# CVE-2026-75596: Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing > Remote attackers can cause CPU exhaustion and service degradation by sending fragmented TLS ClientHello records to applications using the default SNI handler. - **Severity:** High 8.7 (`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`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 19 Aug 2026 - **Updated:** 20 Aug 2026 - **Also known as:** CLEANSTART-2026-BB16699, CLEANSTART-2026-BL22559, CLEANSTART-2026-BP52877, CLEANSTART-2026-CQ05064, CLEANSTART-2026-EO12643, CLEANSTART-2026-LV23492, CLEANSTART-2026-OS42010, CLEANSTART-2026-XL02389, DEBIAN-CVE-2026-75596, ECHO-01bb-63a2-3001, GHSA-fccg-mwvh-qqg4, MINI-2wcq-j599-xcrc, MINI-32rf-23mg-c6q4, MINI-3497-rgmq-fqgr, MINI-34vg-fwcq-w6v6, MINI-3pwh-r8jx-9rcj, MINI-3rh7-2783-5gc5, MINI-44c6-2v9q-hxrj, MINI-47hf-jff5-x5cg, MINI-4cw3-p9hp-g845, MINI-4pw6-58r5-85q2, MINI-52x5-w7cr-j43m, MINI-53cp-fq22-px8x, MINI-53w4-qcc2-rcqc, MINI-57cr-jxq4-v4m4, MINI-58f9-vjq5-gppm, MINI-5j7q-q382-7m26, MINI-5m99-gwrc-j2cj, MINI-5xcr-mcmw-43cg, MINI-6358-6q6r-9424, MINI-64pj-xxh6-4xrq, MINI-6g57-7fx9-5gm9, MINI-6hgp-3mj9-2ff9, MINI-6pj5-qw7g-5vvh, MINI-6rqx-3r9h-3m64, MINI-7363-v7cm-qx43, MINI-736x-m87g-cc22, MINI-7627-vw2r-2jg3, MINI-78c8-wx48-7wh9, MINI-78cj-3r8v-3pmv, MINI-78jf-cf3m-wfxx, MINI-7fmc-rw26-3fm4, MINI-7jq4-p6r5-q8qp, MINI-7v48-xxwx-j89p, MINI-7vh9-f6rm-fjh4, MINI-849c-xp5f-38p2, MINI-85q2-ch5j-qx2j, MINI-868w-prcr-32v7, MINI-8j74-pfvh-q8vw, MINI-8pw8-q3pf-3fq4, MINI-8vwg-q52w-h9p8, MINI-8wrj-ff7x-p7x7, MINI-92rj-h525-64j3, MINI-959g-gpww-3fj4, MINI-98m9-94qh-2vg7, MINI-9h7v-296h-6g8f, MINI-9vvv-vm58-24mc, MINI-9xmc-q25g-phc7, MINI-c2xc-g6ph-9vf3, MINI-c4f9-pr8v-7phx, MINI-c588-4qqv-4rjh, MINI-c8mq-935p-446f, MINI-f3m8-cpfx-x578, MINI-f779-rj2w-h463, MINI-fghq-xf3j-ff93, MINI-fhrx-q9gj-334p, MINI-fp7f-c7f3-rm38, MINI-fqpc-jqj7-gjcv, MINI-frrv-p37j-gp9g, MINI-fw26-gxmm-w9mr, MINI-fw8p-99h8-r8q4, MINI-g3v8-mh9v-hj59, MINI-g5cr-qg3c-7wrw, MINI-g5w9-6v67-2wv8, MINI-g8mj-x7h9-9gwf, MINI-gf4w-j7gr-h2x5, MINI-gq2h-548x-wgm8, MINI-h35j-652r-prhx, MINI-h6g2-9j64-82cc, MINI-hc4r-g8xm-m5w3, MINI-hf84-h4pg-4cqh, MINI-hfv9-jhv7-3272, MINI-hprq-927j-6xx4, MINI-hq88-h9rc-m5xq, MINI-hxc8-v22f-rpv9, MINI-j6g7-rpch-qh2p, MINI-j6rq-75fq-cpxv, MINI-j7jm-jr3p-4q4r, MINI-j9mc-6ph6-pvfg, MINI-jghg-3465-fpvr, MINI-jpr3-gjg7-9gh4, MINI-jvm8-5x7h-r83c, MINI-m8qm-mrmv-cx54, MINI-mmh2-53c7-hhxq, MINI-mpj8-hhmc-vx3c, MINI-p393-6cf5-xr5f, MINI-p4wh-rwx9-gv8c, MINI-p7ch-cq8p-pq6g, MINI-ppp4-w4pr-78c7, MINI-pqxr-c2jv-q7p8, MINI-pvmx-rcww-x9gg, MINI-qgvq-4vxq-c6wp, MINI-qp3m-439r-jm7c, MINI-qw98-777w-92wh, MINI-r5g8-85gp-7gh2, MINI-rfg3-c36m-pfrq, MINI-rgw9-9qgc-65gr, MINI-rj84-67r2-x7w4, MINI-rrhc-jwvg-39wx, MINI-v2x5-x2q5-39vx, MINI-v775-93qw-jr97, MINI-v7w8-7m57-c6r6, MINI-vqff-9hf5-mj5h, MINI-wfwh-j947-q68r, MINI-wm62-4p5g-f75f, MINI-wqrf-7r6q-2xp8, MINI-wrc9-89ch-vjrq, MINI-wrfq-vgcw-6h4q, MINI-wrh6-63r4-px6j, MINI-wv38-5hhr-2h27, MINI-wxr4-qr87-m684, MINI-x23c-xvm3-6g8m, MINI-x3f4-wc2w-29mp, MINI-xp3m-vg58-34h6, UBUNTU-CVE-2026-75596 ## What is CVE-2026-75596? CVE-2026-75596 is a high-severity denial of service vulnerability in Netty Netty, published on 19 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official 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. Weakness: CWE-407 (Inefficient Algorithmic Complexity) ## Which versions are affected by CVE-2026-75596? 4 open source packages ship an affected version, including io.netty:netty-handler. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `io.netty:netty-handler` | Maven | All versions | 4.1.137.Final | | `io.netty:netty-handler` | Maven | All versions | 4.1.137.Final | | `io.netty:netty-handler` | Maven | From 4.2.0.Final | 4.2.17.Final | | `io.netty:netty-handler` | Maven | From 4.2.0.Final | 4.2.17.Final | Products: Netty Netty. ## How do I fix CVE-2026-75596? Upgrade io.netty:netty-handler to a fixed version for your release line: 4.1.137.Final or 4.2.17.Final. ## Is CVE-2026-75596 being exploited? There is no known exploitation of CVE-2026-75596 in the wild and no public exploit code. ## Other vulnerabilities in Netty Netty - [CVE-2026-75595](https://insights.sbomify.com/CVE-2026-75595/): Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext (critical 9.1) - [CVE-2026-59901](https://insights.sbomify.com/CVE-2026-59901/): Netty Bzip2Decoder: Infinite Loop in RLE State Machine Leads to Event-Loop Thread Hang (high 8.7) - [CVE-2026-55851](https://insights.sbomify.com/CVE-2026-55851/): Netty codec-haproxy: Signed-Byte Sentinel Collision in HAProxyMessageDecoder Leads to Unbounded Memory Exhaustion (high 8.7) - [CVE-2026-56745](https://insights.sbomify.com/CVE-2026-56745/): Netty SpdyHttpDecoder: ByteBuf Reference Leak on RST_STREAM Leads to Native Memory Exhaustion (high 8.7) - [CVE-2026-45674](https://insights.sbomify.com/CVE-2026-45674/): Netty Vulnerable to DNS Cache Poisoning via Missing Bailiwick Checks in CNAME Records (high 8.7) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-75596 and fix it if it is affected. What it is: CVE-2026-75596 is a high-severity denial of service vulnerability in Netty Netty, published on 19 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: There is no known exploitation of CVE-2026-75596 in the wild and no public exploit code. Affected packages and fixed versions: - io.netty:netty-handler (Maven): affected all versions; fixed in 4.1.137.Final - io.netty:netty-handler (Maven): affected all versions; fixed in 4.1.137.Final - io.netty:netty-handler (Maven): affected from 4.2.0.Final; fixed in 4.2.17.Final - io.netty:netty-handler (Maven): affected from 4.2.0.Final; fixed in 4.2.17.Final Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-75596/ (Markdown: https://insights.sbomify.com/CVE-2026-75596/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-75596 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-75596 - OSV: https://osv.dev/vulnerability/CVE-2026-75596 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-75596/ Summaries are generated and may contain mistakes; the official description above is the record.