Cyber Resilience

CVE-2026-56821

Published
29 July 2026
Modified
30 July 2026
CVSS Score v3.1 7.4
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
EPSS Score 0.0014 4th percentile
Risk Priority 55 floored blend · peak EPSS

Summary

CVE-2026-56821 is a high-severity Improper Check for Certificate Revocation (CWE-299) vulnerability. Its CVSS base score is 7.4 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Adversary-in-the-Middle (T1557); ranked at the 4th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to SI-2 (Flaw Remediation) and IA-5 (Authenticator Management) — see the control section below for these in your framework.

OWASP Top 10 for Web (2025)

A07:2025 Authentication Failures

EU & UK References

Vulnerability Data

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 Prior to versions 4.1.136.Final and 4.2.16.Final, the OcspServerCertificateValidator flags an out-of-date OCSP response but does not stop processing it, so an expired GOOD response is still reported as VALID, letting an on-path…

more

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(s)
CWE-299

Related Threats

MITRE ATT&CK Enterprise TechniquesAI

T1557 Adversary-in-the-Middle Credential Access
Adversaries may attempt to position themselves between two or more networked devices using an adversary-in-the-middle (AiTM) technique to support follow-on behaviors such as [Network Sniffing](https://attack.
attack.mitre.org →
Why these techniques?

Vulnerability enables on-path MitM by allowing replay of stale OCSP GOOD responses to bypass certificate revocation checks.

Confidence: MEDIUM · MITRE ATT&CK Enterprise v19.0

CVEs Like This One

CVE-2026-4428Shared CWE-299
CVE-2026-6899Shared CWE-299
CVE-2025-36057Shared CWE-299
CVE-2026-9636Shared CWE-299
CVE-2025-11955Shared CWE-299
CVE-2024-56138Shared CWE-299
CVE-2025-3085Shared CWE-299
CVE-2023-23690Shared CWE-299

Affected Assets

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)
  • SI-2 Flaw Remediation
  • IA-5 Authenticator Management
  • SC-17 Public Key Infrastructure Certificates
Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V12.1.4

Mitigating Controls (NIST 800-53 r5) AI

SI-2 Flaw Remediation good match
prevent

Directly addresses the flaw by requiring application of the vendor patch (4.1.136.Final/4.2.16.Final) that enforces OCSP response freshness before accepting a GOOD status.

IA-5 Authenticator Management partial match
prevent

Requires management of authenticators including validation of certificate revocation status, which would reject the replayed stale OCSP response if properly implemented.

SC-17 Public Key Infrastructure Certificates partial match
prevent

Mandates proper issuance, management and revocation handling for PKI certificates, preventing acceptance of certificates whose revocation status is not freshly verified.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.AA-03 mostly match
prevents

Certificate-based authentication requires valid, non-revoked certificates; proper revocation checking is a core implementation detail of this control.

PR.DS-02 mostly match
prevents

Protecting data-in-transit commonly relies on TLS certificate validation that includes revocation status checks.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.24 Use of cryptography mostly match
prevents

Mandates use of cryptography including certificate validation and revocation checking.

A.8.29 Security testing in development and acceptance mostly match
detects

Security testing can detect missing or incorrect certificate revocation checks.

A.8.28 Secure coding partial match
prevents

Secure coding practices should include proper certificate revocation verification.

A.8.5 Secure authentication partial match
prevents

Requires secure authentication mechanisms that rely on valid certificates.

A.8.20 Networks security none match
none

Network security controls can enforce certificate validation at network boundaries.

A.8.21 Security of network services none match
none

Security of network services includes certificate-based authentication and revocation checks.

References