CVE-2026-42577

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-native-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

CVE-2026-42577 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.2.13.Final); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-42577? CVE-2026-42577 is a high-severity security vulnerability in io.netty:netty-transport-native-epoll (maven), affecting versions >= 4.2.0.Final, < 4.2.13.Final. It is fixed in 4.2.13.Final.
2. How severe is CVE-2026-42577? CVE-2026-42577 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of io.netty:netty-transport-native-epoll are affected by CVE-2026-42577? io.netty:netty-transport-native-epoll (maven) versions >= 4.2.0.Final, < 4.2.13.Final is affected.
4. Is there a fix for CVE-2026-42577? Yes. CVE-2026-42577 is fixed in 4.2.13.Final. Upgrade to this version or later.
5. Is CVE-2026-42577 exploitable, and should I be worried? Whether CVE-2026-42577 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-42577 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-42577? Upgrade io.netty:netty-transport-native-epoll to 4.2.13.Final or later.