High severityNVD Advisory· Published Aug 26, 2022· Updated Aug 3, 2024

CVE-2021-3859

Description

A flaw was found in Undertow that tripped the client-side invocation timeout with certain calls made over HTTP2. This flaw allows an attacker to carry out denial of service attacks.

AI Insight

LLM-synthesized narrative grounded in this CVE's description and references.

Undertow HTTP/2 client-side invocation timeout can be tripped via crafted continuation frames, enabling denial of service.

Vulnerability

Overview

CVE-2021-3859 is a flaw in Undertow, a Java-based web server and servlet container. The vulnerability arises from incorrect handling of HTTP/2 continuation frames in the client-side HTTP/2 implementation. When certain calls are made over HTTP2, the client-side invocation timeout is improperly triggered, leading to a denial of service (DoS) condition. [1]

Exploitation

An attacker can exploit this vulnerability by sending specially crafted HTTP/2 continuation frames to an Undertow server. The issue is specifically related to how the HTTP client parses continuation frames; the commit message confirms that "continuation frames are not read correctly" [2]. This manipulation causes the server's client-side timeout logic to activate prematurely, effectively terminating the connection or stalling further processing.

Impact

Successful exploitation allows an attacker to cause a denial of service, making the Undertow-based service unavailable to legitimate users. The attack does not require authentication, as the vulnerable component is accessible during the initial HTTP/2 handshake or data exchange. [1]

Mitigation

Red Hat has acknowledged the issue and a fix was developed in commit e43f0ada3f4da6e8579e0020cec3cb1a81e487c2 and merged via pull request #1296 [2][3][4]. Users should update to a version of Undertow containing the patch. Red Hat's advisory provides guidance for affected products [1].

References

AI Insight generated on May 21, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.

Affected packages

Versions sourced from the GitHub Security Advisory.

Package	Affected versions	Patched versions
io.undertow:undertow-coreMaven	< 2.2.15	2.2.15

Affected products

Undertow/Undertowdescription
ghsa-coords
pkg:maven/io.undertow/undertow-core
Range: < 2.2.15

Patches

db0f5be43f8e

Merge pull request #1296 from fl4via/UNDERTOW-1979

https://github.com/undertow-io/undertowFlavia RainoneFeb 5, 2022via ghsa

commit

3 files changed · +321 −3

core/src/main/java/io/undertow/protocols/http2/HpackEncoder.java+1 −1 modified

@@ -163,6 +163,7 @@ public State encode(HeaderMap headers, ByteBuffer target) {
             if (headers != currentHeaders) {
                 throw new IllegalStateException();
             }
+            it = headers.fiNext(it);
         }
         while (it != -1) {
             HeaderValues values = headers.fiCurrent(it);
@@ -239,7 +240,6 @@ public State encode(HeaderMap headers, ByteBuffer target) {
                         }
                     }
                     if(overflowing) {
-                        it = headers.fiNext(it);
                         this.headersIterator = it;
                         this.overflowLength = current.position();
                         return State.OVERFLOW;

core/src/main/java/io/undertow/protocols/http2/Http2Channel.java+10 −2 modified

@@ -538,6 +538,16 @@ protected AbstractHttp2StreamSourceChannel createChannelImpl(FrameHeaderData fra
 
     @Override
     protected FrameHeaderData parseFrame(ByteBuffer data) throws IOException {
+        Http2FrameHeaderParser frameParser;
+        do {
+            frameParser = parseFrameNoContinuation(data);
+            // if the frame requires continuation and there is remaining data in the buffer
+            // it should be consumed cos spec ensures the next frame is the continuation
+        } while(frameParser != null && frameParser.getContinuationParser() != null && data.hasRemaining());
+        return frameParser;
+    }
+
+    private Http2FrameHeaderParser parseFrameNoContinuation(ByteBuffer data) throws IOException {
         if (prefaceCount < PREFACE_BYTES.length) {
             while (data.hasRemaining() && prefaceCount < PREFACE_BYTES.length) {
                 if (data.get() != PREFACE_BYTES[prefaceCount]) {
@@ -575,10 +585,8 @@ protected FrameHeaderData parseFrame(ByteBuffer data) throws IOException {
         }
         if (frameParser.getContinuationParser() != null) {
             this.continuationParser = frameParser.getContinuationParser();
-            return null;
         }
         return frameParser;
-
     }
 
     protected void lastDataRead() {

core/src/test/java/io/undertow/client/http/H2CUpgradeContinuationTestCase.java+310 −0 added

@@ -0,0 +1,310 @@
+/*
+ * JBoss, Home of Professional Open Source.
+ * Copyright 2021 Red Hat, Inc., and individual contributors
+ * as indicated by the @author tags.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package io.undertow.client.http;
+
+import io.undertow.Undertow;
+import io.undertow.UndertowOptions;
+import io.undertow.client.ClientCallback;
+import io.undertow.client.ClientConnection;
+import io.undertow.client.ClientExchange;
+import io.undertow.client.ClientRequest;
+import io.undertow.client.ClientResponse;
+import io.undertow.client.UndertowClient;
+import io.undertow.connector.ByteBufferPool;
+import io.undertow.io.Receiver;
+import io.undertow.io.Sender;
+import io.undertow.protocols.ssl.UndertowXnioSsl;
+import io.undertow.server.DefaultByteBufferPool;
+import io.undertow.server.HttpHandler;
+import io.undertow.server.HttpServerExchange;
+import io.undertow.server.handlers.PathHandler;
+import io.undertow.server.protocol.http2.Http2UpgradeHandler;
+import io.undertow.testutils.DebuggingSlicePool;
+import io.undertow.testutils.DefaultServer;
+import io.undertow.testutils.HttpOneOnly;
+import io.undertow.util.AttachmentKey;
+import io.undertow.util.HeaderValues;
+import io.undertow.util.Headers;
+import io.undertow.util.HttpString;
+import io.undertow.util.Methods;
+import io.undertow.util.StatusCodes;
+import io.undertow.util.StringReadChannelListener;
+import io.undertow.util.StringWriteChannelListener;
+import java.io.IOException;
+import java.net.URI;
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import org.junit.AfterClass;
+import org.junit.Assert;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.xnio.ChannelListeners;
+import org.xnio.IoUtils;
+import org.xnio.OptionMap;
+import org.xnio.Options;
+import org.xnio.Xnio;
+import org.xnio.XnioWorker;
+import org.xnio.channels.StreamSinkChannel;
+
+/**
+ * <p>Test that uses H2C upgrade and tries to send different number of headers
+ * for a GET/POST request. The idea is that the byte buffer used in the client
+ * and server is small. That way when sending a big number of headers the
+ * HEADERS frame is not enough to contain all the data and some CONTINUATION
+ * frames are needed. The test method also tries with different sizes of DATA
+ * to force several DATA frames to be sent when using POST method.</p>
+ *
+ * @author rmartinc
+ */
+@RunWith(DefaultServer.class)
+@HttpOneOnly
+public class H2CUpgradeContinuationTestCase {
+
+    private static final String HEADER_PREFFIX = "custom-header-";
+    private static final String ECHO_PATH = "/echo";
+    private static final AttachmentKey<String> RESPONSE_BODY = AttachmentKey.create(String.class);
+
+    private static ByteBufferPool smallPool;
+    private static XnioWorker worker;
+    private static Undertow server;
+
+    /**
+     * Just a handler that receives the request and sends back all the custom
+     * headers received and (if data received) returns the same data (empty
+     * response otherwise).
+     * @param exchange The HttpServerExchange
+     */
+    private static void sendEchoResponse(final HttpServerExchange exchange) {
+        exchange.setStatusCode(StatusCodes.OK);
+        // add the custom headers received
+        for (HeaderValues header : exchange.getRequestHeaders()) {
+            if (header.getFirst().startsWith(HEADER_PREFFIX)) {
+                exchange.getResponseHeaders().putAll(header.getHeaderName(), header.subList(0, header.size()));
+            }
+        }
+        // response using echo or empty string
+        if (exchange.getRequestContentLength() > 0) {
+            exchange.getRequestReceiver().receiveFullString(new Receiver.FullStringCallback() {
+                @Override
+                public void handle(HttpServerExchange exchange, String message) {
+                    exchange.getResponseSender().send(message);
+                }
+            });
+        } else {
+            final Sender sender = exchange.getResponseSender();
+            sender.send("");
+        }
+    }
+
+    /**
+     * Initializes the server with the H2C handler and adds the echo handler to
+     * manage the requests.
+     * @throws IOException Some error
+     */
+    @BeforeClass
+    public static void beforeClass() throws IOException {
+        // server and client pool is using 1024 for the buffer size
+        smallPool = new DebuggingSlicePool(new DefaultByteBufferPool(true, 1024, 1000, 10, 100));
+
+        final PathHandler path = new PathHandler()
+                .addExactPath(ECHO_PATH, new HttpHandler() {
+                    @Override
+                    public void handleRequest(HttpServerExchange exchange) throws Exception {
+                        sendEchoResponse(exchange);
+                    }
+                });
+
+        server = Undertow.builder()
+                .setByteBufferPool(smallPool)
+                .addHttpListener(DefaultServer.getHostPort("default") + 1, DefaultServer.getHostAddress("default"), new Http2UpgradeHandler(path))
+                .setSocketOption(Options.REUSE_ADDRESSES, true)
+                .build();
+        server.start();
+
+        // Create xnio worker
+        final Xnio xnio = Xnio.getInstance();
+        final XnioWorker xnioWorker = xnio.createWorker(null, OptionMap.builder()
+                .set(Options.WORKER_IO_THREADS, 8)
+                .set(Options.TCP_NODELAY, true)
+                .set(Options.KEEP_ALIVE, true)
+                .getMap());
+        worker = xnioWorker;
+    }
+
+    /**
+     * Stops server and worker.
+     */
+    @AfterClass
+    public static void afterClass() {
+        if (server != null) {
+            server.stop();
+        }
+        if (worker != null) {
+            worker.shutdown();
+        }
+        if (smallPool != null) {
+            smallPool.close();
+            smallPool = null;
+        }
+    }
+
+    /**
+     * Method that sends a GET or POST request adding count number of custom
+     * headers and sending contentLength data. GET is used if no content length
+     * is passed, POST if contentLength is greater than 0.
+     * @param connection The connection to use
+     * @param requestCount The number of requests to send
+     * @param headersCount The number of custom headers to send
+     * @param contentLength The content length to send (POST method used if >0)
+     * @throws Exception Some error
+     */
+    private void sendRequest(ClientConnection connection, int requestCount, int headersCount, int contentLength) throws Exception {
+        final CountDownLatch latch = new CountDownLatch(requestCount);
+        final List<ClientResponse> responses = new CopyOnWriteArrayList<>();
+        StringBuilder sb = new StringBuilder();
+        for (int i = 0; i < contentLength; i++) {
+            sb.append(i % 10);
+        }
+        final String content = sb.length() > 0? sb.toString() : null;
+        connection.getIoThread().execute(new Runnable() {
+            @Override
+            public void run() {
+                for (int i = 0; i < requestCount; i++) {
+                    final ClientRequest request = new ClientRequest()
+                            .setMethod(contentLength > 0 ? Methods.POST : Methods.GET)
+                            .setPath(ECHO_PATH);
+                    request.getRequestHeaders().put(Headers.HOST, DefaultServer.getHostAddress());
+                    if (contentLength > 0) {
+                        request.getRequestHeaders().put(Headers.CONTENT_LENGTH, contentLength);
+                    }
+                    for (int j = 0; j < headersCount; j++) {
+                        request.getRequestHeaders().put(new HttpString(HEADER_PREFFIX + j), HEADER_PREFFIX + j);
+                    }
+                    connection.sendRequest(request, createClientCallback(responses, latch, content));
+                }
+            }
+        });
+
+        latch.await(10, TimeUnit.SECONDS);
+
+        Assert.assertEquals("No responses received from server in 10s", requestCount, responses.size());
+        for (int i = 0; i < requestCount; i++) {
+            Assert.assertEquals("Response " + i + " code was not OK", StatusCodes.OK, responses.get(i).getResponseCode());
+            Assert.assertEquals("Incorrect data received for response " + i, contentLength > 0 ? content : "", responses.get(i).getAttachment(RESPONSE_BODY));
+            int headersReturned = 0;
+            for (HeaderValues header : responses.get(i).getResponseHeaders()) {
+                if (header.getFirst().startsWith(HEADER_PREFFIX)) {
+                    headersReturned += header.size();
+                }
+            }
+            Assert.assertEquals("Incorrect number of headers returned for response " + i, headersCount, headersReturned);
+        }
+    }
+
+    /**
+     * The real test that sends several GET and POST requests with different
+     * number of headers and different content length.
+     * @throws Exception  Some error
+     */
+    @Test
+    public void testDifferentSizes() throws Exception {
+        final UndertowClient client = UndertowClient.getInstance();
+
+        // the client connection uses the small byte-buffer of 1024 to force the continuation frames
+        final ClientConnection connection = client.connect(
+                new URI("http://" + DefaultServer.getHostAddress() + ":" + (DefaultServer.getHostPort("default") + 1)),
+                worker, new UndertowXnioSsl(worker.getXnio(), OptionMap.EMPTY, DefaultServer.getClientSSLContext()),
+                smallPool, OptionMap.create(UndertowOptions.ENABLE_HTTP2, true)).get();
+        try {
+            // the first request triggers the upgrade to H2C
+            sendRequest(connection, 1, 0, 0);
+            // send several requests with different sizes for headers and data
+            sendRequest(connection, 10, 10, 0);
+            sendRequest(connection, 10, 100, 0);
+            sendRequest(connection, 10, 150, 0);
+            sendRequest(connection, 10, 1, 10);
+            sendRequest(connection, 10, 0, 2000);
+            sendRequest(connection, 10, 150, 2000);
+        } finally {
+            IoUtils.safeClose(connection);
+        }
+    }
+
+    /**
+     * Create the callback to receive the response and assign it to the list.
+     * @param responses The list where the response will be added
+     * @param latch The latch to count down when the response is received
+     * @param message The message to send if it's a POST message (if null nothing is send)
+     * @return The created callback
+     */
+    private static ClientCallback<ClientExchange> createClientCallback(final List<ClientResponse> responses, final CountDownLatch latch, String message) {
+        return new ClientCallback<ClientExchange>() {
+            @Override
+            public void completed(ClientExchange result) {
+                if (message != null) {
+                    new StringWriteChannelListener(message).setup(result.getRequestChannel());
+                }
+                result.setResponseListener(new ClientCallback<ClientExchange>() {
+                    @Override
+                    public void completed(final ClientExchange result) {
+                        responses.add(result.getResponse());
+                        new StringReadChannelListener(result.getConnection().getBufferPool()) {
+
+                            @Override
+                            protected void stringDone(String string) {
+                                result.getResponse().putAttachment(RESPONSE_BODY, string);
+                                latch.countDown();
+                            }
+
+                            @Override
+                            protected void error(IOException e) {
+                                e.printStackTrace();
+                                latch.countDown();
+                            }
+                        }.setup(result.getResponseChannel());
+                    }
+
+                    @Override
+                    public void failed(IOException e) {
+                        e.printStackTrace();
+                        latch.countDown();
+                    }
+                });
+                try {
+                    result.getRequestChannel().shutdownWrites();
+                    if (!result.getRequestChannel().flush()) {
+                        result.getRequestChannel().getWriteSetter().set(ChannelListeners.<StreamSinkChannel>flushingChannelListener(null, null));
+                        result.getRequestChannel().resumeWrites();
+                    }
+                } catch (IOException e) {
+                    e.printStackTrace();
+                    latch.countDown();
+                }
+            }
+
+            @Override
+            public void failed(IOException e) {
+                e.printStackTrace();
+                latch.countDown();
+            }
+        };
+    }
+}

e43f0ada3f4d

[UNDERTOW-1979] CVE-2021-3859 continuation frames are not read correctly

https://github.com/undertow-io/undertowrmartincSep 16, 2021via ghsa

commit

3 files changed · +321 −3

core/src/main/java/io/undertow/protocols/http2/HpackEncoder.java+1 −1 modified

@@ -163,6 +163,7 @@ public State encode(HeaderMap headers, ByteBuffer target) {
             if (headers != currentHeaders) {
                 throw new IllegalStateException();
             }
+            it = headers.fiNext(it);
         }
         while (it != -1) {
             HeaderValues values = headers.fiCurrent(it);
@@ -239,7 +240,6 @@ public State encode(HeaderMap headers, ByteBuffer target) {
                         }
                     }
                     if(overflowing) {
-                        it = headers.fiNext(it);
                         this.headersIterator = it;
                         this.overflowLength = current.position();
                         return State.OVERFLOW;

core/src/main/java/io/undertow/protocols/http2/Http2Channel.java+10 −2 modified

@@ -538,6 +538,16 @@ protected AbstractHttp2StreamSourceChannel createChannelImpl(FrameHeaderData fra
 
     @Override
     protected FrameHeaderData parseFrame(ByteBuffer data) throws IOException {
+        Http2FrameHeaderParser frameParser;
+        do {
+            frameParser = parseFrameNoContinuation(data);
+            // if the frame requires continuation and there is remaining data in the buffer
+            // it should be consumed cos spec ensures the next frame is the continuation
+        } while(frameParser != null && frameParser.getContinuationParser() != null && data.hasRemaining());
+        return frameParser;
+    }
+
+    private Http2FrameHeaderParser parseFrameNoContinuation(ByteBuffer data) throws IOException {
         if (prefaceCount < PREFACE_BYTES.length) {
             while (data.hasRemaining() && prefaceCount < PREFACE_BYTES.length) {
                 if (data.get() != PREFACE_BYTES[prefaceCount]) {
@@ -575,10 +585,8 @@ protected FrameHeaderData parseFrame(ByteBuffer data) throws IOException {
         }
         if (frameParser.getContinuationParser() != null) {
             this.continuationParser = frameParser.getContinuationParser();
-            return null;
         }
         return frameParser;
-
     }
 
     protected void lastDataRead() {

core/src/test/java/io/undertow/client/http/H2CUpgradeContinuationTestCase.java+310 −0 added

@@ -0,0 +1,310 @@
+/*
+ * JBoss, Home of Professional Open Source.
+ * Copyright 2021 Red Hat, Inc., and individual contributors
+ * as indicated by the @author tags.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package io.undertow.client.http;
+
+import io.undertow.Undertow;
+import io.undertow.UndertowOptions;
+import io.undertow.client.ClientCallback;
+import io.undertow.client.ClientConnection;
+import io.undertow.client.ClientExchange;
+import io.undertow.client.ClientRequest;
+import io.undertow.client.ClientResponse;
+import io.undertow.client.UndertowClient;
+import io.undertow.connector.ByteBufferPool;
+import io.undertow.io.Receiver;
+import io.undertow.io.Sender;
+import io.undertow.protocols.ssl.UndertowXnioSsl;
+import io.undertow.server.DefaultByteBufferPool;
+import io.undertow.server.HttpHandler;
+import io.undertow.server.HttpServerExchange;
+import io.undertow.server.handlers.PathHandler;
+import io.undertow.server.protocol.http2.Http2UpgradeHandler;
+import io.undertow.testutils.DebuggingSlicePool;
+import io.undertow.testutils.DefaultServer;
+import io.undertow.testutils.HttpOneOnly;
+import io.undertow.util.AttachmentKey;
+import io.undertow.util.HeaderValues;
+import io.undertow.util.Headers;
+import io.undertow.util.HttpString;
+import io.undertow.util.Methods;
+import io.undertow.util.StatusCodes;
+import io.undertow.util.StringReadChannelListener;
+import io.undertow.util.StringWriteChannelListener;
+import java.io.IOException;
+import java.net.URI;
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import org.junit.AfterClass;
+import org.junit.Assert;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.xnio.ChannelListeners;
+import org.xnio.IoUtils;
+import org.xnio.OptionMap;
+import org.xnio.Options;
+import org.xnio.Xnio;
+import org.xnio.XnioWorker;
+import org.xnio.channels.StreamSinkChannel;
+
+/**
+ * <p>Test that uses H2C upgrade and tries to send different number of headers
+ * for a GET/POST request. The idea is that the byte buffer used in the client
+ * and server is small. That way when sending a big number of headers the
+ * HEADERS frame is not enough to contain all the data and some CONTINUATION
+ * frames are needed. The test method also tries with different sizes of DATA
+ * to force several DATA frames to be sent when using POST method.</p>
+ *
+ * @author rmartinc
+ */
+@RunWith(DefaultServer.class)
+@HttpOneOnly
+public class H2CUpgradeContinuationTestCase {
+
+    private static final String HEADER_PREFFIX = "custom-header-";
+    private static final String ECHO_PATH = "/echo";
+    private static final AttachmentKey<String> RESPONSE_BODY = AttachmentKey.create(String.class);
+
+    private static ByteBufferPool smallPool;
+    private static XnioWorker worker;
+    private static Undertow server;
+
+    /**
+     * Just a handler that receives the request and sends back all the custom
+     * headers received and (if data received) returns the same data (empty
+     * response otherwise).
+     * @param exchange The HttpServerExchange
+     */
+    private static void sendEchoResponse(final HttpServerExchange exchange) {
+        exchange.setStatusCode(StatusCodes.OK);
+        // add the custom headers received
+        for (HeaderValues header : exchange.getRequestHeaders()) {
+            if (header.getFirst().startsWith(HEADER_PREFFIX)) {
+                exchange.getResponseHeaders().putAll(header.getHeaderName(), header.subList(0, header.size()));
+            }
+        }
+        // response using echo or empty string
+        if (exchange.getRequestContentLength() > 0) {
+            exchange.getRequestReceiver().receiveFullString(new Receiver.FullStringCallback() {
+                @Override
+                public void handle(HttpServerExchange exchange, String message) {
+                    exchange.getResponseSender().send(message);
+                }
+            });
+        } else {
+            final Sender sender = exchange.getResponseSender();
+            sender.send("");
+        }
+    }
+
+    /**
+     * Initializes the server with the H2C handler and adds the echo handler to
+     * manage the requests.
+     * @throws IOException Some error
+     */
+    @BeforeClass
+    public static void beforeClass() throws IOException {
+        // server and client pool is using 1024 for the buffer size
+        smallPool = new DebuggingSlicePool(new DefaultByteBufferPool(true, 1024, 1000, 10, 100));
+
+        final PathHandler path = new PathHandler()
+                .addExactPath(ECHO_PATH, new HttpHandler() {
+                    @Override
+                    public void handleRequest(HttpServerExchange exchange) throws Exception {
+                        sendEchoResponse(exchange);
+                    }
+                });
+
+        server = Undertow.builder()
+                .setByteBufferPool(smallPool)
+                .addHttpListener(DefaultServer.getHostPort("default") + 1, DefaultServer.getHostAddress("default"), new Http2UpgradeHandler(path))
+                .setSocketOption(Options.REUSE_ADDRESSES, true)
+                .build();
+        server.start();
+
+        // Create xnio worker
+        final Xnio xnio = Xnio.getInstance();
+        final XnioWorker xnioWorker = xnio.createWorker(null, OptionMap.builder()
+                .set(Options.WORKER_IO_THREADS, 8)
+                .set(Options.TCP_NODELAY, true)
+                .set(Options.KEEP_ALIVE, true)
+                .getMap());
+        worker = xnioWorker;
+    }
+
+    /**
+     * Stops server and worker.
+     */
+    @AfterClass
+    public static void afterClass() {
+        if (server != null) {
+            server.stop();
+        }
+        if (worker != null) {
+            worker.shutdown();
+        }
+        if (smallPool != null) {
+            smallPool.close();
+            smallPool = null;
+        }
+    }
+
+    /**
+     * Method that sends a GET or POST request adding count number of custom
+     * headers and sending contentLength data. GET is used if no content length
+     * is passed, POST if contentLength is greater than 0.
+     * @param connection The connection to use
+     * @param requestCount The number of requests to send
+     * @param headersCount The number of custom headers to send
+     * @param contentLength The content length to send (POST method used if >0)
+     * @throws Exception Some error
+     */
+    private void sendRequest(ClientConnection connection, int requestCount, int headersCount, int contentLength) throws Exception {
+        final CountDownLatch latch = new CountDownLatch(requestCount);
+        final List<ClientResponse> responses = new CopyOnWriteArrayList<>();
+        StringBuilder sb = new StringBuilder();
+        for (int i = 0; i < contentLength; i++) {
+            sb.append(i % 10);
+        }
+        final String content = sb.length() > 0? sb.toString() : null;
+        connection.getIoThread().execute(new Runnable() {
+            @Override
+            public void run() {
+                for (int i = 0; i < requestCount; i++) {
+                    final ClientRequest request = new ClientRequest()
+                            .setMethod(contentLength > 0 ? Methods.POST : Methods.GET)
+                            .setPath(ECHO_PATH);
+                    request.getRequestHeaders().put(Headers.HOST, DefaultServer.getHostAddress());
+                    if (contentLength > 0) {
+                        request.getRequestHeaders().put(Headers.CONTENT_LENGTH, contentLength);
+                    }
+                    for (int j = 0; j < headersCount; j++) {
+                        request.getRequestHeaders().put(new HttpString(HEADER_PREFFIX + j), HEADER_PREFFIX + j);
+                    }
+                    connection.sendRequest(request, createClientCallback(responses, latch, content));
+                }
+            }
+        });
+
+        latch.await(10, TimeUnit.SECONDS);
+
+        Assert.assertEquals("No responses received from server in 10s", requestCount, responses.size());
+        for (int i = 0; i < requestCount; i++) {
+            Assert.assertEquals("Response " + i + " code was not OK", StatusCodes.OK, responses.get(i).getResponseCode());
+            Assert.assertEquals("Incorrect data received for response " + i, contentLength > 0 ? content : "", responses.get(i).getAttachment(RESPONSE_BODY));
+            int headersReturned = 0;
+            for (HeaderValues header : responses.get(i).getResponseHeaders()) {
+                if (header.getFirst().startsWith(HEADER_PREFFIX)) {
+                    headersReturned += header.size();
+                }
+            }
+            Assert.assertEquals("Incorrect number of headers returned for response " + i, headersCount, headersReturned);
+        }
+    }
+
+    /**
+     * The real test that sends several GET and POST requests with different
+     * number of headers and different content length.
+     * @throws Exception  Some error
+     */
+    @Test
+    public void testDifferentSizes() throws Exception {
+        final UndertowClient client = UndertowClient.getInstance();
+
+        // the client connection uses the small byte-buffer of 1024 to force the continuation frames
+        final ClientConnection connection = client.connect(
+                new URI("http://" + DefaultServer.getHostAddress() + ":" + (DefaultServer.getHostPort("default") + 1)),
+                worker, new UndertowXnioSsl(worker.getXnio(), OptionMap.EMPTY, DefaultServer.getClientSSLContext()),
+                smallPool, OptionMap.create(UndertowOptions.ENABLE_HTTP2, true)).get();
+        try {
+            // the first request triggers the upgrade to H2C
+            sendRequest(connection, 1, 0, 0);
+            // send several requests with different sizes for headers and data
+            sendRequest(connection, 10, 10, 0);
+            sendRequest(connection, 10, 100, 0);
+            sendRequest(connection, 10, 150, 0);
+            sendRequest(connection, 10, 1, 10);
+            sendRequest(connection, 10, 0, 2000);
+            sendRequest(connection, 10, 150, 2000);
+        } finally {
+            IoUtils.safeClose(connection);
+        }
+    }
+
+    /**
+     * Create the callback to receive the response and assign it to the list.
+     * @param responses The list where the response will be added
+     * @param latch The latch to count down when the response is received
+     * @param message The message to send if it's a POST message (if null nothing is send)
+     * @return The created callback
+     */
+    private static ClientCallback<ClientExchange> createClientCallback(final List<ClientResponse> responses, final CountDownLatch latch, String message) {
+        return new ClientCallback<ClientExchange>() {
+            @Override
+            public void completed(ClientExchange result) {
+                if (message != null) {
+                    new StringWriteChannelListener(message).setup(result.getRequestChannel());
+                }
+                result.setResponseListener(new ClientCallback<ClientExchange>() {
+                    @Override
+                    public void completed(final ClientExchange result) {
+                        responses.add(result.getResponse());
+                        new StringReadChannelListener(result.getConnection().getBufferPool()) {
+
+                            @Override
+                            protected void stringDone(String string) {
+                                result.getResponse().putAttachment(RESPONSE_BODY, string);
+                                latch.countDown();
+                            }
+
+                            @Override
+                            protected void error(IOException e) {
+                                e.printStackTrace();
+                                latch.countDown();
+                            }
+                        }.setup(result.getResponseChannel());
+                    }
+
+                    @Override
+                    public void failed(IOException e) {
+                        e.printStackTrace();
+                        latch.countDown();
+                    }
+                });
+                try {
+                    result.getRequestChannel().shutdownWrites();
+                    if (!result.getRequestChannel().flush()) {
+                        result.getRequestChannel().getWriteSetter().set(ChannelListeners.<StreamSinkChannel>flushingChannelListener(null, null));
+                        result.getRequestChannel().resumeWrites();
+                    }
+                } catch (IOException e) {
+                    e.printStackTrace();
+                    latch.countDown();
+                }
+            }
+
+            @Override
+            public void failed(IOException e) {
+                e.printStackTrace();
+                latch.countDown();
+            }
+        };
+    }
+}

Vulnerability mechanics

Generated on May 9, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.

References

News mentions

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cvss	0.455
epss	0.000
exploit	0.000
kev	0.000
patch	-0.070
ransomware	0.000