diff --git a/src/iot/mqtt/server/Mqtt.cpp b/src/iot/mqtt/server/Mqtt.cpp
index 0ff1afbb8a..7280a61aba 100644
--- a/src/iot/mqtt/server/Mqtt.cpp
+++ b/src/iot/mqtt/server/Mqtt.cpp
@@ -253,6 +253,11 @@ namespace iot::mqtt::server {
                 << connectionName << " MQTT Broker:   Wrong Protocol Level: " << MQTT_VERSION_3_1_1 << " != " << connect.getLevel();
             sendConnack(MQTT_CONNACK_UNACEPTABLEVERSION, MQTT_SESSION_NEW);
 
+            mqttContext->close();
+        } else if ((connect.getConnectFlags() & 0x01) != 0) {
+            iot::mqtt::semantic::mqttServerLog().error()
+                << connectionName << " MQTT Broker:   CONNECT reserved flag bit set";
+
             mqttContext->close();
         } else if (connect.isFakedClientId() && !connect.getCleanSession()) {
             iot::mqtt::semantic::mqttServerLog().error() << connectionName << " MQTT Broker:   Resume session but no ClientId present";
diff --git a/src/iot/mqtt/server/packets/Publish.cpp b/src/iot/mqtt/server/packets/Publish.cpp
index 2c12a73fa0..9e36738712 100644
--- a/src/iot/mqtt/server/packets/Publish.cpp
+++ b/src/iot/mqtt/server/packets/Publish.cpp
@@ -56,7 +56,7 @@ namespace iot::mqtt::server::packets {
         this->dup = (flags & 0x08) != 0;
         this->retain = (flags & 0x01) != 0;
 
-        error = this->qoS > 2;
+        error = this->qoS > 2 || (this->qoS == 0 && this->dup);
     }
 
     std::size_t Publish::deserializeVP(iot::mqtt::MqttContext* mqttContext) {
@@ -69,6 +69,11 @@ namespace iot::mqtt::server::packets {
                     break;
                 }
 
+                if (topic.size() == 0) {
+                    error = true;
+                    break;
+                }
+
                 state++;
                 [[fallthrough]];
             case 1:
@@ -77,9 +82,18 @@ namespace iot::mqtt::server::packets {
                     if (!packetIdentifier.isComplete()) {
                         break;
                     }
+                    if (packetIdentifier == 0) {
+                        error = true;
+                        break;
+                    }
+                }
+
+                if (getConsumed() + consumed > getRemainingLength()) {
+                    error = true;
+                    break;
                 }
 
-                message.setSize(static_cast<uint16_t>(getRemainingLength() - getConsumed() - consumed));
+                message.setSize(getRemainingLength() - getConsumed() - consumed);
 
                 state++;
                 [[fallthrough]];
diff --git a/src/iot/mqtt/server/packets/Subscribe.cpp b/src/iot/mqtt/server/packets/Subscribe.cpp
index bc5cfdc9e5..da53da866b 100644
--- a/src/iot/mqtt/server/packets/Subscribe.cpp
+++ b/src/iot/mqtt/server/packets/Subscribe.cpp
@@ -84,6 +84,10 @@ namespace iot::mqtt::server::packets {
                 if (!qoS.isComplete()) {
                     break;
                 }
+                if (static_cast<uint8_t>(qoS) > 2) {
+                    error = true;
+                    break;
+                }
                 topics.emplace_back(topic, qoS);
                 topic.reset();
                 qoS.reset();
diff --git a/src/iot/mqtt/types/UIntV.cpp b/src/iot/mqtt/types/UIntV.cpp
index 2ddfecdd30..e4dfc647a3 100644
--- a/src/iot/mqtt/types/UIntV.cpp
+++ b/src/iot/mqtt/types/UIntV.cpp
@@ -69,10 +69,10 @@ namespace iot::mqtt::types {
             if (consumed > 0) {
                 value.push_back(byte);
 
-                if (value.size() > sizeof(uint32_t)) {
+                complete = (byte & 0x80) == 0;
+                if (value.size() > sizeof(uint32_t) || (value.size() == sizeof(uint32_t) && !complete)) {
                     error = true;
-                } else {
-                    complete = (byte & 0x80) == 0;
+                    complete = false;
                 }
             }
         } while (consumed > 0 && !complete && !error);
diff --git a/tests/unit/CMakeLists.txt b/tests/unit/CMakeLists.txt
index 2be7f82148..d1f542bb54 100644
--- a/tests/unit/CMakeLists.txt
+++ b/tests/unit/CMakeLists.txt
@@ -46,4 +46,6 @@ add_subdirectory(http)
 
 add_subdirectory(log)
 
+add_subdirectory(mqtt)
+
 add_subdirectory(utils)
diff --git a/tests/unit/mqtt/CMakeLists.txt b/tests/unit/mqtt/CMakeLists.txt
new file mode 100644
index 0000000000..59ce87599e
--- /dev/null
+++ b/tests/unit/mqtt/CMakeLists.txt
@@ -0,0 +1,5 @@
+snodec_add_test(Mqtt311PacketValidationTest Mqtt311PacketValidationTest.cpp)
+target_include_directories(Mqtt311PacketValidationTest PRIVATE ${PROJECT_SOURCE_DIR})
+target_compile_features(Mqtt311PacketValidationTest PRIVATE cxx_std_20)
+target_link_libraries(Mqtt311PacketValidationTest PRIVATE snodec-test-support snodec::mqtt snodec::mqtt-packets snodec::mqtt-server snodec::mqtt-server-packets)
+set_tests_properties(Mqtt311PacketValidationTest PROPERTIES LABELS "unit;mqtt;mqtt311;validation" TIMEOUT 10)
diff --git a/tests/unit/mqtt/Mqtt311PacketValidationTest.cpp b/tests/unit/mqtt/Mqtt311PacketValidationTest.cpp
new file mode 100644
index 0000000000..ca82972ab4
--- /dev/null
+++ b/tests/unit/mqtt/Mqtt311PacketValidationTest.cpp
@@ -0,0 +1,218 @@
+#include "iot/mqtt/MqttContext.h"
+#include "iot/mqtt/Mqtt.h"
+#include "iot/mqtt/ControlPacketDeserializer.h"
+#include "iot/mqtt/packets/Publish.h"
+#include "iot/mqtt/FixedHeader.h"
+#include "iot/mqtt/server/packets/Connect.h"
+#include "iot/mqtt/server/packets/Publish.h"
+#include "iot/mqtt/server/packets/Pubrel.h"
+#include "iot/mqtt/server/packets/Subscribe.h"
+#include "iot/mqtt/server/packets/Unsubscribe.h"
+#include "tests/support/TestResult.h"
+
+#include <algorithm>
+#include <cstdint>
+#include <string>
+#include <vector>
+
+namespace {
+
+class DummyMqtt : public iot::mqtt::Mqtt {
+public:
+    DummyMqtt() : iot::mqtt::Mqtt("test") {}
+    bool onSignal(int) override { return true; }
+    iot::mqtt::ControlPacketDeserializer* createControlPacketDeserializer(iot::mqtt::FixedHeader&) const override { return nullptr; }
+    void deliverPacket(iot::mqtt::ControlPacketDeserializer*) override {}
+    void distributePublish(const iot::mqtt::packets::Publish&) override {}
+};
+
+class BufferContext : public iot::mqtt::MqttContext {
+public:
+    explicit BufferContext(const std::vector<char>& input)
+        : iot::mqtt::MqttContext(new DummyMqtt())
+        , input(input) {
+    }
+
+    std::size_t recv(char* chunk, std::size_t chunklen) override {
+        const std::size_t available = input.size() - offset;
+        const std::size_t count = std::min(chunklen, available);
+        std::copy_n(input.data() + offset, count, chunk);
+        offset += count;
+        return count;
+    }
+
+    void send(const char*, std::size_t) override {}
+    core::socket::stream::SocketConnection* getSocketConnection() const override { return nullptr; }
+    void end() override {}
+    void close() override { closed = true; }
+
+    bool closed = false;
+
+private:
+    std::vector<char> input;
+    std::size_t offset = 0;
+};
+
+void appendString(std::vector<char>& data, const std::string& value) {
+    data.push_back(static_cast<char>((value.size() >> 8) & 0xff));
+    data.push_back(static_cast<char>(value.size() & 0xff));
+    data.insert(data.end(), value.begin(), value.end());
+}
+
+std::vector<char> connectVp(uint8_t level, uint8_t flags) {
+    std::vector<char> data;
+    appendString(data, "MQTT");
+    data.push_back(static_cast<char>(level));
+    data.push_back(static_cast<char>(flags));
+    data.push_back(0);
+    data.push_back(60);
+    appendString(data, "client");
+    return data;
+}
+
+std::vector<char> subscribeVp(std::initializer_list<std::pair<std::string, uint8_t>> topics) {
+    std::vector<char> data{0, 1};
+    for (const auto& [topic, qos] : topics) {
+        appendString(data, topic);
+        data.push_back(static_cast<char>(qos));
+    }
+    return data;
+}
+
+std::vector<char> publishVp(const std::string& topic, const std::string& payload, bool includePacketId = false, uint16_t pid = 1) {
+    std::vector<char> data;
+    appendString(data, topic);
+    if (includePacketId) {
+        data.push_back(static_cast<char>((pid >> 8) & 0xff));
+        data.push_back(static_cast<char>(pid & 0xff));
+    }
+    data.insert(data.end(), payload.begin(), payload.end());
+    return data;
+}
+
+template <typename Packet>
+bool deserializePacket(Packet& packet, const std::vector<char>& data) {
+    BufferContext context(data);
+    while (!packet.isComplete() && !packet.isError()) {
+        if (packet.deserialize(&context) == 0) {
+            break;
+        }
+    }
+    return packet.isComplete() && !packet.isError();
+}
+
+} // namespace
+
+int main() {
+    tests::support::TestResult result;
+
+    {
+        iot::mqtt::packets::Connect normal("client", 60, true, "", "", 0, false, "", "", false);
+        const auto wire = normal.serialize();
+        result.expectEqual(0x04, static_cast<uint8_t>(wire[8]), "client CONNECT without loop prevention sends level 0x04");
+    }
+    {
+        iot::mqtt::packets::Connect bridge("client", 60, true, "", "", 0, false, "", "", true);
+        const auto wire = bridge.serialize();
+        result.expectEqual(0x84, static_cast<uint8_t>(wire[8]), "client CONNECT with loop prevention sends private bridge level 0x84");
+    }
+
+    {
+        const auto data = connectVp(0x04, 0x02);
+        iot::mqtt::server::packets::Connect connect(data.size(), 0x00);
+        result.expectTrue(deserializePacket(connect, data), "normal MQTT 3.1.1 CONNECT parses");
+        result.expectEqual(0x04, connect.getLevel(), "normal CONNECT level remains 0x04");
+        result.expectTrue(connect.getReflect(), "normal CONNECT reflects publications");
+    }
+    {
+        const auto data = connectVp(0x84, 0x02);
+        iot::mqtt::server::packets::Connect connect(data.size(), 0x00);
+        result.expectTrue(deserializePacket(connect, data), "private bridge CONNECT parses");
+        result.expectEqual(0x04, connect.getLevel(), "private bridge CONNECT masks level to 0x04");
+        result.expectTrue(!connect.getReflect(), "private bridge CONNECT disables reflection");
+    }
+    {
+        const auto data = connectVp(0x84, 0x03);
+        iot::mqtt::server::packets::Connect connect(data.size(), 0x00);
+        result.expectTrue(deserializePacket(connect, data), "CONNECT parser preserves reserved flag for server validation");
+        result.expectEqual(0x03, connect.getConnectFlags(), "CONNECT reserved flag remains visible");
+    }
+
+    for (uint8_t qos = 0; qos <= 2; ++qos) {
+        const auto data = subscribeVp({{"a/b", qos}});
+        iot::mqtt::server::packets::Subscribe subscribe(data.size(), 0x02);
+        result.expectTrue(deserializePacket(subscribe, data), "SUBSCRIBE QoS 0/1/2 parses");
+    }
+    for (uint8_t qos : {uint8_t{0x03}, uint8_t{0x08}, uint8_t{0x0c}, uint8_t{0x80}}) {
+        const auto data = subscribeVp({{"a/b", qos}});
+        iot::mqtt::server::packets::Subscribe subscribe(data.size(), 0x02);
+        result.expectTrue(!deserializePacket(subscribe, data), "SUBSCRIBE invalid QoS/options rejected");
+    }
+    {
+        const auto data = subscribeVp({{"a/b", 0}, {"c/d", 0x80}});
+        iot::mqtt::server::packets::Subscribe subscribe(data.size(), 0x02);
+        result.expectTrue(!deserializePacket(subscribe, data), "SUBSCRIBE rejects invalid QoS on second topic");
+    }
+    {
+        iot::mqtt::server::packets::Subscribe subscribe(5, 0x00);
+        result.expectTrue(subscribe.isError(), "SUBSCRIBE fixed-header flags other than 0x02 rejected");
+        iot::mqtt::server::packets::Unsubscribe unsubscribe(5, 0x00);
+        result.expectTrue(unsubscribe.isError(), "UNSUBSCRIBE fixed-header flags other than 0x02 rejected");
+        iot::mqtt::server::packets::Pubrel pubrel(2, 0x00);
+        result.expectTrue(pubrel.isError(), "PUBREL fixed-header flags other than 0x02 rejected");
+    }
+
+    {
+        const auto data = publishVp("a/b", "payload");
+        iot::mqtt::server::packets::Publish publish(data.size(), 0x00);
+        result.expectTrue(deserializePacket(publish, data), "PUBLISH QoS0 DUP0 parses");
+        result.expectTrue(!publish.getRetain(), "PUBLISH retain false preserved");
+    }
+    {
+        const auto data = publishVp("a/b", "payload");
+        iot::mqtt::server::packets::Publish publish(data.size(), 0x01);
+        result.expectTrue(deserializePacket(publish, data), "PUBLISH retain true parses");
+        result.expectTrue(publish.getRetain(), "PUBLISH retain true preserved");
+    }
+    {
+        const auto data = publishVp("a/b", "payload");
+        iot::mqtt::server::packets::Publish publish(data.size(), 0x08);
+        result.expectTrue(publish.isError(), "PUBLISH QoS0 DUP1 rejected at fixed header");
+    }
+    {
+        const auto data = publishVp("a/b", "payload", true, 1);
+        iot::mqtt::server::packets::Publish publish(data.size(), 0x06);
+        result.expectTrue(publish.isError(), "PUBLISH QoS3 rejected at fixed header");
+    }
+    {
+        const auto data = publishVp("a/b", "payload", true, 0);
+        iot::mqtt::server::packets::Publish publish(data.size(), 0x02);
+        result.expectTrue(!deserializePacket(publish, data), "PUBLISH QoS1 packet id zero rejected");
+    }
+    {
+        const auto data = publishVp("", "payload");
+        iot::mqtt::server::packets::Publish publish(data.size(), 0x00);
+        result.expectTrue(!deserializePacket(publish, data), "PUBLISH empty topic rejected");
+    }
+    {
+        const auto data = publishVp("a/b", std::string(70000, 'x'));
+        iot::mqtt::server::packets::Publish publish(data.size(), 0x00);
+        result.expectTrue(deserializePacket(publish, data), "PUBLISH payload larger than 65535 is not truncated");
+        result.expectEqual(70000, static_cast<int>(publish.getMessage().size()), "large PUBLISH payload size preserved");
+    }
+
+    {
+        BufferContext context({char(0x30), char(0xff), char(0xff), char(0xff), char(0xff), char(0x7f)});
+        iot::mqtt::FixedHeader fixedHeader;
+        fixedHeader.deserialize(&context);
+        result.expectTrue(fixedHeader.isError(), "Remaining Length overlong sequence rejected");
+    }
+    {
+        BufferContext context({char(0x30), char(0xff), char(0xff), char(0xff), char(0xff)});
+        iot::mqtt::FixedHeader fixedHeader;
+        fixedHeader.deserialize(&context);
+        result.expectTrue(fixedHeader.isError(), "Remaining Length continuation on fourth byte rejected");
+    }
+
+    return result.processResult();
+}