CVE-2026-100662 in Netty
Summary
by MITRE • 09/26/2026
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.MAX_VALUE (~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, DEFAULT_MAX_FIELD_SECTION_SIZE) 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.
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Analysis
by VulDB Data Team • 09/26/2026
The vulnerability identified in Netty's HTTP/3 codec module affects versions ranging from 4.2.0.Final through 4.2.17.Final and represents a critical uncontrolled resource consumption flaw within the QPACK encoder-stream instruction decoder, specifically located in the QpackEncoderHandler class. This handler is responsible for processing instructions on peer-initiated unidirectional streams designated as type 0x02, which are essential for maintaining header compression state during HTTP/3 communication sessions. The core technical deficiency lies in the handling of the Insert With Literal Name instruction defined by RFC 9204 section 4.3.3. When this handler processes incoming data, it accepts a string-literal length value declared by the remote peer without imposing any per-instruction or per-literal size constraints. Consequently, an attacker can specify a literal length field with a maximum integer value of approximately two gigabytes, effectively instructing the decoder to allocate memory for a massive buffer that far exceeds reasonable operational limits.
The severity of this flaw is exacerbated by the fact that existing HTTP/3 configuration parameters designed to limit resource usage are not applied to this specific handler. Standard safeguards such as maxHeaderListSize, which restricts the total size of header blocks, and DEFAULT_MAX_FIELD_SECTION_SIZE, which caps field section sizes, along with maxUnknownFramePayloadLength for frame payload restrictions, fail to mitigate the impact of this vulnerability because they do not govern the internal buffer management logic within QpackEncoderHandler. As a result, when an unauthenticated remote peer establishes a valid HTTP/3 connection and sends these oversized instructions, the Netty ByteToMessageDecoder MERGE cumulator retains the incoming data in memory while waiting for the full declared amount of bytes to arrive. If the attacker chooses to trickle fewer bytes than declared or simply stalls the connection after declaring the large length, the cumulative buffer continues to grow without bound, leading directly to unbounded heap expansion on the server side.
The operational impact of this vulnerability is a severe denial of service condition resulting from OutOfMemoryError exceptions within the Java Virtual Machine hosting the Netty application. By repeatedly initiating connections and sending these malformed or oversized QPACK instructions, an attacker can exhaust the available memory resources of the target system. This does not require authentication or complex exploitation techniques beyond establishing a standard HTTP/3 connection, making it highly accessible to malicious actors aiming to disrupt service availability. The vulnerability aligns with CWE-400 Uncontrolled Resource Consumption and is categorized under ATT&CK technique T1498 Network Denial of Service, specifically reflecting the behavior associated with resource exhaustion via protocol-level abuse rather than volumetric traffic flooding.
To mitigate this risk, organizations running affected versions must upgrade to Netty version 4.2.18.Final or later, where the QpackEncoderHandler has been patched to enforce strict length limits on literal fields and prevent unbounded buffer accumulation. In environments where immediate patching is not feasible, network-level controls such as rate limiting HTTP/3 connections or restricting header size expectations at a reverse proxy layer may provide partial mitigation by dropping malformed frames before they reach the application server logic. However, relying solely on perimeter defenses is insufficient given that authenticated internal traffic could also trigger this flaw if it originates from compromised endpoints. Regular security audits of dependency versions and monitoring for abnormal memory growth patterns in Netty-based services are recommended to detect potential exploitation attempts early.