CVE-2021-28166Public exploitNetwork attack vector

Authenticated broker crash via malformed CONNACK

Published Apr 7, 2021 · Updated Aug 3, 2024

NULL pointer dereference in Eclipse Mosquitto 2.0.0 through 2.0.9 allows remote authenticated users to crash the broker via MQTT v5. The handle__connack path accepts a CONNACK packet from a normal client and processes MQTT v5 properties through a null bridge context. A valid authenticated CONNECT must precede the malformed CONNACK; triggering the flaw terminates the broker process and interrupts service.

CVSS severity6.5
Medium
EPSS probability0.97%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

NULL pointer dereference in Eclipse Mosquitto 2.0.0 through 2.0.9 allows remote authenticated users to crash the broker via MQTT v5. The handle__connack path accepts a CONNACK packet from a normal client and processes MQTT v5 properties through a null bridge context. A valid authenticated CONNECT must precede the malformed CONNACK; triggering the flaw terminates the broker process and interrupts service.

The record

CVE
CVE-2021-28166
Published
Apr 7, 2021
Updated
Aug 3, 2024
Vendor
The Eclipse Foundation
Product
Eclipse Mosquitto
Classifications
CWE-476, T1499.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Apr 7, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=2.0 <unspecified
  2. Affected versionversion=unspecified <=2.0.9

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

The Eclipse Foundation · Eclipse Mosquittoversion=2.0 <unspecified; version=unspecified <=2.0.9

Published CVSS scores

4.0NIST NVDAV:N/AC:L/Au:S/C:N/I:N/A:P
6.5Eclipse FoundationCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-476NULL Pointer Dereference
T1499.004Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo