The Tunnel daemon creates an encrypted tunnel between your origin web server and Cloudflare’s nearest data center, all without opening any public inbound ports.

From https://www.cloudflare.com/products/tunnel/

The good news is that in order to exploit the new vulnerability, the attacker first has to obtain kernel level access to the system somehow - by exploiting some other vulnerabilities perhaps.

The bad news is once Sinkclose attack is performed, it can be hard to detect and mitigate: it can even survive an OS reinstall.