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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.
Priority
LOW
Exploitation
NONE
PoC
NONE
Patch
NONE
Momentum
STABLE
🚨 HIGH - Stack Buffer Overflow in Fluent Bit (CVE-2026-61674) A stack buffer overflow vulnerability in Fluent Bit (plugins/out_forward/forward.c) allows a malicious or impersonated Secure Forward server to send an oversized PONG handshake reason. This overwrites stack control data and can lead to remote code execution as the Fluent Bit process user. 👉 Affected: Fluent Bit prior to 5.0.8 | Upgrade to v5.0.8 or later immediately
Your log shipper can get owned by the destination it trusts. CVE-2026-61674: Fluent Bit Secure Forward copies an oversized PONG into a 32-byte stack buffer (CVSS 9.2). Impersonate the Shared_Key peer and you crash or RCE as the flb user. Ship 5.0.8+, then audit every out_forward Shared_Key destination. https://app.opencve.io/cve/CVE-2026-61674
Fluent Bit's Secure Forward made the client prove it knew the shared key and never checked that the server did. The 9.2 patch fixed the overflow, not the trust. No exploitation reported. https://severitydaily.com/fluent-bit-cve-2026-61674-secure-forward-pong-overflow-5-0-9-mutual-auth/