CVE-2026-56817: Netty is a network application framework for development of protocol servers and clients

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CRITICAL 9.8REMOTEUPGRADE

CVE-2026-56817 is a critical-severity flaw (CVSS 9.8) affecting netty. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Netty is a network application framework for development of protocol servers and clients. In versions 4.2.0.Final through 4.2.15.Final and 4.1.0.Final through 4.1.135.Final, any caller that can deliver bytes to a Netty channel pipeline containing `XmlDecoder` can send XML with a `DOCTYPE` declaration to an `AsyncXMLInputFactory` instantiated with no security configuration, leaving DTD and entity handling active depending on Aalto XML async parser

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.7% (top 47%)
CISA exploited list	No
Weakness	CWE-611
Published	2026-07-21
Last modified	2026-07-30

Publisher (CNA): GitHub, Inc. · Vendor: netty

Last updated 2026-07-30 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-21 23:17 UTCPublished CVE record published
2026-07-30 14:48 UTCUpdated Record last modified

netty netty UPGRADE

Affected: >= 4.1.0 and < 4.1.136 · >= 4.2.0 and < 4.2.16

Fixed in: 4.1.136, 4.2.16

Safe to downgrade to: a release before 4.1.0

1. Check installed version

apt-cache policy netty || rpm -q netty || netty --version

2. UPGRADE to 4.1.136 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade netty

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade netty

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 4.1.0 (Debian / Ubuntu)

apt-cache madison netty sudo apt install netty=<VERSION_BEFORE_4.1.0> sudo apt-mark hold netty

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list netty sudo dnf downgrade netty-<VERSION_BEFORE_4.1.0> sudo dnf versionlock add netty

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-56817?

netty: upgrade to 4.1.136, 4.2.16. Exact commands for each OS are on this page.

Is CVE-2026-56817 being exploited?

CVE-2026-56817 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-56817 affect?

CVE-2026-56817 affects netty. Severity is critical (CVSS 9.8). Netty is a network application framework for development of protocol servers and clients. In versions 4.2.0.Final through 4.2.15.Final and 4.1.0.Final through 4.1.135.Final, any caller that can deliver bytes to a Netty channel pipeline cont

How urgent is CVE-2026-56817?

Priority 52/100. Critical severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-42587 HIGH 7.5 UPGRADE Netty is an asynchronous, event-driven network application framework
CVE-2026-42584 HIGH 7.3 UPGRADE Netty is an asynchronous, event-driven network application framework
CVE-2026-42583 HIGH 7.5 UPGRADE Netty is an asynchronous, event-driven network application framework
CVE-2026-42580 MEDIUM 6.5 UPGRADE Netty is an asynchronous, event-driven network application framework
CVE-2026-42585 MEDIUM 6.5 UPGRADE Netty is an asynchronous, event-driven network application framework

References

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