The Bluetooth BR/EDR specification up to and including version 5.1 permits sufficiently low encryption key length and does not prevent an attacker from influencing the key length negotiation. This allows practical brute-force attacks (aka "KNOB") that can decrypt traffic and inject arbitrary ciphertext without the victim noticing.
Exploitability
AV:AAC:LPR:NUI:NScope
S:UImpact
C:HI:HA:N8.1/CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:NOther