CVE-2020-36423: 
Mbed TLS vulnerability analysis and mitigation

A vulnerability (CVE-2020-36423) was discovered in Arm Mbed TLS versions before 2.23.0. The issue relates to an ineffective Lucky 13 countermeasure that fails to properly consider hardware accelerator cases. This vulnerability was reported by Luc Perneel and was disclosed in July 2021 (Mbed Release, NVD).

The vulnerability stems from a flaw in the Lucky 13 countermeasure implementation when hardware accelerators are used (specifically when using MBEDTLSSHAxxxALT macros). The issue has been assigned a CVSS v3.1 base score of 7.5 (HIGH), with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N, indicating high confidentiality impact with network vector and low attack complexity (NVD).

When exploited, this vulnerability allows an active network attacker to recover plaintext after repeated timing measurements under specific conditions. The vulnerability is particularly impactful when hardware accelerators are used, as it renders the Lucky 13 countermeasure ineffective (Mbed Release).

The vulnerability has been fixed in Mbed TLS version 2.23.0 and version 2.16.7 for the LTS branch. Users are recommended to upgrade to these or later versions to address the issue. Debian has also released security updates to address this vulnerability in their distributions (Debian Advisory).