CVE-2026-59903

Q: How do I fix CVE-2026-59903?

Upgrade netty to 4.2.17 or later.

Q: Is CVE-2026-59903 being actively exploited?

Not that we know of. CVE-2026-59903 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.25% is the estimated probability that it will be exploited in the next 30 days. That is higher than 15% of all scored CVEs.

Q: How severe is CVE-2026-59903?

CVE-2026-59903 has a CVSS v3 base score of 6.5, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

Q: What does CVE-2026-59903 affect?

Published advisories record a fix for netty (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

MEDIUM

6.5

CVSS v3

NVD

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.

CWE-524

How to fix

Remediation Available

nettyNVD

Affected:>= 4.2.0, < 4.2.17Fixed in:4.2.17CVE-2026-59903derived from NVD

Affected:< 4.1.137Fixed in:4.1.137CVE-2026-59903derived from NVD

TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityHigh

Privileges RequiredNone

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityHigh

IntegrityLow

AvailabilityNone

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

Exploit Intelligence

0.25%probability of exploitation in 30 days

15thpercentile

Low risk: more likely to be exploited than 15% of all known CVEs.

References

Patch2

Vendor Advisory1

github.com

Release Notes2

Related Vulnerabilities

Other CWE-524 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2019-9494	Medium	5.9	3.9%	-	Fix
CVE-2021-24027	High	7.5	3.8%	-	Fix
CVE-2019-9495	Low	3.7	3.5%	-	Fix
CVE-2019-14997	Medium	4.3	1.2%	-	Fix
CVE-2026-25703	High	7.3	0.8%	-	-
CVE-2024-0874	Medium	5.3	0.8%	-	-

Common questions

How do I fix CVE-2026-59903?

Upgrade netty to 4.2.17 or later.

Is CVE-2026-59903 being actively exploited?

Not that we know of. CVE-2026-59903 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.25% is the estimated probability that it will be exploited in the next 30 days. That is higher than 15% of all scored CVEs.

How severe is CVE-2026-59903?

CVE-2026-59903 has a CVSS v3 base score of 6.5, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

What does CVE-2026-59903 affect?

Published advisories record a fix for netty (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

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Markdown

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-09-23.