CVE-2026-59921: Netty is an asynchronous, event-driven network application framework

Routine

MEDIUM 5.7UPGRADE

CVE-2026-59921 is a medium-severity flaw (CVSS 5.7) affecting netty. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.136.Final and 4.2.16.Final, HttpPostRequestEncoder constructs multipart HTTP request bodies by directly concatenating user-supplied filenames and field names into Content-Disposition MIME headers without validating or sanitizing CRLF characters (\r\n). Since MIME headers are delimited by CRLF, an attacker who controls the filename can inject arbitrary MIME

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	5.7 (Medium)
Attack vector	Adjacent network
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.5% (top 60%)
CISA exploited list	No
Weakness	CWE-93
Published	2026-07-28
Last modified	2026-08-07

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

Last updated 2026-08-07 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-28 23:17 UTCPublished CVE record published
2026-08-07 15:05 UTCUpdated Record last modified

netty netty UPGRADE

Affected: < 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 4.2.0

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 4.2.0 (Debian / Ubuntu)

apt-cache madison netty sudo apt install netty=<VERSION_BEFORE_4.2.0> 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_4.2.0> 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-2026-59921?

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

Is CVE-2026-59921 being exploited?

CVE-2026-59921 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-2026-59921 affect?

CVE-2026-59921 affects netty. Severity is medium (CVSS 5.7). Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.136.Final and 4.2.16.Final, HttpPostRequestEncoder constructs multipart HTTP request bodies by directly concatenating user-supplied filenames and fi

How urgent is CVE-2026-59921?

Priority 30/100. No elevated risk signals yet.

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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