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CVE-2026-61674— Fluent Bit: Remote stack buffer overflow in Fluent Bit `out_forward` Secure-Forward `PONG` handler

Quick assessment

Affected
fluent fluent-bit
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.

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 ms

CVSS 9.2 · Critical EPSS 0.65% · P50

Affected Version Matrix 1

VendorProduct Version RangeStatus
fluent fluent-bit >= 0.11.0, < 5.0.8 affected
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I. Basic Information for CVE-2026-61674

Vulnerability Information

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Vulnerability Title
Fluent Bit: Remote stack buffer overflow in Fluent Bit `out_forward` Secure-Forward `PONG` handler
Source: CVE Program / CVE List V5
Vulnerability Description
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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
栈缓冲区溢出
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
fluent fluent-bit >= 0.11.0, < 5.0.8 -

II. Public POCs for CVE-2026-61674

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III. Intelligence Information for CVE-2026-61674

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Patches & Fixes for CVE-2026-61674 (2)

Other References for CVE-2026-61674 (2)

IV. Related Vulnerabilities

Same vendor: fluent
Same weakness: CWE-121

V. Comments for CVE-2026-61674

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