Affected: Maven: io.netty:netty-transport-classes-epoll

## Summary

Netty's epoll transport fails to detect and close TCP connections that receive a RST after being half-closed, leading to stale channels that are never cleaned up and, in some code paths, a 100% CPU busy-loop in the event loop thread.

## Affected versions

All versions of 4.2.x `netty-transport-classes-epoll` up to and including 4.2.12.Final

## Fixed in

4.2.13.Final (fix merged into the `4.2` branch via [#16689](https://github.com/netty/netty/pull/16689); release not yet cut as of 2026-04-25).

## Severity

**Medium** — Denial of Service (resource exhaustion / CPU spin)

**CVSS:** 3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - **7.5**

**CWE:** CWE-772: Missing Release of Resource after Effective Lifetime

## Description

When a TCP connection using Netty's epoll transport has `ALLOW_HALF_CLOSURE` enabled (or is in a half-closed state via the HTTP codec), and the remote peer:

1. Sends a FIN (half-close), causing the server to mark the input as shutdown, then 2. Sends a RST (e.g. by closing with `SO_LINGER=0`)

the server-side channel is never closed. This happens because:

- `epollOutReady()` is a no-op when there is no pending flush. - `epollInReady()` short-circuits via `shouldBreakEpollInReady()` because input is already marked as shutdown. - The `EPOLLERR`/`EPOLLHUP` error condition is therefore never processed, and `channelInactive` is never fired.

Depending on the Netty version and configuration, this results in:

- **Stale channels**: The connection is neve…