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.
https://www.bluetooth.com/security/statement-key-negotiation-of-bluetooth/
https://www.usenix.org/conference/usenixsecurity19/presentation/antonioli
http://www.cs.ox.ac.uk/publications/publication12404-abstract.html
https://www.kb.cert.org/vuls/id/918987/
http://seclists.org/fulldisclosure/2019/Aug/13
http://seclists.org/fulldisclosure/2019/Aug/14
http://seclists.org/fulldisclosure/2019/Aug/11
http://seclists.org/fulldisclosure/2019/Aug/15
http://www.huawei.com/en/psirt/security-advisories/huawei-sa-20190828-01-knob-en
https://usn.ubuntu.com/4115-1/
https://usn.ubuntu.com/4118-1/
https://lists.debian.org/debian-lts-announce/2019/09/msg00014.html
https://lists.debian.org/debian-lts-announce/2019/09/msg00015.html
https://lists.debian.org/debian-lts-announce/2019/09/msg00025.html
https://usn.ubuntu.com/4147-1/
https://access.redhat.com/errata/RHSA-2019:2975
http://lists.opensuse.org/opensuse-security-announce/2019-10/msg00037.html
http://lists.opensuse.org/opensuse-security-announce/2019-10/msg00036.html
https://access.redhat.com/errata/RHSA-2019:3055
https://access.redhat.com/errata/RHSA-2019:3076
https://access.redhat.com/errata/RHSA-2019:3089
https://access.redhat.com/errata/RHSA-2019:3187
https://access.redhat.com/errata/RHSA-2019:3217
https://access.redhat.com/errata/RHSA-2019:3218
https://access.redhat.com/errata/RHSA-2019:3165
https://access.redhat.com/errata/RHSA-2019:3220
https://access.redhat.com/errata/RHSA-2019:3231
https://access.redhat.com/errata/RHSA-2019:3309
Source: MITRE
Published: 2019-08-14
Updated: 2021-11-04
Type: CWE-327
Base Score: 4.8
Vector: AV:A/AC:L/Au:N/C:P/I:P/A:N
Impact Score: 4.9
Exploitability Score: 6.5
Severity: MEDIUM
Base Score: 8.1
Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Impact Score: 5.2
Exploitability Score: 2.8
Severity: HIGH