CVE-2026-100660: Netty before 4.2.18.Final QpackEncoder Unbounded Memory Retention
Netty's HTTP/3 codec (io.netty:netty-codec-http3) from 4.2.0.Final through 4.2.17.Final retains unbounded per-stream QPACK encoder state. QpackEncoder stores a queue and a dynamic-table index tracker for every encoded field section that references the QPACK dynamic table, keyed by the peer-controlled QUIC stream ID, and these entries are released only when the remote decoder sends a Section Acknowledgment or Stream Cancellation instruction — not when the HTTP/3 stream completes. There is no limit on the number of tracked streams, field sections, or retained bytes. A remote, unauthenticated HTTP/3 client can advertise a non-zero QPACK dynamic-table capacity, acknowledge the table insertion so the server reuses a dynamically indexed response header, and then omit all mandatory Section Acknowledgments while issuing sequential requests over a single QUIC connection, bypassing concurrent-stream limits and causing unbounded heap growth until the server exhausts memory (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
Who is exposed to this issue?
Servers using io.netty:netty-codec-http3 versions 4.2.0.Final through 4.2.17.Final are exposed when they accept HTTP/3 connections. A remote unauthenticated client can trigger the condition over a single QUIC connection.
What does an attacker need to do to cause the denial of service?
The client must advertise a non-zero QPACK dynamic-table capacity, acknowledge a table insertion so dynamically indexed response headers are reused, then omit Section Acknowledgments while sending sequential requests. This avoids reliance on concurrent-stream limits and causes retained encoder state to accumulate.
Is completing an HTTP/3 stream enough to release the retained memory?
No. The tracked per-stream state is released only after the remote decoder sends a QPACK Section Acknowledgment or Stream Cancellation instruction, not when the HTTP/3 stream completes.
What is the remediation?
Upgrade io.netty:netty-codec-http3 to 4.2.18.Final, which fixes the issue.