Stack-based buffer overflow in BIRD BGP AS_PATH mask matching

Small repository containing a Python proof-of-concept (`poc.py`) plus documentation (`README.md`, `advisory.md`) for CVE-2026-49943, a stack-based buffer overflow in BIRD/BIRD2 BGP AS_PATH mask matching. The exploit is a standalone Python script, not tied to a common framework. Its structure is simple: helper functions build and send BGP OPEN, KEEPALIVE, and a malicious UPDATE message; `main()` handles CLI arguments, TCP connection setup, minimal BGP handshake, and exploit delivery. The exploit capability is denial of service against a vulnerable BIRD instance. It connects to a target BGP service (default TCP/179), negotiates capabilities including Extended Message support, and sends a crafted UPDATE whose AS_PATH contains 2295 4-byte ASNs across 9 AS_SEQUENCE segments. This is intended to exceed the vulnerable fixed-size `pm_pos pos[2048 + 1]` stack buffer described in the advisory when the target evaluates an AS path mask filter. The script does not provide code execution or a shell; it aims to crash the daemon, inferred by connection reset/timeout after sending the UPDATE. Repository purpose is to document and demonstrate the vulnerability conditions and a practical crash trigger. The advisory explains the vulnerable code path, affected versions, attack prerequisites, and mitigation guidance. Based on the included code and documentation, this is a real exploit PoC for a network-reachable but authenticated/peer-required BGP attack scenario, with operational exploit logic but a fixed, hardcoded payload.