IPv4 mapped address (RFC2553/3493)

::ffff:127.0.0.1
Representation of 127.0.0.1 on top of AF_INET6 socket
(real peer is IPv4, 127.0.0.1)

Not distinguishable between the following two:
IPv4 packet goes into AF_INET6 socket (kernel translates the address)
Real IPv6 packet with ::ffff:127.0.0.1

Ambiguity leads to Security hole
Malicious party could circumvent access control
Malicious party could make you generate unexpected IPv4 packet
Services that flip src/dst - DNS server, udp echo, ...

Solution
Do not use IPv4 mapped address by setsockopt(IPv6_V6ONLY, 1), open AF_INET socket for IPv4
Specification picked insecure behavior as default!
Some OSes (OpenBSD, NetBSD, FreeBSD) are cautious and picked safer default