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.
5.13.0-19.195.3.0-18.195.3.0-24.265.4.0-9.124.13.0-16.194.13.0-17.204.13.0-25.294.13.0-32.354.15.0-10.114.15.0-12.134.15.0-13.144.15.0-15.164.15.0-19.204.15.0-20.21+24 more4.15.0-60.673.11.0-12.193.12.0-1.33.12.0-2.53.12.0-2.73.12.0-3.83.12.0-3.93.12.0-4.103.12.0-4.123.12.0-5.133.12.0-7.15+170 more4.2.0-16.194.2.0-17.214.2.0-19.234.3.0-1.104.3.0-2.114.3.0-5.164.3.0-6.174.3.0-7.184.4.0-10.254.4.0-101.124+85 more4.4.0-159.1876.5.0-9.96.11.0-8.85.13.0-1005.64.4.0-1001.104.4.0-1003.124.4.0-1004.134.4.0-1007.164.4.0-1009.184.4.0-1011.204.4.0-1012.214.4.0-1013.224.4.0-1016.254.4.0-1017.26+44 more4.4.0-1090.1016.5.0-1008.8Exploitability
AV:AAC:LPR:NUI:NScope
S:UImpact
C:HI:HA:NCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N