VulnerabilityPublished 22 Jul 2026

CVE-2026-56821: Netty · Out-of-date OCSP Responses Accepted by OcspServerCertificateValidator

Affects io.netty:netty-handler-ssl-ocspSource GCVE Database

Not known to be exploited · CVSS 7.4 High : upgrade to 4.2.16.Final soon.

Data as of 26 Oct 2026, 17:16 UTC

CVSS 3.1 base score

7.4High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 12% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 4.2.16.Final, 4.1.136.Final

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: High
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 22 Jul 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade io.netty:netty-handler-ssl-ocsp to 4.2.16.Final; io.netty:netty-handler-ssl-ocsp to 4.1.136.Final.

Affected software

Product	Affected	Fixed in
Mavenio.netty:netty-handler-ssl-ocsp	Affected >=4.2.0.Final <4.2.16.Final	Fixed in 4.2.16.Final
Mavenio.netty:netty-handler-ssl-ocsp	Affected <4.1.136.Final	Fixed in 4.1.136.Final

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

### Summary `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. ### Details In `io.netty.handler.ssl.ocsp.OcspServerCertificateValidator#userEventTriggered` the freshness check has no `return`, so execution falls through and a `VALID` `OcspValidationEvent` is still fired: ```java if (!(current.after(response.getThisUpdate()) && current.before(response.getNextUpdate()))) { ctx.fireExceptionCaught(new IllegalStateException("OCSP Response is out-of-date")); } ``` Nonce validation is optional and off by default, so freshness is the only replay defense — and it is not enforced. Additionally `getNextUpdate()` may be `null`, making `current.before(null)` throw `NullPointerException`. https://datatracker.ietf.org/doc/html/rfc6960#section-3.2 ``` 5. The time at which the status being indicated is known to be correct (thisUpdate) is sufficiently recent; 6. When available, the time at or before which newer information will be available about the status of the certificate (nextUpdate) is greater than the current time. ``` ### PoC Add the test below to `io.netty.handler.ssl.ocsp.OcspServerCertificateValidatorTest` ```java @Test void staleOcspResponseIsRejected() throws Exception { X509Bundle caRoot = new CertificateBuilder() .algorithm(CertificateBuilder.Algorithm.rsa2048) .subject("CN=TrustedRootCA") .setIsCertificateAuthority(true) .buildSelfSigned(); GeneralName ocspName = new GeneralName(GeneralName.uniformResourceIdentifier, "http://localhost/"); AuthorityInformationAccess aia = new AuthorityInformationAccess( new AccessDescription(AccessDescription.id_ad_ocsp, ocspName)); X509Bundle targetCert = new CertificateBuilder() .algorithm(CertificateBuilder.Algorithm.rsa2048) .subject("CN=TargetServer") .addExtensionOctetString("1.3.6.1.5.5.7.1.1", false, aia.getEncoded()) .buildIssuedBy(caRoot); Date past = new Date(System.currentTimeMillis() - TimeUnit.DAYS.toMillis(7)); CertificateID certId = new CertificateID( new JcaDigestCalculatorProviderBuilder().build().get(CertificateID.HASH_SHA1), new JcaX509CertificateHolder(caRoot.getCertificate()), targetCert.getCertificate().getSerialNumber()); BasicOCSPRespBuilder respBuilder = new BasicOCSPRespBuilder( new RespID(new JcaX509CertificateHolder(caRoot.getCertificate()).getSubject())); respBuilder.addResponse(certId, CertificateStatus.GOOD, past, past); BasicOCSPResp expiredBasicResp = respBuilder.build( new JcaContentSignerBuilder("SHA256withRSA").build(caRoot.getKeyPair().getPrivate()), new X509CertificateHolder[0], past); final byte[] responseEncoded = new OCSPRespBuilder() .build(OCSPRespBuilder.SUCCESSFUL, expiredBasicResp).getEncoded(); IoTransport defaultTransport = createDefaultTransport(); IoTransport mockTransport = IoTransport.create(defaultTransport.eventLoop(), () -> { NioSocketChannel channel = new NioSocketChannel(); channel.pipeline().addFirst(new ChannelOutboundHandlerAdapter() { @Override public void connect(ChannelHandlerContext ctx, SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise) { promise.setSuccess(); ctx.executor().execute(() -> { ctx.pipeline().fireChannelActive(); DefaultFullHttpResponse httpResponse = new DefaultFullHttpResponse( HttpVersion.HTTP_1_1, HttpResponseStatus.OK, Unpooled.wrappedBuffer(responseEncoded)); httpResponse.headers().set(HttpHeaderNames.CONTENT_TYPE, "application/ocsp-response"); httpResponse.headers().set(HttpHeaderNames.CONTENT_LENGTH, httpResponse.content().readableBytes()); ctx.pipeline().fireChannelRead(httpResponse); }); } }); return channel; }, defaultTransport.datagramChannel()); SslContext serverSslCtx = SslContextBuilder .forServer(targetCert.getKeyPair().getPrivate(), targetCert.getCertificate(), caRoot.getCertificate

CWE-299Improper Check for Certificate Revocation (opens in a new tab)
Base weaknessLikelihood of exploit: MediumOWASP Top 10 A07:2025: Authentication Failures (opens in a new tab)
The product does not check or incorrectly checks the revocation status of a certificate, which may cause it to use a certificate that has been compromised.
Architecture and Design
Ensure that certificates are checked for revoked status.
Implementation
If certificate pinning is being used, ensure that all relevant properties of the certificate are fully validated before the certificate is pinned, including the revoked status.
OWASP Cheat Sheets
OWASP Authentication (opens in a new tab)
OWASP ASVS 4.0.3
V9.2.4 (opens in a new tab)
V2.8.5 (opens in a new tab)
V2.8.4 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

WeaknessesCWE-299

Technical details

References

Community reviews

Related threats