---
id: CVE-2026-61674
title: >-
  Fluent Bit is a fast and lightweight logs, metrics, and traces processor for
  Linux, BSD, macOS, and Windows
summary: >-
  Fluent Bit is a fast and lightweight logs, metrics, and traces processor for
  Linux, BSD, macOS, and Windows. From 0.11.0 until 5.0.8,
  plugins/out_forward/forward.c secure_forward_pong copies the server-controlled
  PONG[2] reason into the …
severity: critical
cvss: 9.2
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N'
cwe:
  - CWE-121
  - CWE-787
  - CWE-843
vendor: fluent
product: fluent-bit
affected:
  - 'fluent-bit >= 0.11.0, < 5.0.8'
published: '2026-09-21'
updated: '2026-09-21'
sourceUpdated: '2026-09-21T17:17:36.663'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-61674'
references:
  - url: >-
      https://github.com/fluent/fluent-bit/commit/45486556be9d023e194665e243f156b228e195e0
    label: security-advisories@github.com
  - url: 'https://github.com/fluent/fluent-bit/pull/11945'
    label: security-advisories@github.com
  - url: 'https://github.com/fluent/fluent-bit/releases/tag/v5.0.8'
    label: security-advisories@github.com
  - url: >-
      https://github.com/fluent/fluent-bit/security/advisories/GHSA-jrp8-r9hx-gf73
    label: security-advisories@github.com
tags:
  - nvd
  - cve.org
cvssSource: cna
ingestedAt: '2026-09-21T16:49:07.876Z'
---

## Overview

Fluent Bit is a fast and lightweight logs, metrics, and traces processor for Linux, BSD, macOS, and Windows. From 0.11.0 until 5.0.8, plugins/out_forward/forward.c secure_forward_pong copies the server-controlled PONG[2] reason into the 32-byte stack buffer msg with memcpy without checking its MessagePack type or length. An attacker who controls or can impersonate an out_forward Secure Forward destination configured with Shared_Key or Empty_Shared_Key can send an oversized reason during the first handshake and overwrite stack control data. Protected builds reliably terminate, while builds without a stack canary or with a disclosure can allow remote code execution as the Fluent Bit process user. When the opt-in --supervisor mode is used, fork-only respawns preserve the canary and address layout, allowing repeated crash-or-survive probes to support code execution on a hardened build; ordinary exec-based or service-manager restarts do not preserve that state. This issue is fixed in version 5.0.8.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.