sys_term.c in telnetd in FreeBSD 7.0-RELEASE and other 7.x versions deletes dangerous environment variables with a method that was valid only in older FreeBSD distributions, which might allow remote attackers to execute arbitrary code by passing a crafted environment variable from a telnet client, as demonstrated by an LD_PRELOAD value that references a malicious library.
https://www.exploit-db.com/exploits/8055
https://exchange.xforce.ibmcloud.com/vulnerabilities/48780
http://security.freebsd.org/advisories/FreeBSD-SA-09:05.telnetd.asc