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NVD / CVE Program· CNA GitHub_M

CVE-2026-75596: Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing

Noroxi assessment

Priority: Monitor (action score 34/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.37% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • According to CISA the attack is automatable: targets can be found by mass scanning.
  • Fixed version: io.netty:netty-handler (Maven) 4.2.17.Final, 4.1.137.Final; netty (openSUSE:Tumbleweed) 4.1.138-1.1; netty-tcnative (SUSE:Linux Enterprise Module for Development Tools 15 SP7) 2.0.84-150200.3.51.1.

Low priority for now. Upgrade to io.netty:netty-handler (Maven) 4.2.17.Final, 4.1.137.Final; netty (openSUSE:Tumbleweed) 4.1.138-1.1; netty-tcnative (SUSE:Linux Enterprise Module for Development Tools 15 SP7) 2.0.84-150200.3.51.1 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

HighCVSS 8.7 · v4.0—No exploit Fix available
Published
Aug 19, 2026
Updated
Sep 22, 2026
EPSS
0.3% · 30th percentile

Report tools

JSON

Action score

34

Monitor

Low priority for now.

CVSS
34 / 40 · 8.7 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.3%

CISA SSVC decision

Exploitation
none
Automatable
yes
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · GitHub_M
8.7
NVD has not scored this yet; the score shown is the CNA’s.

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

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2026-75596

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
nettynetty

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • netty nettybefore 4.1.137
  • netty netty4.2.0 and later · before 4.2.17

Versions reported by the vendor

Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.

  • netty netty

    • < 4.1.137.Final, >= 4.2.0.Final, < 4.2.17.Finalaffected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Debian:12nettyfrom 0—
Mavenio.netty:netty-handlerfrom 4.2.0.Final · before 4.2.17.Final4.2.17.Final
Mavenio.netty:netty-handlerbefore 4.1.137.Final4.1.137.Final
openSUSE:Tumbleweednettybefore 4.1.138-1.14.1.138-1.1
SUSE:Linux Enterprise Module for Development Tools 15 SP7netty-tcnativebefore 2.0.84-150200.3.51.12.0.84-150200.3.51.1
SUSE:Linux Enterprise Module for Package Hub 15 SP7nettybefore 4.1.138-150200.4.56.14.1.138-150200.4.56.1

Same product

netty: all records

Other highest-scoring records for the same primary product.

  • CVE-2026-47691Netty has Insufficient Bailiwick Validation for NS Records
    40Plan
  • CVE-2026-45674Netty Vulnerable to DNS Cache Poisoning via Missing Bailiwick Checks in CNAME Records
    40Plan
  • CVE-2019-20445HttpObjectDecoder.java in Netty before 4.1.44 allows a Content-Length header to be accompanied by a second Content-Length header, or by a Tr
    40Plan

Published before and after this one in the same product

  • ← EarlierCVE-2026-59903Aug 17, 2026 · Netty: Cache Poisoning and Information Disclosure via CORS Vary Header Overwrite
    30Monitor
  • ← EarlierCVE-2026-75595Aug 19, 2026 · Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext
    36Monitor
  • Later →CVE-2026-62243Aug 22, 2026 · Netty 4.2.0 through 4.2.16 TLS Hostname Verification Bypass
    34Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
Maven:io.netty:netty-handler4.1.137.FinalPackage registry (OSV)
Maven:io.netty:netty-handler4.2.17.FinalPackage registry (OSV)
opensuse:netty4.1.138-1.1 · openSUSE:TumbleweedPackage registry (OSV)
suse:netty4.1.138-150200.4.56.1 · SUSE:Linux Enterprise Module for Package Hub 15 SP7Package registry (OSV)
suse:netty-tcnative2.0.84-150200.3.51.1 · SUSE:Linux Enterprise Module for Development Tools 15 SP7Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
Integrity
Availability
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

CWE-407 · Inefficient Algorithmic Complexity

CVSS vector

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/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

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

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References

Show all references (6 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records