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.
4.4.0-159.1874.4.0-1090.1014.15.0-1047.49~16.04.14.15.0-1041.434.15.0-60.67~16.04.14.15.0-1022.25~16.04.14.4.0-1118.1274.4.0-1122.1284.15.0-60.674.15.0-1047.49Exploitability
AV:AAC:LAT:PPR:NUI:NVulnerable System
VC:LVI:LVA:NSubsequent System
SC:NSI:NSA:NCVSS:4.0/AV:A/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N