Cyber Resilience

CVE-2026-56821

Netty ≤ 4.1.136

Public PoC
Published
29 July 2026
Modified
07 August 2026
Patch / advisory
View advisory
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 0.0016 6th percentile
Risk Priority 53 floored blend · peak EPSS

CVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.

Summary

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

Operationally, exploitation aligns with the MITRE ATT&CK technique Subvert Trust Controls (T1553); ranked at the 6th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.

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 Prior to versions 4.1.136.Final and 4.2.16.Final, the OcspServerCertificateValidator flags an out-of-date OCSP response but does not stop processing it, so an expired GOOD response is still reported as VALID, letting an on-path…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

T1553 Subvert Trust Controls Defense Impairment
Adversaries may undermine security controls that will either warn users of untrusted activity or prevent execution of untrusted programs.
attack.mitre.org →
T1553.002 Code Signing Defense Impairment
Adversaries may create, acquire, or steal code signing materials to sign their malware or tools.
attack.mitre.org →
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 →
T1649 Steal or Forge Authentication Certificates Credential Access
Adversaries may steal or forge certificates used for authentication to access remote systems or resources.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2026-50010Same product: Netty Netty
CVE-2026-56820Same product: Netty Netty
CVE-2026-47691Same product: Netty Netty
CVE-2026-45674Same product: Netty Netty
CVE-2026-44894Same product: Netty Netty
CVE-2026-42580Same product: Netty Netty
CVE-2026-44891Same product: Netty Netty
CVE-2026-42584Same product: Netty Netty
CVE-2026-48748Same product: Netty Netty
CVE-2026-50560Same product: Netty Netty

Affected Assets

netty
netty
≤ 4.1.136 · 4.2.0 — 4.2.16

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

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

IA-5 Authenticator Management partial match

Authenticator management requires verification and lifecycle handling of certificates, which structurally prevents use of revoked ones when revocation status is part of the process.

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
finds

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.

References