PSIRT Advisory

PMKID attack on WPA/WPA2 WiFi networks


Makers of popular wifi hacking tool hashcat have discovered a way to improve WPA/WPA2 password brute-forcing: Leveraging the PMKID served by Access Points in WPA/WPA2 enabled WiFi networks, attackers gain knowledge of a Pre-shared Key hash, which can be used to brute-force the WPA/WPA2 password.

This however requires a set of conditions to work:

o WPA/WPA2 must be "Personal security (Pre-shared Key)". Other types (eg: Enterprise) are not vulnerable

o The PMKID must be included in the first EAPOL message of the 4 way handshake

o 802.11r and PMKID caching must be enabled


Information Disclosure

Affected Products

FortiOS supports WPA/WPA2 WiFi and is only affected under special configurations [1]

FortiAP supports WPA/WPA2 WiFi and is only affected under special configurations [1]

FortiAnalyzer is not affected

FortiSwitch is not affected

[1] when 802.11r/fast-bss-transition is enabled and security is set to wpa2-only-personal.


Since this is a protocol level attack facilitating brute-force cracking, there exists mitigation to disable it altogether, or drastically lower its practical feasibility:

1. When enabling the 801.11r/fast-bss-transition feature on FortiOS/FortiAP, avoid using wpa2-only-personal security, and use wpa2-only-enterprise instead. This effectively prevents the attack completely.

2. If the above is not acceptable given the environment, a minimum of 12 high-entropy random ASCII characters should be used as the password (with 20 characters being preferable). This renders the attack unpractical in the current state of computing power available for brute-force cracking.

Starting with FortiOS 6.2.1, the following CLI commands (for pre-shared key for WPA/WPA2-Personal SSID) will ask to input at least 12 bytes in length when editing the [vap-name] if the existing passphrase is shorter than 12 bytes:

config wireless-controller vap
edit [vap-name]
set passphrase [psk] /* minimum 12 bytes psk when wfa-compatibility is disabled [1] */

[1] To compatible with WPA3™-SAE Test Plan Version 1.0, a new CLI were introduced at same time to allow minimum 8 bytes pre-shared key which the default value is disabled (enable is not suggested in production environment).

config wireless-controller setting
    set wfa-compatibility enable /* disable is the default value */

Revision History:

07-23-2019 pre-shared key length for WPA/WPA2 personal SSID by default needs at least 12 bytes
09-10-2018 Initial version