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
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-21409 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,166 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
io.netty:netty-codec-http2☕org.jboss.netty:netty☕io.netty:nettyReal-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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | io.netty:netty-codec-http2 | ≥ 4.0.0&&< 4.1.61.Final | 4.1.61.Finalio.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 |
Detection & mitigation playbook
Open-source dependencyDetect
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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-21409 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-21409. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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 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
Is CVE-2021-21409 in your dependencies?
O3 Security finds CVE-2021-21409 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.