Evil Packets

When the hacker knows the actual structure of your encrypted packet,
such as the header field, he can send “evil” packets into your network to
change commands, spoof addresses, and perform many other tasks.
Encrypted packets have an integrity check to make certain they have
not been altered, but the integrity check within WEP can be changed so
that it will actually be “valid” for the “evil” packet to be accepted by the
receiver. When the hacker knows the location of the receiver, the
address can be modified to reflect an unknown packet; thus the new destination
can now be controlled by the hacker. If the packet is transmitted
on a wireless network, then the access point will actually decrypt
the packet and send it along to its false destination.