Netty HTTP/3 QPACK encoder-stream unbounded memory exhaustion DoS

Summary

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.

Timeline

1. Sep 26, 2026, 1:23 PM UTCCVE published
2. Sep 30, 2026, 2:53 PM UTCCISA SSVC assessment

   Exploitation: poc · Automatable: yes · Technical impact: partial

3. Sep 30, 2026, 2:53 PM UTCRecord last updated

Affected versions

• netty netty

  From 4.2.0.Final · fixed in 4.2.18.Final

References 2