Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing
Published Aug 19, 2026
8.7
HIGHCVSS 4.0
EPSS 0.37%
Description
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.
Affected products
-
- Version < 4.1.137.FinalStatusaffectedConstraints-
- Version >= 4.2.0.Final, < 4.2.17.FinalStatusaffectedConstraints-
- Version
No data.
No data.
No Red Hat product state for this CVE.
No package ranges for this CVE.
Remediation
No remediation recorded yet.
Metrics
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
2 other sources (GitHub, NVD) ▾
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
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
No CVSS v3.1 score for this CVE.
No CVSS v3.0 score for this CVE.
No CVSS v2.0 score for this CVE.
This CVE is not in the KEV list.
CISA SSVC (Vulnrichment)
Stakeholder-Specific Vulnerability Categorization from CISA ADP.
Exploitation
NoneAutomatable
YesTechnical Impact
PartialDecision
n/aAssessed Aug 20, 2026 · SSVC 2.0.3
Estimated probability of exploitation in the wild in the next 30 days (FIRST EPSS). As of Oct 3, 2026.
Score over time
Aug–Oct 2026- EPSS v5
Percentile over time
- EPSS v5
Table of values (2 key points)
Flat stretches are collapsed; showing up to 120 newest points.
| Date | Score | Percentile | Model |
|---|---|---|---|
| Oct 3, 2026 | 0.37% (0.00374) | 29.03th | v5 (v2026.06.15) |
| Aug 20, 2026 | 0.35% (0.00350) | 28.26th | v5 (v2026.06.15) |
References (9)
- https://github.com/advisories/GHSA-fccg-mwvh-qqg4 Advisory
- https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7 x_refsource_MISC
- https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961 x_refsource_MISC
- https://github.com/netty/netty/pull/17213 x_refsource_MISC
- https://github.com/netty/netty/pull/17217 x_refsource_MISC
- https://github.com/netty/netty/releases/tag/netty-4.1.137.Final x_refsource_MISC
- https://github.com/netty/netty/releases/tag/netty-4.2.17.Final x_refsource_MISC
- https://github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4 x_refsource_CONFIRM
- https://nvd.nist.gov/vuln/detail/CVE-2026-75596
| Link | Providers | Tags |
|---|---|---|
| https://github.com/advisories/GHSA-fccg-mwvh-qqg4 | Advisory | |
| https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7 | x_refsource_MISC | |
| https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961 | x_refsource_MISC | |
| https://github.com/netty/netty/pull/17213 | x_refsource_MISC | |
| https://github.com/netty/netty/pull/17217 | x_refsource_MISC | |
| https://github.com/netty/netty/releases/tag/netty-4.1.137.Final | x_refsource_MISC | |
| https://github.com/netty/netty/releases/tag/netty-4.2.17.Final | x_refsource_MISC | |
| https://github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4 | x_refsource_CONFIRM | |
| https://nvd.nist.gov/vuln/detail/CVE-2026-75596 |
Change history (0)
No recorded changes yet.