diff --git a/third_party/tlslite/tlslite/constants.py b/third_party/tlslite/tlslite/constants.py
index 82e8c075fe2a..8fb75d0948e4 100644
--- a/third_party/tlslite/tlslite/constants.py
+++ b/third_party/tlslite/tlslite/constants.py
@@ -58,6 +58,7 @@ class ExtensionType:    # RFC 6066 / 4366
     signed_cert_timestamps = 18  # RFC 6962
     extended_master_secret = 23  # RFC 7627
     token_binding = 24           # draft-ietf-tokbind-negotiation
+    supported_versions = 43      # draft-ietf-tls-tls13-18
     tack = 0xF300
     supports_npn = 13172
     channel_id = 30032
diff --git a/third_party/tlslite/tlslite/messages.py b/third_party/tlslite/tlslite/messages.py
index ac7e563021d9..b29db939c2a8 100644
--- a/third_party/tlslite/tlslite/messages.py
+++ b/third_party/tlslite/tlslite/messages.py
@@ -140,6 +140,7 @@ class ClientHello(HandshakeMsg):
         self.tb_client_params = []
         self.support_signed_cert_timestamps = False
         self.status_request = False
+        self.has_supported_versions = False
         self.ri = False
 
     def create(self, version, random, session_id, cipher_suites,
@@ -251,6 +252,11 @@ class ClientHello(HandshakeMsg):
                         if extLength != 1 or p.getFixBytes(extLength)[0] != 0:
                             raise SyntaxError()
                         self.ri = True
+                    elif extType == ExtensionType.supported_versions:
+                        # Ignore the extension, but make a note of it for
+                        # intolerance simulation.
+                        self.has_supported_versions = True
+                        _ = p.getFixBytes(extLength)
                     else:
                         _ = p.getFixBytes(extLength)
                     index2 = p.index
diff --git a/third_party/tlslite/tlslite/tlsconnection.py b/third_party/tlslite/tlslite/tlsconnection.py
index 8ba1c6e636ab..2309d4fa8f3a 100644
--- a/third_party/tlslite/tlslite/tlsconnection.py
+++ b/third_party/tlslite/tlslite/tlsconnection.py
@@ -1457,6 +1457,15 @@ class TLSConnection(TLSRecordLayer):
         self._handshakeDone(resumed=False)
 
 
+    def _isIntolerant(self, settings, clientHello):
+        if settings.tlsIntolerant is None:
+            return False
+        clientVersion = clientHello.client_version
+        if clientHello.has_supported_versions:
+            clientVersion = (3, 4)
+        return clientVersion >= settings.tlsIntolerant
+
+
     def _serverGetClientHello(self, settings, certChain, verifierDB,
                                 sessionCache, anon, fallbackSCSV):
         #Tentatively set version to most-desirable version, so if an error
@@ -1480,8 +1489,7 @@ class TLSConnection(TLSRecordLayer):
                 yield result
 
         #If simulating TLS intolerance, reject certain TLS versions.
-        elif (settings.tlsIntolerant is not None and
-              clientHello.client_version >= settings.tlsIntolerant):
+        elif self._isIntolerant(settings, clientHello):
             if settings.tlsIntoleranceType == "alert":
                 for result in self._sendError(\
                     AlertDescription.handshake_failure):