WLAN Security
Antti Miettinen
(modified by JJ)
What is WLAN?
• A wireless data communication system
implemented as an extension to, or alternative
for, a wired local area network.
• Operates at uncontrolled ISM (Industrial,
Scientific and Medical) band
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What is WLAN? (cont.)
• Standards by IEEE for 802.11
– 802.11 First standard, up to 2Mbps@2.4Ghz
– 802.11a Accepted standard, up to
54Mbps@5GHz
– 802.11b Accepted standard, up to
11Mbps@2.4GHz
– 802.11d MAC Enhancements for wider use of
802.11
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What is WLAN? (cont.)
• Standards by IEEE for 802.11 (cont.)
– 802.11e MAC Enhancements for Quality of
Service
– 802.11f Recommended Practice for Inter
Access Point Protocol = Roaming & hand over
– 802.11g Accepted standard, up to
54Mbps@2.4Ghz
– 802.11i Improved WEP and EAP (802.1X)
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What is WLAN? (cont.)
• Standards by ETSI
– HiperLAN/1 23,5Mbps@5GHz
• published 1999
– HiperLAN/2 54Mbps@5Ghz
(http://www.hiperlan2.com/)
• Asynchronous data communication
• Support for QoS (real-time voice & video)
• support Transmit Power Control and Dynamic
Frequency Selection (required in Europe at 5GHz)
• Uses 56 bit to 168 bit key encryption (DES)
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WLAN structure
• Two possibility, either ad-hoc or Access
Point
BSS or ESS
ad-hoc network IBSS
IBSS: Independent Basic Service Set (ad hoc
BSS: (Infrastructure) Basic Service Set
ESS: Extended Service Set
AP: Access Point
Access Point network
Fix to: http://www.comlab.hut.fi/opetus/423/2002/9
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802.11 WLAN security features
• DSSS (Direct sequence Spread Spectrum)
– Isn’t very secure, although theoretically it could be a good security
feature. AP transmits the hop sequence in plain.
• ESSID (Extended Service Set Identifier)
– By default all stations are broadcasting ESSID
– Can be passively received, when legitimate user associates with
Access Point
• WEP (Wired Equivalent Privacy)
– By default is turned off
– Includes flaws (AirSnort attack: collect weak initialization vectors)
• MAC-address controlled authorization to Access Point
– MAC-address is easy to spoof (command line)
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WEP
• Goals
– Access control: To prevent unauthorized users
who lack a correct WEP key from gaining
access to the network.
– Privacy: To protect wireless LAN data streams
by encrypting them and allowing decryption
only by users with the correct WEP keys.
• Includes security flaws!
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WEP Authentication
• Access request by client
• Challenge text sent to client by AP
• Challenge text encoded by client using a
shared secret then sent to AP
• If challenge text encoded properly AP
allows access else denied
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WEP (cont.)
• Based on symmetric RC4-encryption algorithm
• Support 40bit and 104bit encryption
• All clients and AP’s in wireless network share the
same encryption key (weakness)
• No protocol for encryption key distribution
(weakness)
• Initialization Vector (IV) transmitted in the clear
(weakness)
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WEP overview
• A master key k0 (either 40 or 104 bits) is shared
between two parties wishing to communicate a priori.
• Each 802.11 packet (header|data) is then protected by:
– An integrity check field IC = h(header|data)
– A random initialization vector (IV)
• The master key and IV are used to generate a
keystream using RC4 in stream cypher mode
k = RC4(k0, IV)
• The data and IC are then encrypted by this keystream
Ek(m) = m  k
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WEP packet
header
data
IC
RC4 generated keystream
header
IV
encrypted
802.11 packet
random
packet = header | IV | Ek(data | IC)
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Possible Attacks
• War-driving, war-walking etc.
– Moving around the city and scanning the
WLANs
– Many of the WLANs are without protection!
• (about in 50% of present WLANs WEP isn’t
enabled)
– Usually used to find networks, not to
penetrate them
• Monitoring
– Just listening the traffic
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Possible Attacks (cont.)
• DOS-attack
– Use high power 2,45Ghz (or 5GHz) signal generator
• for instance, a microwave oven
– Send continuous streams of CLS (clear-to-send)
frames to a fictitious user
• Legitimate users won’t be able to access the medium
– Send deassociate frame in name of others (MACaddress can be faked)
• It is possible!
– Take the Access Point down!
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Possible Attacks (cont.)
• Man-in-the-middle attack
– If WEP is used, the secret key must first be
solved
– Set up fake Access Point
• No authentication required (from Access Points)
• Legitimate users change their Access Point to
yours, if it has better SNR. You can e.g.
deassociate them from the real Access Point.
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Why is WLAN still used?
• It is fast and easy to set up
• It supports mobility
• Reduced installation time and costs
compared with cable
• Broadband connection, up to 54Mbps
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WLAN is fast
Transmission rate (kbit/s)
100 000
Fixed
LAN
50 000
10 000
802.11a, 802.11g and
HiperLAN2
802.11b/WiFi
1000
500
Bluetooth
UMTS
GPRS
50
GSM
Stationary
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Walking
speed
Source: Public Wireless LAN Access: A Threat to
Mobile Operators, Analysys Research, 2001
Driving
speed
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How to check security of your
WLAN-network?
• AirSnort (http://airsnort.shmoo.com/)
– For Linux and Windows
– Recovers encryption keys
– Operates by passively
• WEPCrack
(http://wepcrack.sourceforge.net/)
– Open source tool for breaking 802.11 WEP
secret keys
– For Linux only
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How to check security of your
WLAN-network?
• Other software:
– Netstumbler (http://www.netstumbler.com/)
• Only for Windows
– Dstumbler
(http://www.dachb0den.com/projects/dstumb
ler.html)
• Only for Linux
– Kismet (http://www.kismetwireless.net/)
• Only for Linux
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WLAN security
• To Be Continued…
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