What is CVE-2021-28166?

CVE-2021-28166 is a medium-severity vulnerability in Eclipse Mosquitto, classified under NULL Pointer Dereference. CVSS score: 6.5/10. Published 2021-04-07.

How severe is CVE-2021-28166?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Is CVE-2021-28166 known to be exploited?

1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

NULL pointer dereference in Eclipse Mosquitto

CVE-2021-28166

In Eclipse Mosquitto version 2.0.0 to 2.0.9, if an authenticated client that had connected with MQTT v5 sent a crafted CONNACK message to the broker, a NULL pointer dereference would occur.

Published 2021-04-07 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H.

EPSS exploit prediction

EPSS: 0.010 (60.3th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 60th percentile — 60.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.010 · newest: 0.010 · change: 0.000

Affected products

Eclipse Mosquitto
The Eclipse Foundation Mosquitto — versions 2.0, unspecified

Weakness classification (CWE)

CWE-476 — NULL Pointer Dereference

Public proof-of-concept exploits

PBearson/FUME-Fuzzing-MQTT-Brokers

References

bugs.eclipse.org/bugs/show_bug.cgi (Patch, Issue Tracking, Vendor Advisory)

Frequently asked questions

What is CVE-2021-28166?: CVE-2021-28166 is a medium-severity vulnerability in Eclipse Mosquitto, classified under NULL Pointer Dereference. CVSS score: 6.5/10. Published 2021-04-07.
How severe is CVE-2021-28166?: Medium severity. CVSS v3 base score is 6.5 out of 10.
Is CVE-2021-28166 known to be exploited?: 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.