From d0a757775795fa0e84dd0c6d5bfede2d5ed634e7 Mon Sep 17 00:00:00 2001
From: pennam <m.pennasilico@arduino.cc>
Date: Mon, 13 Jul 2026 15:50:30 +0200
Subject: [PATCH] WebSocketClient: check extended length/mask bytes are
 available

Only 2 header bytes were guaranteed before reading the extended length and
mask, so a partial header made read() return -1 and corrupt iRxSize. Bail
out until all header bytes have arrived.
---
 src/WebSocketClient.cpp | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/src/WebSocketClient.cpp b/src/WebSocketClient.cpp
index f7c1ddc..e97d4a9 100644
--- a/src/WebSocketClient.cpp
+++ b/src/WebSocketClient.cpp
@@ -206,6 +206,26 @@ int WebSocketClient::parseMessage()
     iRxMasked = (length & 0x80);
     length &= 0x7f;
 
+    // work out how many extended length and mask bytes follow the
+    // initial opcode + length bytes
+    int extendedLength = 0;
+    if (length == 126)
+    {
+        extendedLength = 2;
+    }
+    else if (length == 127)
+    {
+        extendedLength = 8;
+    }
+    int maskLength = iRxMasked ? (int)sizeof(iRxMaskKey) : 0;
+
+    // make sure the remaining header bytes are available before reading
+    // them, otherwise the frame header is incomplete
+    if (HttpClient::available() < (extendedLength + maskLength))
+    {
+        return 0;
+    }
+
     // read the RX size
     iRxSize = length;
     if (length == 126)