How severe is CVE-2021-21409?

CVE-2021-21409 has a CVSS score of 5.9/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21409?

CVE-2021-21409 affects the following packages: io.netty:netty-codec-http2 (Maven), org.jboss.netty:netty (Maven), io.netty:netty (Maven). Ecosystems affected: Maven.

How do I fix CVE-2021-21409?

Update io.netty:netty-codec-http2 to 4.1.61.Final or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21409 is resolved across your whole dependency graph.

How do I detect CVE-2021-21409 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.netty:netty-codec-http2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21409 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-21409 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21409 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21409?

CVE-2021-21409 has an EPSS (Exploit Prediction Scoring System) score of 4.9%, placing it in the 92th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21409?

CVE-2021-21409 is classified as CWE-444 (CWE-444). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21409 published, and has it been updated?

CVE-2021-21409 was published on March 30, 2021 and was last updated on October 24, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕

☕ Maven

Not in CISA KEV

MEDIUM severity

CVE-2021-21409 — netty-codec-http2

MEDIUMFix: netty/netty@b0fa4d5

CVE-2021-21409 is a medium-severity (CVSS 5.9) CWE-444 vulnerability in io.netty:netty-codec-http2. A fix is available for io.netty:netty-codec-http2 — see the affected versions and patch details below.

Possible request smuggling in HTTP/2 due missing validation of content-length

Also known asGHSA-f256-j965-7f32

Published

Mar 30, 2021

Updated

Oct 24, 2026

Affected

3 pkgs

Patched

1 / 3

Exploits

None indexed

Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

4.9%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs92th percentile — riskier than 92% of all scored CVEsHighest risk

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

3 pkgs affected

☕io.netty:netty-codec-http2☕org.jboss.netty:netty☕io.netty:netty

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

Netty is an open-source, asynchronous event-driven network application framework for rapid development of maintainable high performance protocol servers & clients. In Netty (io.netty:netty-codec-http2) before version 4.1.61.Final there is a vulnerability that enables request smuggling. The content-length header is not correctly validated if the request only uses a single Http2HeaderFrame with the endStream set to to true. This could lead to request smuggling if the request is proxied to a remote peer and translated to HTTP/1.1. This is a followup of GHSA-wm47-8v5p-wjpj/CVE-2021-21295 which did miss to fix this one case. This was fixed as part of 4.1.61.Final.

Affected Packages

3 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`io.netty:netty-codec-http2`	≥ 4.0.0&&< 4.1.61.Final	4.1.61.Final`io.netty:netty-codec-http2:4.1.61.Final`
☕Maven	`org.jboss.netty:netty`	all versions	No fix
☕Maven	`io.netty:netty`	all versions	No fix

Affected Products

18 products · 28 configurations

debian linuxdebian

1 version

10.0

Application

oncommand api servicesnetapp

all

Application

oncommand workflow automationnetapp

all

Application

nettynetty

< 4.1.61

range

Application

banking corporate lending process managementoracle

3 versions

14.2.014.3.014.5.0

Application

banking credit facilities process managementoracle

3 versions

14.2.014.3.014.5.0

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.netty:netty-codec-http2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update io.netty:netty-codec-http2 to 4.1.61.Final or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21409 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

Red Hat Satellite ships a vulnerable Netty version embedded in Candlepin. However, it is not directly vulnerable since the HTTP requests are handled by Tomcat and not by Netty. Red Hat OpenStack Platform's OpenDaylight will not be updated for this flaw because it was deprecated as of OpenStack Platform 14 and is only…

Product	Fixed in	Advisory
AMQ Clients 2.y for RHEL 7	`qpid-proton-0:0.33.0-6.el7_9`	RHSA-2021:1511
EAP 7.3.8	`netty`	RHSA-2021:2696
OpenShift Logging 5.1	`openshift-logging/elasticsearch6-rhel8:v6.8.1-67`	RHSA-2021:5128
OpenShift Logging 5.2	`openshift-logging/elasticsearch6-rhel8:v6.8.1-66`	RHSA-2021:5127
OpenShift Logging 5.3	`openshift-logging/elasticsearch6-rhel8:v6.8.1-65`	RHSA-2021:5129
Red Hat AMQ 7.8.2	`netty`	RHSA-2021:2689
Red Hat AMQ 7.9.0	see advisory	RHSA-2021:3700
Red Hat AMQ Streams 1.8.0	`netty`	RHSA-2021:3225

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2021-21409 in your dependencies?

Find it across Maven, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVE-2021-21409 — netty-codec-http2

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Fixing This On Your OS

Frequently Asked Questions

Is CVE-2021-21409 in your dependencies?