CVE-2021-34431
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedIn Eclipse Mosquitto version 1.6 to 2.0.10, if an authenticated client that had connected with MQTT v5 sent a crafted CONNECT message to the broker a memory leak would occur, which could be used to provide a DoS attack against the broker.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceEclipse Mosquitto versions 1.6 through 2.0.10 contain a memory leak vulnerability in the MQTT v5 CONNECT message handling path. When an authenticated client sends a specially crafted CONNECT message using the MQTT v5 protocol, memory is allocated but not properly freed, accumulating over time and potentially leading to resource exhaustion and denial of service.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data>= 1.6, <= 2.0.10CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify installed Mosquitto versionRun 'mosquitto --version' or check the package manager (dpkg -l mosquitto, rpm -qi mosquitto)Affected if Version is 1.6.x through 2.0.10 inclusive
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Confirm MQTT v5 protocol support is enabledExamine mosquitto.conf for 'protocol mqtt' or default settings - MQTT v5 is enabled by default in Mosquitto 2.x. Check for any 'mqtt_version' directive.Affected if MQTT v5 is allowed (default in versions 2.x; must be explicitly enabled in 1.6.x via 'protocol mqtt')
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Verify broker accepts authenticated client connectionsReview mosquitto.conf for 'allow_anonymous' setting and any password/file-based authentication configuration (password_file or psk_file directives)Affected if Anonymous access is allowed or password/PSK authentication is configured and active
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Monitor for accumulating memory usageUse system tools like 'ps aux' or 'top' to observe mosquitto broker process memory over time with active v5 client connectionsAffected if Memory usage continuously grows without stabilizing during normal MQTT v5 client activity
You are affected if Mosquitto version is between 1.6 and 2.0.10 inclusive AND the broker accepts MQTT v5 connections from authenticated clients, as the memory leak only occurs in the v5 CONNECT handling path for authenticated sessions.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataUpgrade Mosquitto to version 2.0.11 or later. As interim measures, consider implementing connection rate limiting, restricting MQTT v5 protocol usage, or monitoring broker memory consumption for anomalies.
- Consultation3.0 h
- Implementation2.0 h
- Testing4.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,072.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-34431 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-34431 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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