Fix various spec compliance gaps by embhorn · Pull Request #504 · wolfSSL/wolfMQTT

embhorn · 2026-04-30T22:11:19Z

Thanks to @LiD0209 for the excellent issue reports

Fix SUBSCRIBE / UNSUBSCIRIBE / PUBREL Fixed-Header Flags Do Not Trigger Malformed Close
- Fixes Invalid SUBSCRIBE Fixed-Header Flags Do Not Trigger Malformed Close #486
- Fixes Invalid UNSUBSCRIBE Fixed-Header Flags Do Not Trigger Malformed Close #487
- Fixes PUBREL Fixed Header Reserved Bits Are Not Validated #488
- Fixes SUBSCRIBE / UNSUBSCRIBE Fixed-Header Reserved Bits #492
- Fixes PUBLISH QoS Bits 11 Are Accepted Instead of Rejected #502
Fix Zero-Length ClientId Unique Assignment
- Fixes Zero-Length ClientId Unique Assignment Is Incomplete #491
- Fixes Zero-Length ClientId Is Not Bound to CleanSession #493
Fix UTF-8 Well-Formed Validation
- Fixes UTF-8 Well-Formed Validation Is Missing #495 (requires rebase after Reject null chars in strings #503)
Fix Unsupported Protocol Level Is Not Rejected with CONNACK 0x01
- Fixes Unsupported Protocol Level Is Not Rejected with CONNACK 0x01 #496
Fix CONNACK Flags Receive-Side Validation
- Fixes CONNACK Flags Receive-Side Validation Is Insufficient #497
Fix PUBLISH DUP Initial-Send and QoS0 Constraint Is Not Enforced by the Generic Encoder
- Fixes PUBLISH DUP Initial-Send and QoS0 Constraint Is Not Enforced by the Generic Encoder #498
Handle QoS2 Duplicate PUBLISH Forwarded Before PUBREL
- Fixes wolfMQTT QoS2 Duplicate PUBLISH Forwarded Before PUBREL #506
Fix Packet Identifier In-Use Collision Is Not Rejected
- Fixes Packet Identifier In-Use Collision Is Not Rejected #507
Fix Packet Identifier Zero Acceptance on Receive Paths
Fixes Packet Identifier Zero Acceptance on Receive Paths #509
Fix Packet Identifier Reuse State Is Not Centrally Modeled
- Fixes Packet Identifier Reuse State Is Not Centrally Modeled #510
Fix Password Flag 0 Extra Payload Is Accepted
- Fixes Password Flag 0 Extra Payload Is Accepted #511
Fix Password Flag Without User Name Flag Is Accepted
- Fixes Password Flag Without User Name Flag Is Accepted #512
Fix CONNECT Flags Cross-Constraint Violations Are Accepted
- Fixes CONNECT Flags Cross-Constraint Violations Are Accepted #513
Fix Empty SUBSCRIBE and UNSUBSCRIBE Payloads Are Accepted
- Fixes Empty SUBSCRIBE and UNSUBSCRIBE Payloads Are Accepted #514
Fix PINGREQ, PINGRESP, and DISCONNECT Nonzero Remaining Length Is Accepted
- Fixes PINGREQ, PINGRESP, and DISCONNECT Nonzero Remaining Length Is Accepted #515
Fix PUBACK, PUBREC, PUBREL, and PUBCOMP Accept Extra Payload in MQTT 3.x
- Fixes PUBREL and PUBCOMP Accept Extra Payload in MQTT 3.x #516
- Fixes PUBACK and PUBREC Accept Extra Payload in MQTT 3.x #517
Fix SUBSCRIBE Requested QoS Options Are Accepted Without Strict Validation
- Fixes SUBSCRIBE Requested QoS Options Are Accepted Without Strict Validation #518
Test DISCONNECT Fixed Header Reserved Bits Are Not Validated
- Fixes DISCONNECT Fixed Header Reserved Bits Are Not Validated #519
Fix Retained Message QoS Is Not Stored
- Fixes Retained Message QoS Is Not Stored #520
Fix SUBACK Return Code issues
- Fixes SUBACK Return Code Receive Validation Is Missing #521
- Fixes SUBACK Return Code Helper Guard Is Incomplete #522
Fix CONNACK Session Present Is Not Set When a Persistent Session Is Resumed
- Fixes CONNACK Session Present Is Not Set When a Persistent Session Is Resumed #523
Fix topic filtering issues
- Fixes Topic Filter Multi-Level Wildcard Validation Is Missing #524
- Fixes Empty Topic Filters Are Accepted #525
- Fixes Wildcard Subscription Rejection Is Missing When Wildcards Are Disabled #526
- Fixes Topic Filter Single-Level Wildcard Validation Is Missing #527
Fix PUBLISH Topic Names validation
- Fixes Empty PUBLISH Topic Names Are Accepted #528
- Fixes PUBLISH Topic Name Wildcard Validation Is Missing on Encode Path #529
Fix CONNECT User Name Validation
- Fixes User Name Flag 0 Extra Payload Is Accepted #530
- Fixes CONNECT User Name UTF-8 Validation Is Missing #531
Fix LWT Validation Issues
- Fixes Will Flag 0 Extra Will Payload Is Accepted #532
- Fixes Reserved Will QoS Value 3 Is Accepted #533
- Fixes Will Topic UTF-8 Validation Is Missing #534
- Fixes Will Topic U+0000 Rejection Is Missing #535

Copilot

Pull request overview

This PR targets MQTT v3.1.1 compliance for fixed-header reserved flags (notably SUBSCRIBE) by validating the first-byte flag nibble during decode/dispatch and ensuring malformed packets lead to connection close in the broker.

Changes:

Add a public helper to validate fixed-header reserved flags (MqttPacket_FixedHeaderFlagsValid) and use it from MqttDecode_FixedHeader.
Update broker dispatch to (a) pre-validate fixed-header flags for packet types not run through decoders and (b) close connections when handlers report malformed/protocol errors.
Add unit tests covering canonical/invalid fixed-header flag permutations and end-to-end decode rejection for SUBSCRIBE/UNSUBSCRIBE/PUBREL and malformed PUBLISH flags.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
`wolfmqtt/mqtt_packet.h`	Exposes new fixed-header flag validation helper as public API.
`src/mqtt_packet.c`	Implements reserved-flag validation and enforces it in `MqttDecode_FixedHeader`.
`src/mqtt_broker.c`	Closes connections on malformed packets by validating flags pre-dispatch and honoring handler error returns.
`tests/test_mqtt_packet.c`	Adds coverage for valid/invalid reserved-flag nibbles and malformed PUBLISH QoS/DUP combinations.

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

Copilot · 2026-04-30T22:20:09Z

+        /* [MQTT-2.2.2-2] Reject malformed fixed-header reserved flags. The
+         * per-type decoders also enforce this (see MqttDecode_FixedHeader),
+         * but PUBACK / PUBCOMP / PINGREQ / DISCONNECT are not run through a
+         * decoder here, so the broker enforces it directly before dispatch. */
+        if (!MqttPacket_FixedHeaderFlagsValid(bc->rx_buf[0])) {
+            WBLOG_ERR(broker,
+                "broker: invalid fixed-header flags type=%u byte=0x%02X "
+                "sock=%d [MQTT-2.2.2-2]",


This check only validates the low-nibble fixed-header flags; it does not ensure the packet type itself is supported in the current session. As a result, packet types that the broker does not handle (e.g., AUTH, or RESERVED/unknown types) can pass this validation and then hit the switch default: (no-op), keeping the connection open and updating last_rx. Consider explicitly treating unsupported/unknown packet types as malformed here (and closing the connection), and additionally rejecting AUTH when bc->protocol_level < MQTT_CONNECT_PROTOCOL_LEVEL_5.

Copilot · 2026-04-30T22:20:09Z

+TEST(fixed_header_flags_valid_canonical_values)
+{
+    /* Canonical first-byte values for each fixed-flag packet type. */
+    ASSERT_EQ(1, MqttPacket_FixedHeaderFlagsValid(0x10)); /* CONNECT */
+    ASSERT_EQ(1, MqttPacket_FixedHeaderFlagsValid(0x20)); /* CONNACK */
+    ASSERT_EQ(1, MqttPacket_FixedHeaderFlagsValid(0x40)); /* PUBACK */
+    ASSERT_EQ(1, MqttPacket_FixedHeaderFlagsValid(0x50)); /* PUBREC */
+    ASSERT_EQ(1, MqttPacket_FixedHeaderFlagsValid(0x62)); /* PUBREL */
+    ASSERT_EQ(1, MqttPacket_FixedHeaderFlagsValid(0x70)); /* PUBCOMP */
+    ASSERT_EQ(1, MqttPacket_FixedHeaderFlagsValid(0x82)); /* SUBSCRIBE */
+    ASSERT_EQ(1, MqttPacket_FixedHeaderFlagsValid(0x90)); /* SUBACK */
+    ASSERT_EQ(1, MqttPacket_FixedHeaderFlagsValid(0xA2)); /* UNSUBSCRIBE */


The new fixed-header flag validator tests don’t cover reserved/unknown packet types (e.g., first byte 0x00 / packet type 0). Since the broker uses this helper as a gate before dispatch, it’s worth adding an assertion that reserved/unknown packet types are treated as invalid (or, if that’s intentionally out of scope for this helper, adding a separate broker-level test that reserved types trigger disconnect).

Copilot · 2026-04-30T22:20:09Z

    return header_len;
 }

+/* [MQTT-2.2.2-1] Required fixed-header reserved-flag values per packet type.


The comment tag references [MQTT-2.2.2-1], but the reserved fixed-header flag requirement being implemented here is [MQTT-2.2.2-2] (as also referenced elsewhere in this PR). Please correct the spec reference to avoid confusion when auditing compliance behavior.

Suggested change

/* [MQTT-2.2.2-1] Required fixed-header reserved-flag values per packet type.

/* [MQTT-2.2.2-2] Required fixed-header reserved-flag values per packet type.

Copilot · 2026-04-30T22:20:10Z

+    /* Unknown/reserved type: this helper validates the flag nibble only.
+     * Callers are responsible for rejecting unknown packet types. */
+    return 1;


MqttPacket_FixedHeaderFlagsValid currently returns 1 for unknown/reserved packet types (including type=0). That makes it unsafe to use as a protocol-level malformed check (e.g., in the broker), because reserved/unknown types will pass this validation and can be silently ignored downstream. Consider returning 0 for MQTT_PACKET_TYPE_RESERVED and for any unrecognized packet type, or otherwise making callers explicitly reject reserved/unknown types before treating a packet as valid.

Suggested change

/* Unknown/reserved type: this helper validates the flag nibble only.

* Callers are responsible for rejecting unknown packet types. */

return 1;

/* Reject reserved/unknown packet types so this helper is safe to use

* as a protocol-level malformed packet check. */

return 0;

…he Generic Encoder

…epted

…ation

…esumed

Fix SUBSCRIBE Fixed-Header Flags Do Not Trigger Malformed Close (wolf…

62b0e51

…SSL#486)

embhorn self-assigned this Apr 30, 2026

Copilot AI review requested due to automatic review settings April 30, 2026 22:11

Copilot started reviewing on behalf of embhorn April 30, 2026 22:14 View session

Copilot AI reviewed Apr 30, 2026

View reviewed changes

embhorn changed the title ~~Fix various issues reported in GH~~ Fix various soec compliance gaps Apr 30, 2026

embhorn changed the title ~~Fix various soec compliance gaps~~ Fix various spec compliance gaps Apr 30, 2026

embhorn added 23 commits April 30, 2026 18:16

Fix Zero-Length ClientId Unique Assignment

2a831b4

Fix UTF-8 handling

f4f1bca

Fix Unsupported Protocol Level Is Not Rejected with CONNACK 0x01

dec01d1

Fix CONNACK Flags Receive-Side Validation

d9b0aa3

Fix PUBLISH DUP Initial-Send and QoS0 Constraint Is Not Enforced by t…

361903b

…he Generic Encoder

Handle QoS2 Duplicate PUBLISH Forwarded Before PUBREL

826795c

Fix Packet Identifier In-Use Collision Is Not Rejected

fec544e

Fix Packet Identifier Zero Acceptance on Receive Paths

4725723

Cleanup

746a493

Fix Packet Identifier Reuse State Is Not Centrally Modeled

1c0b461

Fix Password Flag 0 Extra Payload Is Accepted

e1fcc91

Cleanup

8762fc0

Password Flag Without User Name Flag Is Accepted

0944f34

Fix CONNECT Flags Cross-Constraint Violations Are Accepted

d07524d

Fix Empty SUBSCRIBE and UNSUBSCRIBE Payloads Are Accepted

1da06f0

Fix PINGREQ, PINGRESP, and DISCONNECT Nonzero Remaining Length Is Acc…

a69a721

…epted

Fix PUBACK, PUBREC, PUBREL, and PUBCOMP Accept Extra Payload in MQTT 3.x

1c5d08f

Fix SUBSCRIBE Requested QoS Options Are Accepted Without Strict Valid…

f754ec9

…ation

Test DISCONNECT Fixed Header Reserved Bits Are Not Validated

fc6cc05

Fix Retained Message QoS Is Not Stored

b8f921f

Fix SUBACK Return Code issues

467d75c

Fix CONNACK Session Present Is Not Set When a Persistent Session Is R…

6ff39f6

…esumed

Fix topic filtering issues

912dbe7

embhorn added 3 commits May 5, 2026 11:37

Fix PUBLISH Topic Names validation

230c6d4

Fix CONNECT User Name Validation

b4fbc2a

Fix LWT Validation Issues

a2359c6

embhorn requested a review from wolfSSL-Fenrir-bot May 5, 2026 17:30

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Fix various spec compliance gaps#504

Fix various spec compliance gaps#504
embhorn wants to merge 27 commits intowolfSSL:masterfrom
embhorn:gh_issues

embhorn commented Apr 30, 2026 •

edited

Loading

Uh oh!

Copilot AI left a comment

Uh oh!

Copilot AI Apr 30, 2026

Uh oh!

Copilot AI Apr 30, 2026

Uh oh!

Copilot AI Apr 30, 2026

Uh oh!

Copilot AI Apr 30, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

	/* [MQTT-2.2.2-1] Required fixed-header reserved-flag values per packet type.
	/* [MQTT-2.2.2-2] Required fixed-header reserved-flag values per packet type.

Conversation

embhorn commented Apr 30, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

Copilot AI Apr 30, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Apr 30, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Apr 30, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Apr 30, 2026

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

embhorn commented Apr 30, 2026 •

edited

Loading