What is CVE-2021-34431?

CVE-2021-34431 is a medium-severity vulnerability in Eclipse Mosquitto, classified under Missing Release of Memory after Effective Lifetime. CVSS score: 6.5/10. Published 2021-07-22.

How severe is CVE-2021-34431?

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

Vulnerability in Eclipse Mosquitto

CVE-2021-34431

In 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.

Published 2021-07-22 · 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: 0.011 (64.6th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 65th percentile — 64.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.011 · newest: 0.011 · change: 0.000

Affected products

Eclipse Mosquitto
The Eclipse Foundation Mosquitto — versions 1.6, unspecified

Weakness classification (CWE)

CWE-401 — Missing Release of Memory after Effective Lifetime

References

bugs.eclipse.org/bugs/show_bug.cgi (Mailing List, Vendor Advisory)

Frequently asked questions

What is CVE-2021-34431?: CVE-2021-34431 is a medium-severity vulnerability in Eclipse Mosquitto, classified under Missing Release of Memory after Effective Lifetime. CVSS score: 6.5/10. Published 2021-07-22.
How severe is CVE-2021-34431?: Medium severity. CVSS v3 base score is 6.5 out of 10.