) {
@@ -2461,3 +2461,12 @@ func (s messageOnlySigner) SignMessage(rand io.Reader, msg []byte, opts crypto.S
 	digest := h.Sum(nil)
 	return s.Signer.Sign(rand, digest, opts)
 }
+
+func TestConfigCloneAutoSessionTicketKeys(t *testing.T) {
+	orig := &Config{}
+	orig.ticketKeys(nil)
+	clone := orig.Clone()
+	if slices.Equal(orig.autoSessionTicketKeys, clone.autoSessionTicketKeys) {
+		t.Fatal("autoSessionTicketKeys slice copied in Clone")
+	}
+}