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CVE-2026-75596

Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing

HIGH
7.5
CVSS Score
Published: August 19, 2026 at 08:56 PM UTC
Updated: October 26, 2026 at 10:10 AM UTC
1 product affected across 1 vendorNetty

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Severity
HIGH7.5
Exploit prediction
EPSS 0.4%
SSVC
No known exploit

Description

Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Final and 4.2.17.Final, the default io.netty.handler.ssl.SniHandler constructors use the pre-handshake ClientHello aggregation path in handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java at io.netty.handler.ssl.SslClientHelloHandler#decode, where handshakeBuffer.clear() and writeBytes() recopy all previously received body bytes for every additional TLS record. An unauthenticated remote peer can advertise a large ClientHello and deliver its body in thousands of tiny records, causing quadratic CPU work on the event loop before the TLS handshake completes and degrading TLS handling for other clients. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Netty· Nettymaven
95%
v < 4.1.1374.2.0 ≤ v < 4.2.17
Affected range stops below 4.1.137, so it looks safe — our inference, not a vendor statement
Affected range stops below 4.1.137.Final, so it looks safe — our inference, not a vendor statement
Affected range stops below 4.2.17, so it looks safe — our inference, not a vendor statement
Affected range stops below 4.2.17.Final, so it looks safe — our inference, not a vendor statement

References (7)

x_refsource_CONFIRMVendor Advisory
Source: cverecord
https://github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4
x_refsource_MISCIssue TrackingPatch
Source: cverecord
https://github.com/netty/netty/pull/17213
x_refsource_MISCIssue TrackingPatch
Source: cverecord
https://github.com/netty/netty/pull/17217
x_refsource_MISCPatch
Source: cverecord
https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7
x_refsource_MISCPatch
Source: cverecord
https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961
x_refsource_MISCRelease Notes
Source: cverecord
https://github.com/netty/netty/releases/tag/netty-4.1.137.Final
x_refsource_MISCRelease Notes
Source: cverecord
https://github.com/netty/netty/releases/tag/netty-4.2.17.Final

SSVC — Action Prioritization

SSVCNo known exploitAutomatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

7.5HIGH[email protected]
Attack Vector
NETWORK
Complexity
LOW
Privileges
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
NONE
Integrity
NONE
Availability
HIGH
Vector?

Quick Info

StatusANALYZED
CWE
CWE-407

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