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.
http://lists.opensuse.org/opensuse-security-announce/2019-10/msg00036.html
http://lists.opensuse.org/opensuse-security-announce/2019-10/msg00037.html
https://access.redhat.com/errata/RHSA-2019:2975
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-2019:3165
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-2019:3220
https://access.redhat.com/errata/RHSA-2019:3231
https://access.redhat.com/errata/RHSA-2019:3309
https://access.redhat.com/errata/RHSA-2019:3517
https://access.redhat.com/errata/RHSA-2020:0204
http://seclists.org/fulldisclosure/2019/Aug/11
http://seclists.org/fulldisclosure/2019/Aug/13
http://seclists.org/fulldisclosure/2019/Aug/14
http://seclists.org/fulldisclosure/2019/Aug/15
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/4115-1/
https://usn.ubuntu.com/4118-1/
https://usn.ubuntu.com/4147-1/
https://www.kb.cert.org/vuls/id/918987/
http://www.huawei.com/en/psirt/security-advisories/huawei-sa-20190828-01-knob-en