When nc(1) tries to connect through an HTTP proxy that requires authentication, nc calls readpassphrase(3) and aborts. Pledging "tty" fixes this problem, but you'll notice that the diff has a lot of nasty branches. My failure to check Pflag when connecting over unix sockets is not an oversight; nc does not support that configuration.
To reproduce the failure without setting up a real HTTP proxy, open two terminals and run nc as a coprocess in the first. The following session causes a core dump: (tty1)$ nc -lk 8080 |& (tty2)$ nc -Xconnect -xlocalhost:8080 -Puser localhost 8081 (tty1)$ print -np "HTTP/1.0 407 Authentication Required\r\n\r\n" (tty2) Abort trap (core dumped) Index: netcat.c =================================================================== RCS file: /cvs/src/usr.bin/nc/netcat.c,v retrieving revision 1.150 diff -u -p -r1.150 netcat.c --- netcat.c 4 Jan 2016 02:18:31 -0000 1.150 +++ netcat.c 28 May 2016 18:33:30 -0000 @@ -323,7 +323,13 @@ main(int argc, char *argv[]) if (pledge("stdio rpath wpath cpath tmppath unix", NULL) == -1) err(1, "pledge"); } else if (Fflag) { - if (pledge("stdio inet dns sendfd", NULL) == -1) + if (Pflag) { + if (pledge("stdio inet dns sendfd tty", NULL) == -1) + err(1, "pledge"); + } else if (pledge("stdio inet dns sendfd", NULL) == -1) + err(1, "pledge"); + } else if (Pflag) { + if (pledge("stdio inet dns tty", NULL) == -1) err(1, "pledge"); } else if (usetls) { if (pledge("stdio rpath inet dns", NULL) == -1) @@ -434,7 +440,10 @@ main(int argc, char *argv[]) if (Kflag && (privkey = tls_load_file(Kflag, &privkeylen, NULL)) == NULL) errx(1, "unable to load TLS key file %s", Kflag); - if (pledge("stdio inet dns", NULL) == -1) + if (Pflag) { + if (pledge("stdio inet dns tty", NULL) == -1) + err(1, "pledge"); + } else if (pledge("stdio inet dns", NULL) == -1) err(1, "pledge"); if (tls_init() == -1)