diff --git a/index.bs b/index.bs
index c77dcea..1946c31 100644
--- a/index.bs
+++ b/index.bs
@@ -112,6 +112,7 @@ interface Socket {
   Promise<undefined> close(optional any reason);
 
   readonly attribute boolean upgraded;
+  readonly attribute String secureTransport;
 
   [NewObject] Socket startTls();
 };
@@ -211,6 +212,16 @@ Cancelling the socket's ReadableStream and closing the socket's WritableStream d
 
 The {{upgraded}} attribute is a boolean flag that indicates whether the socket has been upgraded to a secure connection (using `startTLS()`).
 
+<h4 id="secureTransport-attribute">secureTransport</h4>
+
+The {{secureTransport}} attribute is a string value indicating the type of secure transport used by the socket. Possible values are:
+
+<ul>
+  <li>"on" - TLS/SSL connection</li>
+  <li>"starttls" - Opportunistic TLS connection</li>
+  <li>"off" - Insecure TLS connection</li>
+</ul>
+
 <h3 id="methods">Methods</h3>
 
 <h4 id="close-method">close(optional any reason)</h4>