CVE-2026-56821: 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 Prior to versions 4.1.136.Final and 4.2.16.Final, the OcspServerCertificateValidator flags an out-of-date OCSP

highCVSS 7.4CVE-2026-56821

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 Prior to versions 4.1.136.Final and 4.2.16.Final, the OcspServerCertificateValidator flags an out-of-date OCSP response but does not stop processing it, so an expired GOOD response is still reported as VALID, letting an on-path attacker replay a stale GOOD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

Details

Source: NVD Recent CVEs (US · database · site)
Severity: high — CVSS 7.4
Published: 2026-07-29
Last updated: 2026-07-31
Exploitation: Not in CISA KEV at last sync

Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-56821

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-568210.14% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 4% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-56821	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

Recent advisories for Netty is an

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

highGHSA-93wv-jw9v-4972: Netty: HTTP/2 decompression leaks ByteBuf reference count when the decompressor channel i…ghsa · 2026-07-31
medium[NEW] [medium] Netty: Multiple vulnerabilitiescert-bund · 2026-07-30
high[NEW] [high] Netty: Multiple vulnerabilitiescert-bund · 2026-07-30
high[UPDATE] [high] Netty: Multiple vulnerabilitiescert-bund · 2026-07-30
unknownCVE-2026-59898: Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.13…nvd · 2026-07-29
mediumCVE-2026-59920: Netty is an asynchronous, event-driven network application framework. In versions prior to 4.1…nvd · 2026-07-29