What’s Vulnerable › Vulnerabilities

CVE-2026-42209: FlashMQ: Division by zero crash when using non-default deferred retained message setting

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-05-08 · updated 2026-10-26
• Weakness: CWE-369

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.

Affected products

• halfgaar FlashMQ: before 1.26.1

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
• https://github.com/halfgaar/FlashMQ/issues/167

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