CVE-2026-100662: Netty HTTP/3 QPACK encoder-stream unbounded memory exhaustion DoS
Netty's HTTP/3 codec (io.netty:netty-codec-http3) versions 4.2.0.Final through 4.2.17.Final contain an uncontrolled resource consumption vulnerability in the QPACK encoder-stream instruction decoder (QpackEncoderHandler, installed on the peer-initiated unidirectional QPACK encoder stream, type 0x02). The handler accepts an attacker-declared string-literal length of up to Integer.MAXVALUE (~2 GiB) for the Name Length and Value Length fields of the "Insert With Literal Name" instruction (RFC 9204 §4.3.3), with no per-instruction or per-literal length cap and no cumulation-size limit; the existing HTTP/3 limits (maxHeaderListSize, maxUnknownFramePayloadLength, DEFAULTMAXFIELDSECTIONSIZE) are not applied to this handler. A remote, unauthenticated peer with an established HTTP/3 connection to a default Netty HTTP/3 server can declare a very large literal length and then trickle fewer bytes than declared, causing the ByteToMessageDecoder MERGE cumulator to retain and grow the per-connection buffer, and ultimately triggering a large byte-array allocation. This leads to unbounded per-connection heap growth and OutOfMemoryError, resulting in denial of service. Fixed in 4.2.18.Final.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
io.netty:netty-codec-http3to a version that resolves this vulnerability.Fixed in 4.2.18.Final
Event History
Frequently Asked Questions
Which deployments are exposed?
Deployments using io.netty:netty-codec-http3 versions 4.2.0.Final through 4.2.17.Final are affected. A default Netty HTTP/3 server is exposed when it accepts an established HTTP/3 connection from a remote peer.
What does an attacker need to exploit this issue?
The attacker needs only an established HTTP/3 connection and does not need authentication or user interaction. They can declare a very large literal length on the peer-initiated QPACK encoder stream and send fewer bytes than declared over time.
What is the practical impact on an affected server?
The per-connection decoder buffer can continue growing until a large byte-array allocation is attempted. This can exhaust heap memory, trigger an OutOfMemoryError, and cause denial of service.
What version resolves the issue?
The issue is fixed in io.netty:netty-codec-http3 4.2.18.Final. Versions from 4.2.0.Final through 4.2.17.Final should be treated as affected.