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Wireless & Mobility · Advanced

Pairwise Master Key Identifier (PMKID)

A hash identifying a cached PMK, used in fast roaming and exploitable for clientless WPA2-PSK cracking.

Definition

The PMKID is an HMAC-SHA1 of the PMK, a fixed label, and the AP and client MAC addresses. Clients include it to resume a cached security association. In 2018 researchers showed that capturing the PMKID from an AP's first EAPOL message enables offline PSK cracking without a client handshake.

Exam tip

The PMKID attack needs no connected client; strong passphrases or WPA3 mitigate it.

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