mTLS bypass via fragmented ClientHello
Published Aug 19, 2026 · Updated Aug 20, 2026
Authentication bypass in Netty before 4.1.137.Final and 4.2.17.Final allows remote attackers to bypass mTLS via a fragmented ClientHello. In io.netty.handler.ssl.SslClientHelloHandler#decode, the pre-read guard checks for the four-byte handshake header at the wrong offset, and the resulting exception path calls select(ctx, null), falling back from the SNI-selected SslContext to the default context. Exploitation requires deployments that rely on per-SNI clientAuth=REQUIRE as the only client-certificate gate while the default context accepts missing certificates and no application-layer certificate check blocks the request.
Summary
What happened
Authentication bypass in Netty before 4.1.137.Final and 4.2.17.Final allows remote attackers to bypass mTLS via a fragmented ClientHello. In io.netty.handler.ssl.SslClientHelloHandler#decode, the pre-read guard checks for the four-byte handshake header at the wrong offset, and the resulting exception path calls select(ctx, null), falling back from the SNI-selected SslContext to the default context. Exploitation requires deployments that rely on per-SNI clientAuth=REQUIRE as the only client-certificate gate while the default context accepts missing certificates and no application-layer certificate check blocks the request.
The record
- CVE
- CVE-2026-75595
- Published
- Aug 19, 2026
- Updated
- Aug 20, 2026
- Vendor
- The Netty Project
- Product
- Netty
- Classifications
- CWE-754, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 19, 2026CVE publishedPublication date reported by the CVE source.
- Aug 20, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 4.1.137.Final
- Affected versionversion=>= 4.2.0.Final, < 4.2.17.Final
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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