CVE-2026-42209
CVE-2026-42209 is a medium-severity vulnerability with a CVSS 3.x base score of 6.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-369.
Key facts
- Severity: Medium (CVSS 3.x base score 6.5)
- EPSS exploit prediction: 0% (28th percentile)
- Actively exploited: Not listed in CISA KEV
- EU (EUVD) id: EUVD-2026-28838
- Weakness: CWE-369
- Published:
- Last modified:
Description
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.
Frequently asked questions
- What is 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.
- How severe is CVE-2026-42209?
- CVE-2026-42209 has a CVSS 3.x base score of 6.5, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
- Is CVE-2026-42209 being actively exploited?
- It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (28th percentile), an estimate of the probability of exploitation in the next 30 days.
- How do I fix CVE-2026-42209?
- Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
- Does CVE-2026-42209 have an EU (EUVD) identifier?
- Yes. CVE-2026-42209 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-28838.
- When was CVE-2026-42209 published?
- CVE-2026-42209 was published on 2026-05-08 and last updated on 2026-06-17.
References
- https://github.com/halfgaar/FlashMQ/commit/193b6e7767889511cfa8e933908ea5e6a1077a1f
- https://github.com/halfgaar/FlashMQ/issues/167
- https://github.com/halfgaar/FlashMQ/releases/tag/v1.26.1
- https://github.com/halfgaar/FlashMQ/security/advisories/GHSA-2789-vfcg-5922
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