diff --git a/src/ngx_http_lua_socket_tcp.c b/src/ngx_http_lua_socket_tcp.c
index 027d534a5e..1822ae55c9 100644
--- a/src/ngx_http_lua_socket_tcp.c
+++ b/src/ngx_http_lua_socket_tcp.c
@@ -2283,6 +2283,14 @@ ngx_http_lua_ffi_socket_tcp_settrustedstore(ngx_http_request_t *r,
         return NGX_ERROR;
     }
 
+    if (u->peer.connection->ssl
+        && u->peer.connection->ssl->handshaked)
+    {
+        *errmsg = "ssl handshake already done; trusted store cannot be "
+                  "changed on an established TLS connection";
+        return NGX_ERROR;
+    }
+
     u->ssl_trusted_store = store;
 
     return NGX_OK;
diff --git a/t/193-ssl-trusted-store.t b/t/193-ssl-trusted-store.t
index 4b937b4907..92e5ba3d3c 100644
--- a/t/193-ssl-trusted-store.t
+++ b/t/193-ssl-trusted-store.t
@@ -352,3 +352,68 @@ settrustedstore: true nil
 [alert]
 [crit]
 [emerg]
+
+
+
+=== TEST 6: settrustedstore is rejected after the TLS handshake has completed
+--- http_config eval: $::tls_http_config
+--- config eval
+"
+    location /t {
+        content_by_lua_block {
+            local f = assert(io.open('$::HtmlDir/mtls_ca.crt'))
+            local ca_pem = f:read('*a')
+            f:close()
+
+            local store1 = assert(load_store_from_pem(ca_pem))
+
+            local sock = ngx.socket.tcp()
+            assert(sock:connect('unix:$::HtmlDir/tls.sock'))
+
+            assert(settrustedstore(sock, store1))
+
+            local sess, err = sock:sslhandshake(nil, 'example.com', true)
+            if not sess then
+                ngx.say('failed to do SSL handshake: ', err)
+                return
+            end
+
+            -- second settrustedstore on an already-handshaked cosocket
+            -- must fail with a clear error rather than silently no-op.
+            local f2 = assert(io.open('$::HtmlDir/unrelated_ca.crt'))
+            local ca_pem2 = f2:read('*a')
+            f2:close()
+            local store2 = assert(load_store_from_pem(ca_pem2))
+
+            local ok, err = settrustedstore(sock, store2)
+            ngx.say('settrustedstore after handshake: ', ok, ' ', err)
+
+            -- connection must still be usable after the rejected call.
+            local bytes, err = sock:send('GET / HTTP/1.0\\r\\nHost: example.com\\r\\n\\r\\n')
+            if not bytes then
+                ngx.say('failed to send: ', err)
+                return
+            end
+
+            local line, err = sock:receive('*l')
+            if not line then
+                ngx.say('failed to receive: ', err)
+                return
+            end
+
+            ngx.say('received: ', line)
+            sock:close()
+        }
+    }
+"
+--- user_files eval: $::tls_user_files
+--- request
+GET /t
+--- response_body_like
+^settrustedstore after handshake: nil ssl handshake already done; trusted store cannot be changed on an established TLS connection
+received: HTTP/1\.[01] 200 OK$
+--- no_error_log
+[error]
+[alert]
+[crit]
+[emerg]