CVE-2026-100666: Netty 4.2.0 through 4.2.16 Response Desynchronization via HttpServerCodec

Severity: high (CVSS 7.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-26 · updated 2026-09-30
Weakness: CWE-444

Description

Netty's HttpServerCodec (io.netty:netty-codec-http) in versions 4.2.0.Final through 4.2.16.Final and in versions up to and including 4.1.136.Final pairs each outbound response with an inbound request by calling pollMethod() once per response, including for 1xx informational responses. If a client pipelines an HTTP/1.1 GET carrying an Expect: 100-continue header followed by a HEAD request, the 100 Continue response consumes the queued GET method, so the subsequent 200 OK for the GET is paired with HEAD and its body is dropped, while the following 200 OK for the HEAD request is written with a body. This desynchronizes HTTP parsing on the connection: the GET entity is never delivered and the HEAD response body is interpreted as the GET body, resulting in response splitting and unsafe connection reuse. Fixed in 4.2.17.Final and 4.1.137.Final.

Affected products

netty: from 4.2.0.Final before 4.2.17.Final; before 4.1.137.Final — fixed in 4.1.137.Final, 4.2.17.Final

How to fix

Upgrade to a fixed version: netty 4.1.137.Final, 4.2.17.Final. Check the vendor advisory for mitigations.

References

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