CVE-2026-61674

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.

An attacker can remotely execute code as the Fluent Bit process user or cause repeated crashes, disrupting log and metric processing. The risk is high in environments using Secure Forward with Shared_Key or Empty_Shared_Key.

Upgrade Fluent Bit to version 5.0.8 or later to fix the buffer overflow in secure_forward_pong. Restrict network access to Secure Forward destinations and avoid --supervisor mode until patched.