Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext (CVE-2026-75595)
Published Last modified Sources: SUSE, Red Hat, BSI, Hersteller, CVE.org, OSV
What is affected, what to do
A flaw was found in Netty, an asynchronous, event-driven network application framework. An unauthenticated remote attacker can exploit fragmented Transport Layer Security (TLS) ClientHello messages to bypass the Server Name Indication (SNI) routing mechanism.
Update Netty to version 4.1.137.Final.
- Affected
- Netty
gte 4.2.0.Final, < 4.2.17.Final..* - Netty
lt *..4.1.137.Final
- Netty
- Fixed in
- Netty
4.1.137.Final
- Netty
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Metrics
9.1
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
9.1
CVSS v3
Severity Critical on the scale from 0 to 10.
Source: SUSE, Red Hat, BSI, Hersteller, CVE.org, OSV
0.3 %
EPSS score
Low — model estimates < 1% exploitation likelihood.
Source: FIRST EPSS, percentile 19.0
Show all metrics
- Severity
- Critical
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 73.01 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- no public PoC known
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 9.1 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 9.1
CVSS v4
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 9.1
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 0.3 %
- Low — model estimates < 1% exploitation likelihood.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 19.0
- Elevated — this CVE ranks above at least 10% of all CVEs scored today.
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- —
- Published
CWE
CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.
Read in glossary- CWE-754
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
A flaw was found in Netty, an asynchronous, event-driven network application framework. An unauthenticated remote attacker can exploit fragmented Transport Layer Security (TLS) ClientHello messages to bypass the Server Name Indication (SNI) routing mechanism. This forces a fallback to a less secure default SslContext, allowing the attacker to bypass mutual TLS authentication requirements and gain unauthorized access to protected resources.
Source: suse_csaf, redhat_csaf, BSI WID Portal, BSI CSAF, GitHub Security Advisories, cvelistv5, NVD (NIST), OSV (Open Source Vulnerabilities)
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| netty | netty |
|
|
| Oracle | Communications |
|
|
Weakness classes (CWE)
CWE-754Improper Check for Unusual or Exceptional Conditions
Class
The product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product.
Entry at MITRE CWE-754
References & sources
- github.comnetty/netty/security/advisories/GHSA-c4c3-7fpv-j4q5x_refsource_CONFIRM
- github.comnetty/netty/pull/17213x_refsource_MISC
- github.comnetty/netty/pull/17217x_refsource_MISC
- github.comnetty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7x_refsource_MISC
- github.comnetty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961x_refsource_MISC
- github.comnetty/netty/releases/tag/netty-4.1.137.Finalx_refsource_MISC
- github.comnetty/netty/releases/tag/netty-4.2.17.Finalx_refsource_MISC
- NVD (NIST)vuln/detail/CVE-2026-75595advisory
- github.comnetty/nettypackage
Reanalysis & status changes
Chronological NVD audit events for this CVE — reanalyses, CVSS updates, CPE diffs.
Initial Analysisnvd@nist.gov
- CVSS V3.1: after AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
- CPE Configuration: after OR *cpe:2.3:a:netty:netty:*:*:*:*:*:*:*:* versions up to (excluding) 4.1.137 *cpe:2.3:a:netty:netty:*:*:*:*:*:*:*:* versions from (including) 4.2.0 up to (excluding) 4.2.17
- Reference Type: after GitHub, Inc.: https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7 Types: Patch
- Reference Type: after GitHub, Inc.: https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961 Types: Patch
CVE Modified134c704f-9b21-4f2e-91b3-4a467353bcc0
- SSVC: after {"id":"CVE-2026-75595","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalI…
New CVE Receivedsecurity-advisories@github.com
- Affected: after netty
- Description: after Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Fina and 4.2.17.Final, io.netty.handler.ssl.SslClientHelloHandler#decode checks the wrong offset before reading the four-byte TLS handshake header, so a ClientHello whose handshake header spans records can cause an IndexOutOfBoundsException and invoke select(ctx, null). This selects the default SslContext instead of the SNI-specific context. In deployments where per-SNI clientAuth=REQUIRE is the sole mutual TLS gate, the default SslContext uses clientAuth=NONE or clientAuth=OPTIONAL, and no application-layer certificate verification exists, an unauthenticated remote attacker can bypass the protected route's mutual TLS requirement. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.
- CVSS V4.0: after AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
- CWE: after CWE-754
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