CVE-2026-59900: Netty request smuggling via Host header duplication in HTTP/2 translation

CVE-2026-59900 · Severity: medium · CVSS 4 · Published 2026-07-29

Netty, a widely used networking framework for Java applications, contains a flaw in how it handles web traffic when converting between different versions of the HTTP protocol. An attacker can send a specially crafted request that contains conflicting 'Host' information, which may confuse security filters or load balancers. This can lead to request smuggling or routing bypasses, potentially allowing unauthorized access to internal web services.

Technologies: io.netty:netty-codec-http2 (Maven), Netty Project Netty Codec HTTP2. Vendors: Maven, Netty Project.

Quick answers

Is CVE-2026-59900 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-59900 being exploited in the wild.
How severe is CVE-2026-59900?
Medium severity, with a CVSS score of 4, published 29 July 2026.
Which products and versions are affected?
io.netty:netty-codec-http2 (Maven) ECOSYSTEM:introduced=4.2.0.Final,fixed=4.2.16.Final; Netty Project Netty Codec HTTP2 < 4.1.136.Final, >= 4.2.0.Final, < 4.2.16.Final.
Is there a fix for CVE-2026-59900?
Yes. io.netty:netty-codec-http2 (Maven): upgrade to 4.2.16.Final or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-59900 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-59900?
Check whether you run io.netty:netty-codec-http2 (Maven) or Netty Project Netty Codec HTTP2 at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • io.netty:netty-codec-http2 (Maven): upgrade to 4.2.16.Final or later
    # set io.netty:netty-codec-http2 to 4.2.16.Final in your pom.xml or build.gradle

Technical details

A vulnerability exists in Netty's HTTP/2-to-HTTP/1.x translation components, specifically `Http2StreamFrameToHttpObjectCodec` and `InboundHttp2ToHttpAdapter`. When an HTTP/2 HEADERS frame contains both an `:authority` pseudo-header and a literal `host` header, the translator fails to validate or deduplicate them. This results in an `HttpRequest` object with two `Host` headers containing different, attacker-controlled values. This inconsistency can be exploited for HTTP request smuggling or to bypass security constraints in downstream proxies that rely on the `Host` header for routing or authorization. The issue is fixed in versions 4.1.136.Final and 4.2.16.Final.

Affected products

  • Netty netty-codec-http2 < 4.1.136.Final, >= 4.2.0.Final, < 4.2.16.Final

Identifiers

  • CVE-2026-59900
  • GHSA-c69g-56f8-xwqj

Timeline

  • 2026-07-07: patched: Release of 4.2.16.Final
  • 2026-07-08: patched: Release of 4.1.136.Final
  • 2026-07-14: advisory: GitHub Security Advisory GHSA-c69g-56f8-xwqj published
  • 2026-07-29: disclosed: CVE-2026-59900 published to NVD

References

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