The vty layer in Quagga before 0.96.4, and Zebra 0.93b and earlier, does not verify that sub-negotiation is taking place when processing the SE marker, which allows remote attackers to cause a denial of service (crash) via a malformed telnet command to the telnet CLI port, which may trigger a null dereference.
http://www.redhat.com/support/errata/RHSA-2003-307.html
http://www.redhat.com/support/errata/RHSA-2003-305.html
http://www.debian.org/security/2004/dsa-415