An amd64-microcode security update has been released for Debian GNU/Linux 9 LTS to add amd-microcode-based IBPB support.

DLA 2743-1: amd64-microcode security update

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Debian LTS Advisory DLA-2743-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Utkarsh Gupta
August 16, 2021   https://wiki.debian.org/LTS
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Package : amd64-microcode
Version : 3.20181128.1~deb9u1
CVE ID : CVE-2017-5715
Debian Bug : 886382

It was discovered that systems with microprocessors utilizing
speculative execution and indirect branch prediction may allow
unauthorized disclosure of information to an attacker with local
user access via a side-channel analysis (Spectre v2).
Multiple fixes were done already in Linux kernel, intel-microcode,
et al. This fix adds amd-microcode-based IBPB support.

For Debian 9 stretch, this problem has been fixed in version
3.20181128.1~deb9u1.

We recommend that you upgrade your amd64-microcode packages.

For the detailed security status of amd64-microcode please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/amd64-microcode

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