Vulnerability in Halfgaar Flashmq

CVE-2026-42209

FlashMQ is a MQTT broker/server, designed for multi-CPU environments. Prior to version 1.26.1, a remote client with retained publish permission can crash the FlashMQ broker when both set_retained_message_defer_timeout and set_retained_message_defer_timeout_spread are configured to non-default values, resulting in denial of service. If anonymous retained publishing is allowed, no authentication is required; otherwise, the attacker needs the corresponding publish permission. This issue has been patched in version 1.26.1.

Published · last modified .

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: 0.006 (50.0th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 50th percentile — 50.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-42209: held from 0.004 to 0.006.

EPSS over last 30 days · oldest: 0.004 · newest: 0.006 · change: +0.003

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-42209?
CVE-2026-42209 is a medium-severity vulnerability in Halfgaar Flashmq, classified under Divide By Zero. CVSS score: 6.5/10. Published 2026-05-08.
How severe is CVE-2026-42209?
Medium severity. CVSS v3 base score is 6.5 out of 10.