From ab07ec1a2fbb210a42b5422e5e3a2661eb248f98 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Matthias=20Walln=C3=B6fer?= <mdw@samba.org>
Date: Mon, 3 Mar 2025 11:34:33 +0100
Subject: [PATCH] MqttMsgBase - implement mqtt message length requirements
 (2.2.3 Remaining Length)

---
 M2Mqtt/Exceptions/MqttClientException.cs | 8 +++++++-
 M2Mqtt/Messages/MqttMsgBase.cs           | 8 ++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/M2Mqtt/Exceptions/MqttClientException.cs b/M2Mqtt/Exceptions/MqttClientException.cs
index cd8303fc..de2c0156 100644
--- a/M2Mqtt/Exceptions/MqttClientException.cs
+++ b/M2Mqtt/Exceptions/MqttClientException.cs
@@ -127,6 +127,12 @@ public enum MqttClientErrorCode
         /// <summary>
         /// Invalid protocol name
         /// </summary>
-        InvalidProtocolName
+        InvalidProtocolName,
+
+        // [v3.1.1]
+        /// <summary>
+        /// Invalid message length
+        /// </summary>
+        InvalidLength
     }
 }
diff --git a/M2Mqtt/Messages/MqttMsgBase.cs b/M2Mqtt/Messages/MqttMsgBase.cs
index 870fe5fe..bc17bf32 100644
--- a/M2Mqtt/Messages/MqttMsgBase.cs
+++ b/M2Mqtt/Messages/MqttMsgBase.cs
@@ -174,6 +174,9 @@ protected int encodeRemainingLength(int remainingLength, byte[] buffer, int inde
                     digit = digit | 0x80;
                 buffer[index++] = (byte)digit;
             } while (remainingLength > 0);
+
+            if (index > 4) // 4 bytes [0..3] is the maximum allowed (2.2.3 Remaining Length)
+                throw new MqttClientException(MqttClientErrorCode.InvalidLength);
             return index;
         }
 
@@ -188,6 +191,7 @@ protected static int decodeRemainingLength(IMqttNetworkChannel channel)
             int value = 0;
             int digit = 0;
             byte[] nextByte = new byte[1];
+            int index = 0;
             do
             {
                 // next digit from stream
@@ -195,6 +199,10 @@ protected static int decodeRemainingLength(IMqttNetworkChannel channel)
                 digit = nextByte[0];
                 value += ((digit & 127) * multiplier);
                 multiplier *= 128;
+                ++index;
+
+                if (index > 4) // 4 bytes [0..3] is the maximum allowed (2.2.3 Remaining Length)
+                    throw new MqttClientException(MqttClientErrorCode.InvalidLength);
             } while ((digit & 128) != 0);
             return value;
         }