A new attack technique would allow attackers to bypass a victim’s NAT, firewall, or router to gain access to any TCP/UDP service hosted on their devices. The attack method, dubbed NAT Slipstreaming, was discovered by security researcher Samy Kamkar. Web browser vendors are planning to block the 5060 and 5061 TCP ports used in this attack by adding them to the restricted list. Chromium developer Adam Rice expects some impact to be observed by browser users including: http://example.com:5060/ or http://www.com/5061/ will no longer work.
Source: https://www.bleepingcomputer.com/news/security/new-slipstream-nat-bypass-attacks-to-be-blocked-by-browsers/