CVE-2026-42209 — Denial of Service

title: FlashMQ Broker Crash via Retained Publish - CVE-2026-42209
id: scw-2026-05-08-ai-1
status: experimental
level: critical
description: |
  Detects attempts to crash the FlashMQ broker (CVE-2026-42209) by sending a PUBLISH request to the MQTT endpoint with specific non-default configurations for 'set_retained_message_defer_timeout' and 'set_retained_message_defer_timeout_spread' in the query parameters. This targets the specific vulnerability that allows a remote client to cause a denial of service.
author: SCW Feed Engine (AI-generated)
date: 2026-05-08
references:
  - https://shimiscyberworld.com/posts/nvd-CVE-2026-42209/
tags:
  - attack.impact
  - attack.t1499
logsource:
    category: webserver
detection:
  selection:
      cs-uri|contains:
          - '/mqtt'
      cs-method|exact: "PUBLISH"
      cs-uri-query|contains:
          - 'set_retained_message_defer_timeout='
          - 'set_retained_message_defer_timeout_spread='
  condition: selection
falsepositives:
  - Legitimate administrative activity

Source: Shimi's Cyber World · License & reuse