Suppose a university with address block 18.104.22.168/16 has a link
connected to AT&T, where the AT&T router forwards packets destined to
22.214.171.124/16 to the university router. Suppose the university router has three forwarding entries: 126.96.36.199/24 out the link to the math
department, 188.8.131.52/24 out the link to the CS department, and a
“default route” for 0.0.0.0/0 pointing to the AT&T router. Suppose a
host in the rest of the Internet sends a packet destined to
184.108.40.206. What would happen to that packet? What could be done to prevent it?
the answer is :
The packets would loop between the AT&T and university routers,
because AT&T would forward the packet to the university (using the
route for 220.127.116.11/16) and the university would forward the packet
back to AT&T (using the default route 0.0.0.0/0). The university
should configure a “null route” to drop all packets matching
18.104.22.168/16 to prevent this. See http://www.nanog.org/mtg-0602/gao.html for details about this issue,
and the security vulnerabilities associated with it.
I can't understand what are the security vulnerabilities associated with it
You could start sort of a DoS attack by sending packets to out of range 22.214.171.124/16 addresses that would get stuck in a routing loop. Eventually you could get enough traffic in the loop that things would start to break.