CVE-2026-42209 FlashMQ: Division by zero crash when using non-default deferred retained message setting
Remote attackers can crash the FlashMQ MQTT broker via division by zero if specific non-default retained message settings are enabled.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-42209?
CVE-2026-42209 is a medium-severity denial of service vulnerability in halfgaar FlashMQ, published on 8 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official 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.
Weakness: CWE-369
Which versions are affected by CVE-2026-42209?
How do I fix CVE-2026-42209?
No fix has been published for CVE-2026-42209 yet. Limit exposure of the affected software until one is available.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-42209 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-42209 exists.
Sources
Also known as GHSA-2789-vfcg-5922.