CVE-2026-75595

Summary

A 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.

Details

In 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:

if (handshakeLength == -1) {
    if (readerIndex + 4 > endOffset) {
        // Need more data to read HandshakeType and handshakeLength (4 bytes)
        return;
    }

When the first record's payload is < 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.

Impact

SNI routing bypass. Escalates to an unauthenticated mTLS bypass only when:

• mTLS is enforced solely via per-SNI SslContext (clientAuth=REQUIRE)
• the default/fallback SslContext is permissive (clientAuth=NONE/OPTIONAL)
• no secondary peer-certificate verification exists at the application layer.

Red Hat Lightwell has released io.netty:netty-handler 4.1.100.Final-rhlw-00001 with backported fixes for CVE-2025-24970, CVE-2026-44249, CVE-2026-45416, CVE-2026-50010, CVE-2026-75595, and CVE-2026-75596.

CVE-2025-24970: SslHandler doesn't correctly validate packets which can lead to native crash when using native SSLEngine

CVE-2026-44249: Netty has an IPv6 Subnet Filter Bypass via Incorrect Comparator Masking

CVE-2026-45416: Netty: SNI handler pre-allocates up to 16 MiB from nine attacker bytes

CVE-2026-50010: Netty's wrapping plain trust manager silently disables hostname verification

CVE-2026-75595: Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext

CVE-2026-75596: Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing

This patched artifact is a drop-in replacement for io.netty:netty-handler 4.1.100.Final from Maven Central.

Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Fina and 4.2.17.Final, io.netty.handler.ssl.SslClientHelloHandler#decode checks the wrong offset before reading the four-byte TLS handshake header, so a ClientHello whose handshake header spans records can cause an IndexOutOfBoundsException and invoke select(ctx, null). This selects the default SslContext instead of the SNI-specific context. In deployments where per-SNI clientAuth=REQUIRE is the sole mutual TLS gate, the default SslContext uses clientAuth=NONE or clientAuth=OPTIONAL, and no application-layer certificate verification exists, an unauthenticated remote attacker can bypass the protected route's mutual TLS requirement. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.

TuxCare rebuilt io.netty:netty-handler to address 2 CVEs. Fixed in 4.1.122.Final-tuxcare.4.

TuxCare rebuilt io.netty:netty-handler to address 2 CVEs. Fixed in 4.1.119.Final-tuxcare.3.

TuxCare rebuilt io.netty:netty-handler to address 1 CVE. Fixed in 4.1.108.Final-tuxcare.3.

TuxCare rebuilt 8 io.netty packages to address 9 CVEs. Fixed in 4.1.115.Final-tuxcare.9.

TuxCare rebuilt 14 io.netty packages to address 26 CVEs. Fixed in 4.1.118.Final-tuxcare.1.

TuxCare rebuilt 13 io.netty packages to address 24 CVEs. Fixed in 4.1.114.Final-tuxcare.1.

TuxCare rebuilt 10 io.netty packages to address 15 CVEs. Fixed in 4.1.113.Final-tuxcare.1.

These are all security issues fixed in the netty-4.1.138-1.1 package on the GA media of openSUSE Tumbleweed.

Root has patched CVE-2026-75595 in the io.netty:netty-handler package for Root:Maven. Multiple fixed versions available.

This update for netty, netty-tcnative fixes the following issues:

• CVE-2026-59902: Netty: Memory Exhaustion in SctpMessageCompletionHandler (bsc#1275501).
• CVE-2026-59903: Netty Vulnerable to Cache Poisoning and Information Disclosure via CORS Vary Header Overwrite (bsc#1275500).
• CVE-2026-62243: TLS hostname verification bypass in Netty OpenSSL client path (bsc#1276455).
• CVE-2026-62380: null byte and CRLF injection in Socks4ClientEncoder and Socks5ClientEncoder (bsc#1276456).
• CVE-2026-75595: fragmented TLS ClientHello causes fallback to default SslContext and allows for SNI routing and mTLS requirement bypass (bsc#1276420).
• CVE-2026-75596: fragmented ClientHellorecords can trigger quadratic pre-handshake reassembly in default SNI parsing (bsc#1276421).
• CVE-2026-76816: missing input validation in MqttEncoder allows for null-byte injection, topic hijacking, and ACL bypassing (bsc#1277243).
• CVE-2026-89044: HTTP request smuggling in Netty via improper validation of final Transfer-Encoding coding (bsc#1280048).
• CVE-2026-93488: Denial of Service via unbounded concurrent SPDY streams (bsc#1282084).
• CVE-2026-93491: Denial of Service via unbounded HttpServerCodec HTTP/1.1 pipeline queue (bsc#1282085).
• CVE-2026-93492: HTTP/2 HpackEncoder DoS with large table size (bsc#1282132).
• CVE-2026-93493: missing nextUpdate field in OCSP responses leads to silent validation bypass (bsc#1281431).
• CVE-2026-93494: ByteBuf Leak in StompSubframeDecoder When a Frame Body Is Never Terminated (bsc#1282506).
• CVE-2026-93558: Unbounded Per-Connection Queue Growth in WebSocketServerExtensionHandler Leads to Denial of Service (bsc#1282140).
• CVE-2026-93560: STOMP codec content-length long-to-int truncation causes infinite decode loop DoS (bsc#1282141).
• CVE-2026-93562: Incomplete validation of malformed Transfer-Encoding allows HTTP request smuggling (bsc#1282362).
• CVE-2026-93563: unbounded multi-line response accumulation in SmtpResponseDecoder leads to memory exhaustion and a DoS (bsc#1281432).
• CVE-2026-93564: HAProxy PROXY-v2 nested-TLV grandchild ByteBuf reference-count leak (bsc#1282363).
• CVE-2026-93565: RtspDecoder Method-Token Smuggling via Trailing Control Byte (bsc#1282364).
• CVE-2026-93566: HTTP Request Smuggling due to control characters in the chunk-size line (bsc#1282366).
• CVE-2026-93567: HTTP/1 authority-form CONNECT is translated to malformed HTTP/2 CONNECT with Host-controlled :authority (bsc#1282367).
• CVE-2026-93568: HTTP/2 and HTTP/3 Extended CONNECT requests are downgraded as regular CONNECT requests (bsc#1282370).
• CVE-2026-93569: HTTP/1 absolute-form Host mismatch is translated to HTTP/2 :authority, overriding the request-target authority (bsc#1282372).
• CVE-2026-93572: multiplication of patched preallocation limits in RedisArrayAggregator nested RESP headers can lead to denial of service (bsc#1281433).
• CVE-2026-93573: Incomplete validation of malformed Transfer-Encoding allows HTTP request smuggling (bsc#1282373).
• CVE-2026-93574: HTTP request smuggling via post-digit whitespace in chunk-size parsing (bsc#1282374).
• CVE-2026-93575: missing validations in the MqttDecoder can lead to excessive resource consumption and a DoS (bsc#1281434).
• CVE-2026-93576: netty-codec-smtp -- SMTP command-name field is not CRLF-validated (bsc#1282507).
• CVE-2026-93578: missing Extended Key Usage (EKU) check in OCSP client allows certificate revocation bypass (bsc#1281435).
• CVE-2026-93579: HTTP/2 header field values are not validated by default (bsc#1282378).

Changes for netty:

• Upgrade to upstream version 4.1.138

Changes for netty-tcnative:

• Upgrade to version 2.0.84 Final