CVE-2026-59903Network attack vector

Information disclosure via Vary header overwrite

Published Aug 17, 2026 · Updated Aug 17, 2026

Information disclosure in Netty before 4.1.137.Final and 4.2.17.Final allows remote attackers to read cached user-specific responses via shared caches. In io.netty.handler.codec.http.cors.CorsHandler, setVaryHeader calls response.headers().set(VARY, ORIGIN), which replaces existing Vary values such as Authorization or Cookie instead of preserving them. Exposure requires a Netty-based application to use CorsHandler, emit cache-separating Vary headers for authenticated content, and sit behind a caching proxy or CDN that can later serve the poisoned entry to a different user.

CVSS severity6.5
Medium
EPSS probability0.24%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Information disclosure in Netty before 4.1.137.Final and 4.2.17.Final allows remote attackers to read cached user-specific responses via shared caches. In io.netty.handler.codec.http.cors.CorsHandler, setVaryHeader calls response.headers().set(VARY, ORIGIN), which replaces existing Vary values such as Authorization or Cookie instead of preserving them. Exposure requires a Netty-based application to use CorsHandler, emit cache-separating Vary headers for authenticated content, and sit behind a caching proxy or CDN that can later serve the poisoned entry to a different user.

The record

CVE
CVE-2026-59903
Published
Aug 17, 2026
Updated
Aug 17, 2026
Vendor
The Netty Project
Product
Netty
Classifications
CWE-524, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 17, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 17, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 4.1.137.Final
  2. Affected versionversion=>= 4.2.0.Final, < 4.2.17.Final

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Netty Project · Nettyversion=< 4.1.137.Final; version=>= 4.2.0.Final, < 4.2.17.Final

Published CVSS scores

6.5GitHub_MCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-524Use of Cache Containing Sensitive Information
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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