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CVE Record

CVE-2026-75595: Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext

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.

CriticalCVSS 9.1Not KEV-listedUpdated
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Glexia's TakeAutomated analysiscritical

Security readout for executives and security teams

Plain-English summary

A flaw in Netty’s TLS hostname routing can send specially fragmented connections to a default security configuration. An attacker may bypass mutual TLS when that default permits clients without certificates and the application performs no separate certificate check. Impact can include unauthorized access to protected data or functions.

Executive priority

Prioritize immediate assessment of internet-facing Netty services that rely on SNI-based mutual TLS. Patch confirmed condition-matching systems urgently because successful bypass could expose protected operations with high confidentiality and integrity impact. Maintain accelerated, risk-based patching for affected systems that do not meet the exploit prerequisites.

Technical view

Affected Netty versions may read a fragmented TLS handshake header at the wrong offset, causing an IndexOutOfBoundsException and select(ctx, null). That path selects the default SslContext rather than the SNI-specific context. The bypass requires a weaker default clientAuth setting and no application-layer certificate verification.

Likely exposure

Exposure is limited to applications using affected Netty versions, SNI-specific SslContext routing, and mutual TLS as the sole route-level control. Systems are most vulnerable when protected hosts require clientAuth but the default context uses NONE or OPTIONAL. Other Netty deployments may not be exploitable.

Exploitation context

The flaw is remotely reachable without privileges or user interaction, but exploitation depends on a particular TLS configuration. The supplied sources do not report active exploitation, and the CVE is not listed as KEV in the provided bundle. Treat internet-facing, condition-matching deployments as urgent.

Researcher notes

CWE-754 reflects improper handling of an exceptional condition. The security boundary failure is not merely the exception: select(ctx, null) converts parsing failure into fallback context selection. Review both dependency reachability and runtime TLS configuration. The supplied evidence establishes fixes but does not establish public exploitation or universal impact across affected versions.

Mitigation direction

  • Upgrade Netty 4.1 deployments to 4.1.137.Final or later.
  • Upgrade Netty 4.2 deployments to 4.2.17.Final or later.
  • Ensure the default SslContext requires client certificates where feasible.
  • Add application-layer certificate authorization for sensitive routes as defense in depth.
  • Review vendor guidance before applying temporary controls when immediate upgrades are impossible.

Validation and detection

  • Inventory direct and transitive Netty versions in deployed applications.
  • Identify services using SNI-specific TLS contexts or hostname-based certificate routing.
  • Compare clientAuth settings between protected SNI contexts and the default SslContext.
  • Confirm sensitive routes independently verify authenticated client identities.
  • After upgrading, verify fragmented TLS handshakes fail closed without selecting a weaker context.
Prepared
Confidence
high
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-754: Exact CWE lookup

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cve · low confidence lookup

CVE-2026-75595 mapping review

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Vulnerability profileCVE Program record
Severity
Critical
CVSS
9.1 (4.0)
Known Exploited
No
Published

Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
3Timeline events
1ADP providers
8Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: total

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
9.1CVSS 4.0CriticalCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N——GitHub_M

Vulnerability scoring details

Base CVSS 4.0 score

9.1Critical
CVSS 4.0 vector shape for CVE-2026-75595Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
nettynetty< 4.1.137.Final, >= 4.2.0.Final, < 4.2.17.FinalListed
Weakness

CWE details

CWE-754

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-754 · source CWE mapping

Improper Check for Unusual or Exceptional Conditions

Improper Check for Unusual or Exceptional Conditions represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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