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PatchSiren cyber security CVE debrief

CVE-2026-56820 netty CVE debrief

CVE-2026-56820 is a high-severity vulnerability in Netty, a network application framework, that allows for replay attacks due to improper OCSP response validation. The issue affects Netty versions 4.2.0.Final through 4.2.15.Final and prior to 4.1.135.Final. The vulnerability is caused by the `OcspClient` not validating that the `CertificateID` in an OCSP response matches the requested `CertificateID`, which can lead to bypass of revocation checks for another certificate.

Vendor
netty
Product
Unknown
CVSS
HIGH 7.4
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-21
Original CVE updated
2026-07-22
Advisory published
2026-07-21
Advisory updated
2026-07-22

Who should care

Users of Netty versions 4.2.0.Final through 4.2.15.Final and prior to 4.1.135.Final should be aware of this vulnerability and take steps to mitigate it. This includes upgrading to a patched version of Netty, specifically 4.1.136.Final or 4.2.16.Final, and performing inventory checks to identify potentially affected systems.

Technical summary

The vulnerability is caused by the `OcspClient` not validating that the `CertificateID` in an OCSP response matches the requested `CertificateID`. This allows for replay attacks, as a legitimately signed `GOOD` status response for an unrelated certificate issued by the same CA can be accepted, bypassing revocation checks for another certificate. The issue is fixed in versions 4.1.136.Final and 4.2.16.Final.

Defensive priority

High

Recommended defensive actions

  • Upgrade to a patched version of Netty, specifically 4.1.136.Final or 4.2.16.Final.
  • Perform inventory checks to identify potentially affected systems.
  • Implement compensating controls, such as monitoring and exception tracking, to detect potential exploitation attempts.
  • Verify the validity of OCSP responses to prevent replay attacks.
  • Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance.
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
  • Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.

Evidence notes

The CVE record was published on 2026-07-21T23:17:52.403Z and has been modified since then. The NVD entry is currently awaiting analysis. The vulnerability has a CVSS score of 7.4 and a severity of HIGH.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-56820 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-56820

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-56820 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-56820

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.