CVE-2026-56821

Q: How do I fix CVE-2026-56821?

Upgrade netty to 4.1.136 or later.

Q: Is CVE-2026-56821 being actively exploited?

Not that we know of. CVE-2026-56821 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.14% is the estimated probability that it will be exploited in the next 30 days. That is higher than 4% of all scored CVEs.

Q: How severe is CVE-2026-56821?

CVE-2026-56821 has a CVSS v3 base score of 7.4, rated high. 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-2026-56821 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.

HIGH

7.4

CVSS v3

NVD

Description

Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.136.Final and 4.2.16.Final, the OcspServerCertificateValidator flags an out-of-date OCSP response but does not stop processing it, so an expired GOOD response is still reported as VALID, letting an on-path attacker replay a stale GOOD response to bypass revocation of a since-revoked certificate. Exploitation can lead to certificate revocation bypass via replay of an expired OCSP response. Any application using OcspServerCertificateValidator is affected; a revoked certificate can be accepted. This issue has been fixed in versions 4.1.136.Final and 4.2.16.Final.

CWE-299

How to fix

Remediation Available

nettyNVD

Affected:>= 4.1.0, < 4.1.136Fixed in:4.1.136CVE-2026-56821derived from NVD

Affected:>= 4.2.0, < 4.2.16Fixed in:4.2.16CVE-2026-56821derived from NVD

Affected:< 4.1.136Fixed in:4.1.136CVE-2026-56821derived 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

IntegrityHigh

AvailabilityNone

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

Exploit Intelligence

0.14%probability of exploitation in 30 days

4thpercentile

Low risk: more likely to be exploited than 4% of all known CVEs.

References

Exploit1

github.com

Related Vulnerabilities

Other CWE-299 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2020-1675	High	8.3	0.6%	-	Fix
CVE-2020-16228	Medium	6.4	0.4%	-	-
CVE-2025-3085	High	8.1	0.3%	-	Fix
CVE-2025-11955	Unscored	-	0.3%	-	-
CVE-2026-4428	High	7.4	0.3%	-	-
CVE-2023-23690	High	7.0	0.2%	-	Fix

Common questions

How do I fix CVE-2026-56821?

Upgrade netty to 4.1.136 or later.

Is CVE-2026-56821 being actively exploited?

Not that we know of. CVE-2026-56821 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.14% is the estimated probability that it will be exploited in the next 30 days. That is higher than 4% of all scored CVEs.

How severe is CVE-2026-56821?

CVE-2026-56821 has a CVSS v3 base score of 7.4, rated high. 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-2026-56821 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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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 2026-08-07.