Netty 4.2.0 through 4.2.16 Response Desynchronization via HttpServerCodec (CVE-2026-100666) | HOL Guard CVE

Medium · CVSS 6.9CVE-2026-100666

Netty 4.2.0 through 4.2.16 Response Desynchronization via HttpServerCodec

Answer in brief

CVE-2026-100666 records a Medium severity (CVSS 6.9) vulnerability in Netty 4.2.0 through 4.2.16 Response Desynchronization via HttpServerCodec. The current sources do not mark it as known exploited. The current feed maps netty/netty (generic), netty/netty (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 26, 2026Updated Sep 27, 2026Source checked Sep 27, 2026First seen by HOL Sep 26, 2026Material review Sep 27, 2026

Upstream Advisory

Key facts

Risk: Medium · CVSS 6.9
Exploitation: Not marked as known exploited
Affected software: 2 mapped packages or products
Fix availability: Available

Why this deserves its current priority

CVSS is 6.9. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps netty/netty (generic), netty/netty (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
netty/nettygeneric	>=4.2.0.Final <4.2.17.Final	4.2.17.Final
netty/nettygeneric	>=0 <4.1.137.Final	4.1.137.Final

Recommended response

Check inventory. Check lockfiles and deployed manifests for netty/netty, netty/netty.
Review the reported fix. Update netty/netty to 4.2.17.Final; netty/netty to 4.1.137.Final if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Sep 26, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Sep 27, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Sep 26, 2026

Sources and claim methodology

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.