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

Quick assessment

Affected
halfgaar FlashMQ
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

FlashMQ是Wiebe Cazemier个人开发者的一个快速轻量级的MQTT代理服务器。 FlashMQ 1.26.1之前版本存在数字错误漏洞，该漏洞源于当set_retained_message_defer_timeout和set_retained_message_defer_timeout_spread配置为非默认值时，具有保留发布权限的远程客户端可能导致FlashMQ代理崩溃，导致拒绝服务。

CVSS 6.5 · Medium EPSS 0.43% · P37

Affected Version Matrix 1

VendorProduct Version RangeStatus
halfgaar FlashMQ < 1.26.1 affected
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I. Basic Information for CVE-2026-42209

Vulnerability Information
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Vulnerability Title
FlashMQ: Division by zero crash when using non-default deferred retained message setting
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
除零错误
Source: CVE Program / CVE List V5
Vulnerability Title
FlashMQ 数字错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
FlashMQ是Wiebe Cazemier个人开发者的一个快速轻量级的MQTT代理服务器。 FlashMQ 1.26.1之前版本存在数字错误漏洞，该漏洞源于当set_retained_message_defer_timeout和set_retained_message_defer_timeout_spread配置为非默认值时，具有保留发布权限的远程客户端可能导致FlashMQ代理崩溃，导致拒绝服务。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
halfgaar FlashMQ < 1.26.1 -

II. Public POCs for CVE-2026-42209

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III. Intelligence Information for CVE-2026-42209

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Patches & Fixes for CVE-2026-42209 (1)

Vendor Advisories for CVE-2026-42209 (1)

News Coverage for CVE-2026-42209 (1)

Vendor Pages for CVE-2026-42209 (1)

IV. Related Vulnerabilities

Same product: FlashMQ
Same vendor: halfgaar
Same weakness: CWE-369