{"id":"GHSA-c4c3-7fpv-j4q5","summary":"Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext","details":"### Summary\nA fragmented TLS ClientHello whose handshake header spans multiple records makes Netty silently fall back to the default SslContext; where per-SNI selection is the sole mTLS gate, an unauthenticated attacker can bypass the route's mTLS requirement.\n\n### Details\nIn `io.netty.handler.ssl.SslClientHelloHandler#decode` the guard that should wait for the 4-byte handshake header checks the wrong offset - it ignores the 5-byte record header that precedes it - and therefore never fires:\n\n```java\nif (handshakeLength == -1) {\n    if (readerIndex + 4 \u003e endOffset) {\n        // Need more data to read HandshakeType and handshakeLength (4 bytes)\n        return;\n    }\n```\n\nWhen the first record's payload is \u003c 4 bytes, `handshakeLength = in.getUnsignedMedium(readerIndex + SslUtils.SSL_RECORD_HEADER_LENGTH + 1);` leads to `IndexOutOfBoundsException `. That is caught by the generic `catch (Exception)` block, which calls `select(ctx, null)` - this is the default `SslContext`. Fallback to default on parse failure is a problem when per-SNI selection is the sole mTLS gate.\n\n### Impact\nSNI routing bypass. Escalates to an unauthenticated mTLS bypass only when:\n- mTLS is enforced solely via per-SNI SslContext (clientAuth=REQUIRE)\n- the default/fallback SslContext is permissive (clientAuth=NONE/OPTIONAL)\n- no secondary peer-certificate verification exists at the application layer.","aliases":["CVE-2026-75595"],"modified":"2026-09-08T18:28:35.321425686Z","published":"2026-09-08T18:16:14Z","database_specific":{"severity":"CRITICAL","github_reviewed":true,"github_reviewed_at":"2026-09-08T18:16:14Z","nvd_published_at":"2026-08-19T21:17:37Z","cwe_ids":["CWE-475","CWE-754"]},"references":[{"type":"WEB","url":"https://github.com/netty/netty/security/advisories/GHSA-c4c3-7fpv-j4q5"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-75595"},{"type":"WEB","url":"https://github.com/netty/netty/pull/17213"},{"type":"WEB","url":"https://github.com/netty/netty/pull/17217"},{"type":"WEB","url":"https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7"},{"type":"WEB","url":"https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961"},{"type":"PACKAGE","url":"https://github.com/netty/netty"},{"type":"WEB","url":"https://github.com/netty/netty/releases/tag/netty-4.1.137.Final"},{"type":"WEB","url":"https://github.com/netty/netty/releases/tag/netty-4.2.17.Final"}],"affected":[{"package":{"name":"io.netty:netty-handler","ecosystem":"Maven","purl":"pkg:maven/io.netty/netty-handler"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.2.0.Final"},{"fixed":"4.2.17.Final"}]}],"versions":["4.2.0.Final","4.2.1.Final","4.2.10.Final","4.2.11.Final","4.2.12.Final","4.2.13.Final","4.2.14.Final","4.2.15.Final","4.2.16.Final","4.2.2.Final","4.2.3.Final","4.2.4.Final","4.2.5.Final","4.2.6.Final","4.2.7.Final","4.2.8.Final","4.2.9.Final"],"database_specific":{"last_known_affected_version_range":"\u003c= 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