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.
Improper Input Validation The product receives input or data, but it does
not validate or incorrectly validates that the input has the
properties that are required to process the data safely and
correctly.