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Vulnerability record · CVE-2026-61674 · published 21 September 2026

CVE-2026-61674: Stack-based buffer overflow vulnerability

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.

9.2 CVSS 4.0 Critical EPSS 0.85% · top 43.4% CWE-121 · Stack-based buffer overflowCWE-787 · Out-of-bounds write Awaiting Analysis
9.2CVSS 4.0 base score
0.85%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
5References
24 Sep 2026Last modified by NVD

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.

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:

References

LinkTags
https://github.com/fluent/fluent-bit/commit/45486556be9d023e194665e243f156b228e195e0
https://github.com/fluent/fluent-bit/pull/11945
https://github.com/fluent/fluent-bit/releases/tag/v5.0.8
https://github.com/fluent/fluent-bit/security/advisories/GHSA-jrp8-r9hx-gf73
https://github.com/fluent/fluent-bit/security/advisories/GHSA-jrp8-r9hx-gf73

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Source: NIST National Vulnerability Database (record CVE-2026-61674), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.