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HIGH severity

CVE-2026-42577 — netty-transport-classes-e…

HIGHFix: netty/netty@0ec3d97

CVE-2026-42577 is a high-severity (CVSS 7.5) CWE-772 vulnerability in io.netty:netty-transport-classes-epoll. A fix is available for io.netty:netty-transport-classes-epoll — see the affected versions and patch details below.

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

Also known asGHSA-rwm7-x88c-3g2p
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 3, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-42577.

EPSS Exploitation Probability

via FIRST.org ↗
0.4%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs35th percentile — riskier than 35% of all scored CVEsHighest risk
0.00%0.36%0.72%1.08%0.1%0.4%Jun 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
☕io.netty:netty-transport-classes-epoll

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

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

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
☕Mavenio.netty:netty-transport-classes-epoll≥ 4.2.0.Final&&< 4.2.13.Final4.2.13.Finalio.netty:netty-transport-classes-epoll:4.2.13.Final

Affected Products

1 product · 1 configurations
Application
nettynetty
≥ 4.2.0 && < 4.2.13
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.netty:netty-transport-classes-epoll, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update io.netty:netty-transport-classes-epoll to 4.2.13.Final or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-42577 is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

## 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:*
O3 Security · Impact-Aware SCA

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CVE-2026-42577: DoS — Fixed in 4.2.13.Final