Wi-Fi is a broadcast
medium that enables any device to eavesdrop and participate either as a
legitimate or rogue device. Management frames such as authentication,
de-authentication, association, dissociation, beacons, and probes are used by
wireless clients to initiate and tear down sessions for network services.
Unlike data traffic, which can be encrypted to provide a level of
confidentiality, these frames must be heard and understood by all clients and
therefore must be transmitted as open or unencrypted. While these frames cannot
be encrypted, they must be protected from forgery to protect the wireless
medium from attacks. For example, an attacker could spoof management frames
from an AP to attack a client associated with the AP.
The 802.11w protocol
applies only to a set of robust management frames that are protected by the
Protected Management Frames ( PMF) service. These include Disassociation,
De-authentication, and Robust Action frames.
that are considered as robust action and therefore protected are the following:
- Spectrum Management
- Block Ack
- Radio Measurement
- Fast BSS Transition
- SA Query
- Protected Dual of Public
- Vendor-specific Protected
When 802.11w is
implemented in the wireless medium, the following occur:
- Client protection is added by
the AP adding cryptographic protection to de-authentication and dissociation
frames preventing them from being spoofed in a DOS attack.
- Infrastructure protection is
added by adding a Security Association (SA) tear down protection mechanism
consisting of an Association Comeback Time and an SA-Query procedure preventing
spoofed association request from disconnecting an already connected client.
introduced a new IGTK Key, which is used to protect broadcast/multicast robust
- IGTK is a random value
assigned by the authenticator STA (WLC) and used to protect MAC management
protocol data units (MMPDUs) from that source STA.
When Management Frame
Protection is negotiated, the AP encrypts the GTK and IGTK values in the
EAPOL-Key frame, which is delivered in Message 3 of 4-way handshake.
Figure 1. IGTK Exchange
in 4-way Handshake
- If the AP later changes the
GTK, it sends the new GTK and IGTK to the client using the Group Key Handshake
802.11w defines a new
Broadcast/Multicast Integrity Protocol (BIP) that provides data integrity and
replay protection for broadcast/multicast robust management frames after
successful establishment of an IGTKSA - It adds a MIC that is calculated using
the shared IGTK key.
Information Elements (IEs)
Figure 2. 802.11w
- Modifications made in the RSN
capabilities field of RSNIE.
- Bit 6: Management Frame
Protection Required (MFPR)
- Bit 7: Management Frame
Protection Capable (MFPC)
- Two new AKM Suites, 5 and 6
are added for AKM Suite Selectors.
- New Cipher Suite with type 6
is added to accommodate BIP.
The WLC adds this
modified RSNIE in association and re-association responses and the APs add this
modified RSNIE in beacons and probe responses.
Wireshark captures shows the RSNIE capabilities and the Group Management Cipher
Figure 3. 802.11w
Association (SA) Teardown Protection
protection is a mechanism to prevent replay attacks from tearing down the
session of an existing client. It consists of an Association Comeback Time and
an SA-Query procedure preventing spoofed association requests from
disconnecting an already connected client.
If a client has a
valid security association, and has negotiated 802.11w, the AP shall reject
another Association Request with status code 30. This status code stands for
"Association request rejected temporarily; Try again later". The AP should not
tear down or otherwise modify the state of the existing association until the
SA-Query procedure determines that the original SA is invalid and shall include
in the Association Response an Association Comeback Time information element,
specifying a comeback time when the AP would be ready to accept an association
with this client.
capture shows the Association Reject message with status code 0x1e (30) and the
Association comeback time set to 10 seconds.
Figure 4. Association
Reject with Comeback Time
Following this, if
the AP is not already engaged in an SA Query with the client, the AP shall
issue an SA Query until a matching SA Query response is received or the
Association Comeback time expires. An AP may interpret reception of a valid
protected frame as an indication of a successfully completed SA Query.
If a SA QUERY
response with a matching transaction identifier within the time period, the AP
shall allow the association process to be started without starting additional
SA Query procedures.
Configuration for Protected Management Frames
security pmf [ association-comeback
association-comeback-time-in-seconds | mandatory | optional |
saquery saquery-time-interval-milliseconds ]
Controller(config-wlan)#security pmf saquery-retry-time 200
the PMF parameters with the following options:
- association-comeback—Configures the 802.11w association. The
range is from 1 through 20 seconds.
- mandatory—Requires clients to
negotiate 802.11w MFP protection on a WLAN.
- optional—Enables 802.11w MFP
protection on a WLAN.
- saquery-retry-time —Time
interval identified in milliseconds in the association response to an already
associated client before the association can be tried again. This time interval
checks if the client is a real client and not a rogue client during the
association comeback time. If the client does not respond within this time, the
client association is deleted from the controller. The saquery retry time is
milliseconds. The range is from 100 to 500 ms. The value must be specified in
multiples of 100 milliseconds.
also contains a new Authenticated Key Management (AKM) type called Protected
Management Frames (PMF)
Controller(config-wlan)#security wpa akm pmf ?
dot1x Configures 802.1x support
psk Configures PSK support
cannot be enabled on WLANs of None, WEP-40, WEP-104, and WPA (AES or TKIP)
The WLC does not
have a GUI configuration for 802.11w.
show wlan name wlan-name
the WLAN parameters on the WLAN. The PMF parameters are displayed. Here is an
PMF Support :Disabled
PMF Association Comeback Timeout : 1
PMF SA Query Time : 200
wlan 11w-psk 6 11w-psk
client vlan 49
security wpa akm psk set-key ascii 0 ciscocisco
security wpa akm pmf psk
security pmf association-comeback 10
security pmf mandatory
security pmf saquery-retry-time 100
debug and Trace commands can be used to troubleshooting this feature:
Controller#debug pmf ?
all debug Protected Management Frame all
events Protected Management Frame events
keys Protected Management Frame keys
Controller#set trace pmf ?
events PMF events debugging
filter Trace Adapted Flag Filter
keys PMF keys debugging
level Trace Level