802.11 Security Architecture and Frame Protection
This domain tests the frame-level plumbing of WLAN security: which frames are protected by which mechanism, how the RSN key hierarchy is built, and where Protected Management Frames fit. Expect questions that describe a captured frame exchange or an attack symptom and ask what protection should have prevented it.
Frame classes and their protection
- Management frames (beacon, probe request and response, authentication, association, action) drive state but carry no user data. In legacy 802.11 they were entirely forgeable.
- Control frames (RTS, CTS, ACK, Block Ack) are unprotected and are used for denial-of-service rather than data theft.
- Data frames are protected after key establishment: unicast by the pairwise temporal key, group-addressed traffic by the GTK.
- 802.11w Protected Management Frames protects disassociation, deauthentication, and designated robust action frames. Unicast robust management frames are protected with the pairwise key; group robust action frames use BIP-CMAC-128 with an IGTK. PMF supplies integrity only, never confidentiality. PMF is mandatory for WPA3 and for operation in the 6 GHz band.
RSN key hierarchy
The 802.1X exchange (or the PSK or SAE exchange) produces a 256-bit PMK. The 4-way handshake derives the PTK from the PMK, both nonces, and both MAC addresses, and splits it into the KCK, KEK, and TK. The GTK is delivered encrypted under the KEK in message three, with the IGTK for management frame protection. PMK caching and fast BSS transition allow roaming without full reauthentication, which is a performance feature with security consequences if the cache is not tightly scoped.
Cipher suites
- TKIP: RC4, 48-bit TSC, Michael MIC, per-packet key mixing. Deprecated and never acceptable in new designs.
- CCMP: AES-128 in CCM mode, 48-bit packet number, 8-octet MIC, 8-byte header.
- GCMP: AES-GCM, introduced with 802.11ac and used in 256-bit form for WPA3-Enterprise 192-bit mode; one key provides both confidentiality and integrity.
Worked example
An analyst sees repeated deauthentication frames from an unknown BSSID and clients dropping. Without PMF, the frames are honored and the attack is trivial. With PMF required, forged deauthentication fails the integrity check and is discarded, forcing the attacker to jam instead, which is louder and easier to locate. Remediation: enable PMF in required mode where the client population supports it, and keep deauthentication floods as a monitored indicator.
Traps that cost marks
- Claiming PMF encrypts management frames.
- Swapping GTK and PTK roles.
- Assuming GCMP replaced CCMP as the baseline; CCMP remains the mandatory common denominator.
- Forgetting that beacons and probe responses remain unprotected even with PMF enabled.