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.

• 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