Vulnerability in Netty
CVE-2026-75596
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.004 (29.0th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 29th percentile — 29.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: +0.000
Affected products
- Netty — versions < 4.1.137.Final, >= 4.2.0.Final, < 4.2.17.Final
Weakness classification (CWE)
References
- github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4 (Vendor Advisory)
- github.com/netty/netty/pull/17213 (Patch, Issue Tracking)
- github.com/netty/netty/pull/17217 (Patch, Issue Tracking)
- github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7 (Patch)
- github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961 (Patch)
- github.com/netty/netty/releases/tag/netty-4.1.137.Final (Release Notes)
- github.com/netty/netty/releases/tag/netty-4.2.17.Final (Release Notes)
Frequently asked questions
- What is CVE-2026-75596?
- CVE-2026-75596 is a high-severity vulnerability in Netty, classified under Inefficient Algorithmic Complexity. CVSS score: 7.5/10. Published 2026-08-19.
- How severe is CVE-2026-75596?
- High severity. CVSS v3 base score is 7.5 out of 10.