A: mbedtls security update has been released for Debian GNU/Linux 9 LTS.

DLA 2826-1: mbedtls security update

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Debian LTS Advisory DLA-2826-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Emilio Pozuelo Monfort
November 23, 2021   https://wiki.debian.org/LTS
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Package : mbedtls
Version : 2.4.2-1+deb9u4
CVE ID : CVE-2018-9988 CVE-2018-9989 CVE-2020-36475 CVE-2020-36476
CVE-2020-36478 CVE-2021-24119

Several vulnerabilities were discovered in mbed TLS, a lightweight crypto
and SSL/TLS library, which could result in denial of service, information
disclosure or side-channel attacks.

For Debian 9 stretch, these problems have been fixed in version
2.4.2-1+deb9u4.

We recommend that you upgrade your mbedtls packages.

For the detailed security status of mbedtls please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/mbedtls

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://wiki.debian.org/LTS