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Medium

Possible request smuggling in HTTP/2 due missing validation

  • Disclosed: 09.03.2021
  • Updated: 11.10.2026 13:19

Impact

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. A sample attack request looks like:

POST / HTTP/2
:authority:: externaldomain.com
Content-Length: 4

asdfGET /evilRedirect HTTP/1.1
Host: internaldomain.com

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
  • These HTTP/1.1 objects are forwarded to another remote peer.

Patches

This has been patched in 4.1.60.Final

Workarounds

The user can do the validation by themselves by implementing a custom ChannelInboundHandler that is put in the ChannelPipeline behind Http2StreamFrameToHttpObjectCodec.

References

Related change to workaround the problem: https://github.com/Netflix/zuul/pull/980

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Java / Kotlin (Maven) io.netty:netty all versions no fix
Java / Kotlin (Maven) io.netty:netty-codec-http2 >= 4.0.0, < 4.1.60.Final 4.1.60.Final
Java / Kotlin (Maven) org.jboss.netty:netty all versions no fix

References

CVSS score 5.9/10
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
Weakness (CWE)
CWE-444
Platform
Java / Kotlin (Maven)
Package
io.netty:netty, io.netty:netty-codec-http2, org.jboss.netty:netty
Disclosed
09.03.2021
Updated
11.10.2026 13:19

Sources and licenses

OSV / GitHub Advisory Database - GHSA-wm47-8v5p-wjpj - CC BY 4.0

Vulnerability history: io.netty:netty-codec-http2

All published vulnerabilities of this package (Java / Kotlin (Maven)), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (11) in the archive
Total entries
11
First → latest
03.2021 07.2026
Without a fix
0
Exploited in attacks
0

Severity breakdown

  • High 5
  • Medium 6

Disclosures per year

Severity:
  1. Netty: HTTP/2 decompression leaks ByteBuf reference count when the decompressor channel is already closed (Direct memory leak / OOM DoS)
    High · 7.5 CVE-2026-56819 >= 4.2.0, < 4.2.16.Final, >= 4.1.0.Final, < 4.1.136.Final 4.2.16.Final, 4.1.136.Final
  2. Netty: HttpContentDecompressor maxAllocation bypass when Content-Encoding set to br/zstd/snappy leads to decompression bomb DoS
    High · 7.5 CVE-2026-42587 >= 4.2.0.Alpha1, < 4.2.13.Final, < 4.1.133.Final 4.2.13.Final, 4.1.133.Final
  3. Netty HTTP/2 CONTINUATION Frame Flood DoS via Zero-Byte Frame Bypass
    High CVE-2026-33871 < 4.1.132.Final, >= 4.2.0.Alpha1, < 4.2.11.Final 4.1.132.Final, 4.2.11.Final
  4. Netty affected by MadeYouReset HTTP/2 DDoS vulnerability
    High · 7.5 CVE-2025-55163 >= 4.2.0.Alpha1, < 4.2.4.Final, < 4.1.124.Final 4.2.4.Final, 4.1.124.Final
  5. io.netty:netty-codec-http2 vulnerable to HTTP/2 Rapid Reset Attack
    High · 7.5 ghsa-xpw8-rcwv-8f8p < 4.1.100.Final 4.1.100.Final
  6. Possible request smuggling in HTTP/2 due missing validation this entry
    Medium · 5.9 CVE-2021-21295 >= 4.0.0, < 4.1.60.Final 4.1.60.Final

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