When using WPA2, EAPOL-Key frames with the Encrypted flag and without the MIC flag set, the data field was decrypted first without verifying the MIC. When the dta field was encrypted using RC4, for example, when negotiating TKIP as a pairwise cipher, the unauthenticated but decrypted data was subsequently processed. This opened wpa_supplicant(8) to abuse by decryption and recovery of sensitive information contained in EAPOL-Key messages.
See
https://w1.fi/security/2018-1/unauthenticated-eapol-key-decryption.txt for a detailed description of the bug.