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CVE-2026-56821

Netty vulnerability

7.4 HIGHPublic exploitEPSS 0.22%
  1. Published28 Jul 2026

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 Prior to versions 4.1.136.Final and 4.2.16.Final, the OcspServerCertificateValidator flags an out-of-date OCSP response but does not stop processing it, so an expired GOOD response is still reported as VALID, letting an on-path attacker replay a stale GOOD response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.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityHigh
Privileges requiredNone
User interactionNone
ScopeUnchanged
ConfidentialityHigh
IntegrityHigh
AvailabilityNone
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Affected products

Netty≥ 4.2.0 < 4.2.16 · < 4.1.136

Remediation

Fixed in
Netty≥ 4.2.16

References

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Frequently asked

Is CVE-2026-56821 being actively exploited?

CVE-2026-56821 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 0.22%.

How severe is CVE-2026-56821?

CVE-2026-56821 has a CVSS 3.1 base score of 7.4, rated high. EPSS estimates a 0.22% probability of exploitation.

Which products are affected by CVE-2026-56821?

Affected products include Netty.

How do I fix CVE-2026-56821?

Upgrade affected software to a fixed version, for example Netty 4.2.16 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.