Passive Attack to Decrypt Traffic
The first attack follows directly from the above observation. A passive eavesdropper can intercept all wireless traffic, until an IV collision occurs. By XORing two packets that use the same IV, the attacker obtains the XOR of the two plaintext messages. The resulting XOR can be used to infer data about the contents of the two messages. IP traffic is often very predictable and includes a lot of redundancy. This redundancy can be used to eliminate many possibilities for the contents of messages. Further educated guesses about the contents of one or both of the messages can be used to statistically reduce the space of possible messages, and in some cases it is possible to determine the exact contents.
When such statistical analysis is inconclusive based on only two messages, the attacker can look for more collisions of the same IV. With only a small factor in the amount of time necessary, it is possible to recover a modest number of messages encrypted with the same key stream, and the success rate of statistical analysis grows quickly. Once it is possible to recover the entire plaintext for one of the messages, the plaintext for all other messages with the same IV follows directly, since all the pairwise XORs are known.
An extension to this attack uses a host somewhere on the Internet to send traffic from the outside to a host on the wireless network installation. The contents of such traffic will be known to the attacker, yielding known plaintext. When the attacker intercepts the encrypted version of his message sent over 802.11, he will be able to decrypt all packets that use the same initialization vector.