mqtt: verify that CONNACK has Remaining Length set to 2

[bagder]

Alternative take to #20503

[Copilot]

Pull request overview

This pull request adds validation to ensure MQTT CONNACK packets have a Remaining Length field set to 2, as required by the MQTT v3.1.1 specification. This is an alternative implementation to PR #20503.

Changes:

• Added validation in mqtt_verify_connack() to verify CONNACK Remaining Length equals 2
• Simplified error handling flow by replacing goto statements with early returns
• Added test infrastructure to allow test server to send malformed CONNACK packets
• Added test case to verify client correctly rejects CONNACK with incorrect Remaining Length

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
lib/mqtt.c	Adds validation that CONNACK Remaining Length is 2 and simplifies error handling with early returns
tests/server/mqttd.c	Adds remlen_connack configuration option to test server for sending malformed CONNACK packets; reorders struct fields cosmetically
tests/data/test1132	New test case verifying client rejects CONNACK with Remaining Length of 3
tests/data/Makefile.am	Adds test1132 to the test suite

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

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[JimFuller-RedHat]

LGTM (and good to see new test).

[bagder]

augment review

[augmentcode]

🤖 Augment PR Summary

Summary: Tightens MQTT protocol validation around CONNACK handling, with additional test coverage for malformed packets.

Changes:

• In lib/mqtt.c, verify CONNACK Remaining Length is exactly 2 and simplify error returns.
• Add a SUBACK Remaining Length sanity check before parsing the SUBACK payload.
• Extend the MQTT test server (tests/server/mqttd.c) with a remlen-CONNACK knob to emit malformed CONNACK packets.
• Add new test tests/data/test1132 and wire it into the test list to validate rejection of bad CONNACK Remaining Length.

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[augmentcode]

Review completed. 2 suggestions posted.

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[bagder]

@aisle-analyzer what do you think?

[aisle-research-bot]

🔒 Aisle Security Analysis

✅ We scanned this PR and did not find any security vulnerabilities.
_{Aisle supplements but does not replace security review.}

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Analyzed PR: #20513 at commit 42fbd1a