diff --git a/quic.go b/quic.go
index 4f24fd2..baaad01 100644
--- a/quic.go
+++ b/quic.go
@@ -229,16 +229,22 @@ func (q *QUICConn) HandleData(level QUICEncryptionLevel, data []byte) error {
 		return nil
 	}
 	// The handshake goroutine has exited.
+	c.handshakeMutex.Lock()
+	defer c.handshakeMutex.Unlock()
 	c.hand.Write(c.quic.readbuf)
 	c.quic.readbuf = nil
 	for q.conn.hand.Len() >= 4 && q.conn.handshakeErr == nil {
 		b := q.conn.hand.Bytes()
 		n := int(b[1])<<16 | int(b[2])<<8 | int(b[3])
-		if 4+n < len(b) {
+		if n > maxHandshake {
+			q.conn.handshakeErr = fmt.Errorf("tls: handshake message of length %d bytes exceeds maximum of %d bytes", n, maxHandshake)
+			break
+		}
+		if len(b) < 4+n {
 			return nil
 		}
 		if err := q.conn.handlePostHandshakeMessage(); err != nil {
-			return quicError(err)
+			q.conn.handshakeErr = err
 		}
 	}
 	if q.conn.handshakeErr != nil {