Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext

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.