CVE-2026-100660 in Netty
Summary
by MITRE • 09/26/2026
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.
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Analysis
by VulDB Data Team • 09/26/2026
The vulnerability identified within Netty's HTTP/3 codec module affects versions ranging from 4.2.0.Final through 4.2.17.Final and represents a critical resource management flaw rooted in the implementation of QPACK encoding state retention. The core technical deficiency lies in how the QpackEncoder manages dynamic table references for encoded field sections. Specifically, the encoder maintains a queue and a dynamic-table index tracker for every section that interacts with the QPACK dynamic table, using the peer-controlled QUIC stream ID as the key. This design choice creates a direct dependency on remote signaling to clean up internal data structures rather than relying on local state transitions such as stream completion. Consequently, entries in this tracking structure are only released when the remote decoder explicitly sends a Section Acknowledgment or Stream Cancellation instruction. Because these acknowledgments are controlled entirely by the client, the server retains full responsibility for managing memory based on external inputs that may be maliciously withheld.
This architectural oversight leads to unbounded heap growth under specific attack conditions. An attacker operating as an unauthenticated remote HTTP/3 client can exploit this behavior by first advertising a non-zero QPACK dynamic-table capacity during connection establishment. Following this, the client acknowledges table insertions, prompting the server to reuse dynamically indexed response headers and thereby retaining references in its internal state. The critical exploitation step involves the client omitting all mandatory Section Acknowledgments while continuing to issue sequential requests over a single QUIC connection. By doing so, the attacker forces the server to accumulate tracking entries for each new request without ever triggering the cleanup mechanism tied to acknowledgments. This behavior effectively bypasses concurrent-stream limits because the server continues to allocate resources for stream-specific state even as the number of active streams increases beyond intended thresholds.
The operational impact of this vulnerability is a severe denial of service resulting from memory exhaustion. As the attacker persists in sending requests without providing necessary acknowledgment signals, the server's heap usage grows linearly with each new request. Since there are no limits imposed on the number of tracked streams, field sections, or retained bytes within the QPACK encoder state, the process will eventually consume all available system memory. This leads to application crashes or unresponsiveness for legitimate users sharing the same infrastructure. The vulnerability is classified under CWE-400 Uncontrolled Resource Consumption and aligns with ATT&CK technique T1498 Network Denial of Service, specifically reflecting behaviors associated with resource exhaustion through protocol-level manipulation rather than simple traffic volume flooding.
Mitigation strategies primarily involve upgrading to Netty version 4.2.18.Final or later, where this issue has been resolved by implementing proper bounds checking and ensuring that state is correctly released upon stream completion regardless of remote acknowledgment status. In environments where immediate patching is not feasible, administrators should consider deploying web application firewalls or reverse proxies capable of terminating HTTP/3 connections and validating QPACK header block integrity before forwarding requests to the backend Netty service. Additionally, configuring strict limits on concurrent streams at the QUIC layer can help mitigate the scale of resource consumption, although this does not fully address the underlying memory leak caused by unacknowledged dynamic table entries. Regular monitoring of heap usage trends for services utilizing affected Netty versions is recommended to detect potential exploitation attempts early.