How severe is CVE-2026-75596?

No CVSS score has been assigned to CVE-2026-75596 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-75596?

CVE-2026-75596 affects the following packages: io.netty:netty-handler (Maven), io.netty:netty-handler (Maven). Ecosystems affected: Maven.

How do I fix CVE-2026-75596?

Update io.netty:netty-handler to 4.2.17.Final or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-75596 is resolved across your whole dependency graph.

How do I detect CVE-2026-75596 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.netty:netty-handler, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-75596 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2026-75596 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-75596 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-75596?

CVE-2026-75596 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 29th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-75596?

CVE-2026-75596 is classified as CWE-407 (CWE-407). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-75596 published, and has it been updated?

CVE-2026-75596 was published on August 19, 2026 and was last updated on September 30, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕ Maven

Not in CISA KEV

CVE-2026-75596 — netty-handler

Fix: netty/netty@1b5abc6

CVE-2026-75596 is a CWE-407 vulnerability in io.netty:netty-handler. A fix is available for io.netty:netty-handler — see the affected versions and patch details below.

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

Also known asGHSA-fccg-mwvh-qqg4

Published

Aug 19, 2026

Updated

Sep 30, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Oct 3, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-75596.

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs29th percentile — riskier than 29% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

2 pkgs affected

☕io.netty:netty-handler☕io.netty:netty-handler

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

Summary

Netty's default SNI entrypoint reparses and recopies previously received ClientHello fragments on every additional TLS handshake record. A remote peer can send a small first record that advertises a large ClientHello length and then drip the body in many tiny records, causing superlinear (quadratic) CPU work before the handshake completes. With 4095 one-byte fragments, the handler recopies 8,386,560 bytes from only 24,579 bytes on the wire — a 341× amplification ratio.

Affected Entrypoints

io.netty.handler.ssl.SniHandler — default constructors
io.netty.handler.ssl.SslClientHelloHandler — pre-handshake ClientHello aggregation path

Vulnerable Code Locations

handler/src/main/java/io/netty/handler/ssl/SniHandler.java:85
handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:75 (decode entry)
handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:165 (handshakeBuffer.clear)
handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:174 (writeBytes re-copy)
codec-base/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java:294 (cumulation retention)

Exploit Path

1. TCP connection → SniHandler → SslClientHelloHandler.decode
2. First record: TLS handshake header declaring large ClientHello length (e.g., 4096 bytes)
3. Attacker sends thousands of tiny follow-on handshake records (1 byte each)
4. On each fragment: handshakeBuffer.clear() + writeBytes() re-copies ALL accumulated body bytes
5. Total bytes copied = n*(n+1)/2 where n = number of body bytes → quadratic
6. Event-loop CPU exhausted before SslHandler takes over

Impact

Vulnerability Type: Inefficient Algorithmic Complexity
An unauthenticated network attacker can drive disproportionate CPU consumption on the Netty event loop
Affects all Netty deployments using SniHandler for TLS termination (the default SNI path)
No privileges, user interaction, or special configuration required
Can degrade or stall TLS connection handling for all clients on the affected event loop
The attack requires only modest bandwidth (~25 KB) to trigger significant CPU work

Credits

Found by a security research team from the University of Sydney, focusing on detecting open source software vulnerabilities. Liyi Zhou: https://lzhou1110.github.io/ Ziyue Wang: https://zyy0530.github.io/ Strick: https://str1ckl4nd.github.io/ Maurice: https://maurice.busystar.org/ Chenchen Yu: https://7thparkk.github.io/

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`io.netty:netty-handler`	≥ 4.2.0.Final&&< 4.2.17.Final	4.2.17.Final`io.netty:netty-handler:4.2.17.Final`
☕Maven	`io.netty:netty-handler`	all versions	4.1.137.Final`io.netty:netty-handler:4.1.137.Final`

Affected Products

1 product · 2 configurations

Application

nettynetty

≥ 4.2.0 && < 4.2.17

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.netty:netty-handler, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update io.netty:netty-handler to 4.2.17.Final or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-75596 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### Summary Netty's default SNI entrypoint reparses and recopies previously received ClientHello fragments on every additional TLS handshake record. A remote peer can send a small first record that advertises a large ClientHello length and then drip the body in many tiny records, causing **superlinear (quadratic) CPU work** before the handshake completes. With 4095 one-byte fragments, the handler recopies **8,386,560 bytes** from only **24,579 bytes** on the wire — a **341× amplification** ratio. ### Affected Entrypoints - `io.netty.handler.ssl.SniHandler` — default constructors - `io.netty

O3 Security · Impact-Aware SCA

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