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Full vulnerability report · 2026
CVE-2026-75596High confidence

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

netty · netty

Official source article: GitHub GHSA-FCCG-MWVH-QQG4 ↗. Check the applicable product and release in the original source.

8.7HighCVSS 4.0
Recommended action
Patch only the product branches with a verified fix

High technical severity; prioritise exposed affected systems while verifying vendor guidance. Verified remediation exists for at least one product or source, but 4 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Fix availability varies by product
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityHighOperational priority:High, unchanged from published severity.unchanged

Evidence used

  • No CISA KEV confirmation is currently recorded.
  • The selected CVSS metric records a network-reachable, unauthenticated path with no user interaction.
  • EPSS is 0.37% for the current model date.

Compensating controls

  • Validate the affected product branch and deploy the verified fixed release.
  • Restrict the affected network interface to trusted sources where business-safe.
  • Monitor vendor guidance and exploitation sources for a material change.

Verification

  1. Confirm that the asset runs netty netty and falls inside the recorded affected range.
  2. Verify the installed build against the product-specific fixed version after deployment.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: Within 30 daysRemediation target: Within 180 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Cross-source reconciliation

Remediation availability differs by product scope

Verified remediation exists for at least one product or source, but 4 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Distribution package intelligence

Release-specific package status

Debian, ubuntu findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.

5 package states
Package result overrides the generic status

BlackTree has verified remediation for at least one product or source, but the relevant distribution still reports no fixed package for 4 affected package states shown here. Treat those rows as affected with no fix until that distribution publishes a fixed version.

Repository candidate not checked

A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.

Distribution releaseSource packageVendor stateFixed versionEvidence
Debian trixietrixie · sourcenettyAffected, no fix publishedDebian currently tracks this release as open.Not published in this feedDebian Security Tracker ↗Source updated 8 Oct 2026
Debian bookwormbookworm · sourcenettyAffected, no fix publishedDebian currently tracks this release as open.Not published in this feedDebian Security Tracker ↗Source updated 8 Oct 2026
Debian forkyforky · sourcenettyAffected, no fix publishedDebian currently tracks this release as open.Not published in this feedDebian Security Tracker ↗Source updated 8 Oct 2026
Debian sidsid · sourcenettyAffected, no fix publishedDebian currently tracks this release as open.Not published in this feedDebian Security Tracker ↗Source updated 8 Oct 2026
Ubuntu 24.04 LTSnoble · esm-appsnettyUnder evaluationCanonical reports that the package might be affected and still needs evaluation or fixing.Not published in this feedCanonical Ubuntu Security ↗Source updated 7 Oct 2026
Open-source package ranges4 source-attributed ranges

These OSV and GitHub advisory ranges apply only to the named package and ecosystem. A listed fixed version is not a universal product patch or proof that an update is installed.

Ecosystem and packageAffected rangeFirst fixed versionEvidence
Mavenio.netty:netty-handlerECOSYSTEM: introduced 4.2.0.Final; fixed 4.2.17.Final4.2.17.FinalOSV record ↗aggregator derived · 8 Sep 2026
Mavenio.netty:netty-handlerECOSYSTEM: introduced 0; fixed 4.1.137.Final4.1.137.FinalOSV record ↗aggregator derived · 8 Sep 2026
mavenio.netty:netty-handler>= 4.2.0.Final, <= 4.2.16.Final4.2.17.FinalGitHub advisory ↗upstream repository advisory · 8 Sep 2026
mavenio.netty:netty-handler<= 4.1.136.Final4.1.137.FinalGitHub advisory ↗upstream repository advisory · 8 Sep 2026
Direct vendor intelligence

Authoritative vendor CSAF and VEX advisories

Structured product status and remediation from the issuing vendor. Product-state explanations are always visible; large lists can be searched or downloaded.

1 current
CVE-2026-75596 · CSAF 2.0 · revision 3 · interimSUSE Product Security TeamCVE-2026-75596
26 known affected

The vendor explicitly identifies these products as affected by this CVE.

  • netty3 as component of SUSE Linux Enterprise Desktop 15 SP7
  • netty3 as component of SUSE Linux Enterprise High Performance Computing 15 SP2
  • netty3 as component of SUSE Linux Enterprise High Performance Computing 15 SP3
  • netty3 as component of SUSE Linux Enterprise High Performance Computing 15 SP4-LTSS
  • netty3 as component of SUSE Linux Enterprise High Performance Computing 15 SP4
  • netty3 as component of SUSE Linux Enterprise High Performance Computing 15 SP5-LTSS
  • netty3 as component of SUSE Linux Enterprise High Performance Computing 15 SP5
  • netty3 as component of SUSE Linux Enterprise Module for Development Tools 15 SP7
  • netty3 as component of SUSE Linux Enterprise Server 15 SP2-LTSS
  • netty3 as component of SUSE Linux Enterprise Server 15 SP2
  • netty3 as component of SUSE Linux Enterprise Server 15 SP3-LTSS
  • netty3 as component of SUSE Linux Enterprise Server 15 SP3
Summary
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.
Remediation
To install this SUSE Security Update use the SUSE recommended installation methods like YaST online_update or "zypper patch".
Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2026-63092

No EUVD known-exploited evidence

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

EUVD state
Present in the current official mapping
Known exploitation
Not present in the current ENISA EUVD known-exploited dataset. This is not proof of no exploitation.
ENISA score
8.7 · CVSS 4.0
Advisory evidence
No linked advisory details stored yet
Recommended actionPatch only the product branches with a verified fix

High technical severity; prioritise exposed affected systems while verifying vendor guidance. Verified remediation exists for at least one product or source, but 4 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Fix availability varies by product
01

What, why and how

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.

What

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.

Why

An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

What

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.

Why

An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
No confirmed evidence

No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
None recorded

No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.

Likely attack path
a network path → Inefficient Algorithmic Complexity → cause the confidentiality, integrity or availability impact described by the vendor
Attack surface
Network
Privileges required
None: unauthenticated exploitation is possible
User interaction
None
Attack complexity
Low: no specialised conditions are recorded
Security boundary
Not a CVSS 4.0 base metric
Weakness
?CWE means Common Weakness Enumeration: a standard category for the underlying weakness.
CWE-407 ↗

CWE-407: Inefficient Algorithmic Complexity. An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.

CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Common Vulnerability Scoring System 4.0: the compact vector below is decoded into plain language.

AVNetworkAttack vector: The vulnerable component can be reached over a network.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.ATNoneAttack requirements: No additional deployment or execution condition is required.PRNonePrivileges required: The attacker does not need an account or existing privileges.UINoneUser interaction: No action by another user is required.VCNoneVulnerable-system confidentiality: No direct loss is represented by this metric.VINoneVulnerable-system integrity: No direct loss is represented by this metric.VAHighVulnerable-system availability: A successful attack can cause a major loss.SCNoneSubsequent-system confidentiality: No direct loss is represented by this metric.SINoneSubsequent-system integrity: No direct loss is represented by this metric.SANoneSubsequent-system availability: No direct loss is represented by this metric.
Post-exploitation / living off the land
No specific living-off-the-land technique is confirmed in the structured sources. Monitor normal administration tools for activity inconsistent with the affected service's baseline.
NetworkUnauthenticatedCWE-407
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

ENISA EUVD · EUVD-2026-63092Official EUVD mapping

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

Official EUVD record ↗
BSI · German · WID-SEC-2026-2712Netty: Mehrere Schwachstellen

Ein Angreifer kann mehrere Schwachstellen in Netty ausnutzen, um Sicherheitsvorkehrungen zu umgehen, um einen Denial of Service Angriff durchzuführen, und um Informationen offenzulegen.

Official advisory ↗
Cyber Security Agency of Singapore · English · CSA-SB-20260826Security Bulletin 26 Aug 2026

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 26 Aug 2026, published on 26 August 2026. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
CERT-FR · French · CERTFR-2026-AVI-1249Multiples vulnérabilités dans les produits VMware

ecord?id=CVE-2026-68569 Référence CVE CVE-2026-68763 https://www.cve.org/CVERecord?id=CVE-2026-68763 Référence CVE CVE-2026-6879 https://www.cve.org/CVERecord?id=CVE-2026-6879 Référence CVE CVE-2026-69152 https://www.cve.org/CVERecord?id=CVE-2026-69152 Référence CVE CVE-2026-69192 https://www.cve.org/CVERecord?id=CVE-2026-69192 Référence CVE CVE-2026-69247 https://www.cve.org/CVERecord?id=CVE-2026-69247 Référence CVE CVE-2026-73180 https://www.cve.org/CVERecord?id=CVE-2026-73180 Référence CVE CVE-2026-7383 https://www.cve.org/CVERecord?id=CVE-2026-7383 Référence CVE CVE-2026-75595 https://www.cve.org/CVERecord?id=CVE-2026-75595 Référence CVE CVE-2026-75596 https://www.cve.org/CVERecord?id=CVE-2026-75596 Référence CVE CVE-2026-75803 https://www.cve.org/CVERecord?id=CVE-2026-75803 Référence CVE CVE-2026-77117 https://www.cve.org/CVERecord?id=CVE-2026-77117 Référence CVE CVE-2026-77310 https://www.cve.org/CVERecord?id=CVE-2026-77310 Référence CVE CVE-2026-80489 https://www.cve.org/CVERecord?id=CVE-2026-80489 Référence CVE CVE-2026-81870 https://www.cve.org/CVERecord?id=CVE-2026-81870 Référence CVE CVE-2026-82049 https://www.cve.org/CVERecord?id=CVE-2026-82049 Référence CVE CVE-2026-8286 https://www.cve.org/CVERecord?id=CVE-2026-8286 Référence CVE CVE-2026-83557 https://www.cve.org/CVERecord?id=CV

Official advisory ↗
CERT-FR · French · CERTFR-2026-AVI-1165Multiples vulnérabilités dans les produits IBM

ord?id=CVE-2026-69247 Référence CVE CVE-2026-6938 https://www.cve.org/CVERecord?id=CVE-2026-6938 Référence CVE CVE-2026-70907 https://www.cve.org/CVERecord?id=CVE-2026-70907 Référence CVE CVE-2026-71290 https://www.cve.org/CVERecord?id=CVE-2026-71290 Référence CVE CVE-2026-71491 https://www.cve.org/CVERecord?id=CVE-2026-71491 Référence CVE CVE-2026-73088 https://www.cve.org/CVERecord?id=CVE-2026-73088 Référence CVE CVE-2026-73089 https://www.cve.org/CVERecord?id=CVE-2026-73089 Référence CVE CVE-2026-73180 https://www.cve.org/CVERecord?id=CVE-2026-73180 Référence CVE CVE-2026-75595 https://www.cve.org/CVERecord?id=CVE-2026-75595 Référence CVE CVE-2026-75596 https://www.cve.org/CVERecord?id=CVE-2026-75596 Référence CVE CVE-2026-75838 https://www.cve.org/CVERecord?id=CVE-2026-75838 Référence CVE CVE-2026-77310 https://www.cve.org/CVERecord?id=CVE-2026-77310 Référence CVE CVE-2026-77413 https://www.cve.org/CVERecord?id=CVE-2026-77413 Référence CVE CVE-2026-77414 https://www.cve.org/CVERecord?id=CVE-2026-77414 Référence CVE CVE-2026-77415 https://www.cve.org/CVERecord?id=CVE-2026-77415 Référence CVE CVE-2026-8384 https://www.cve.org/CVERecord?id=CVE-2026-8384 Référence CVE CVE-2026-8400 https://www.cve.org/CVERecord?id=CVE-2026-8400 Référence CVE CVE-2026-84305 https://www.cve.org/CVERecord?id=CVE-

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Fix availability varies by product
Affected
Fixed
An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
Action
Use the product-specific evidence above. Patch only products with a verified fixed release, and keep every affected or under-investigation state without a matching fix in the remediation queue.
Workaround
No verified workaround is recorded. If business-safe, reduce exposure to the affected interface and allow only trusted sources until authoritative guidance is available.
Official vendor articles3 stored documents

Linked articles are downloaded and versioned as source evidence. A CVE mention or approved update relationship does not, by itself, verify a fix for every product branch.

04

Evidence and provenance

Published 19 Aug 2026 · Last source change 20 Aug 2026, 19:59 UTC · CWE-407 · Inefficient Algorithmic Complexity

CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-23
European sourceENISA EUVD · EUVD-2026-63092
Product sourceVendor CSAF · SUSE Product Security Team
Remediation sourceVendor CSAF · SUSE Product Security Team
CWE sourceCNA
NVD statusNVD enriched

Core structured fields are present and their contributing authorities are shown above.

Material change intelligence

What changed after publication

View recent updates ↗
  1. Vendor guidanceAuthoritative vendor guidance changed: added patch: github.com/17213; added patch: github.com/17217; 4 further additions.
    Before
    no authoritative guidance recorded
    After
    patch: github.com/17213 · patch: github.com/17217 · patch: github.com/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7 · 3 more references
    security-advisories@github.com ↗
  2. Affected versionsThe structured affected or fixed version information changed.
    Before
    < 4.1.137.Final; >= 4.2.0.Final, < 4.2.17.Final · Fixed: No fixed version is explicitly recorded in the structured CVE data.
    After
    netty: < 4.1.137.Final, >= 4.2.0.Final, < 4.2.17.Final · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
    CNA ↗
  3. Remediation statusRemediation status changed from Awaiting fix to Patch available.
    Before
    Awaiting fix
    After
    Patch available
    security-advisories@github.com ↗
  4. Catalogue recordCVE added to the BlackTree catalogue.
    CNA ↗
Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Fixed release or patch reference recorded
ENISA EUVD · EUVD-2026-63092↗Vendor CSAF · SUSE Product Security Team↗CVE authority or vendor reference · GitHub GHSA-FCCG-MWVH-QQG4↗Patch or update reference↗Patch or update reference↗Patch or update reference↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2026-75596 · cve.blacktree.nl