Netty epoll transport denial of service via RST on half-closed TCP connection

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; 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 never closed or deregistered. An unauthenticated remote attacker can repeat the sequence to accumulate stale connections, exhausting file descriptors, memory, or connection-count limits.
• CPU busy-loop: In code paths where clearEpollIn0() is not called during the ChannelInputShutdownReadComplete event, epoll_wait returns immediately on every iteration for the affected fd, causing 100% CPU utilization on the event loop thread and starving all other connections multiplexed on it.

Mitigation

• Upgrade to 4.2.13.Final when released (or build from the 4.2 branch at commit 0ec3d97).
• If upgrading is not immediately possible, configure idle timeouts on connections to limit the lifetime of stale channels.

References

• Issue: https://github.com/netty/netty/issues/16683
• Fix: https://github.com/netty/netty/pull/16689