A bind9 security update has been released for both Debian GNU/Linux 9 and 10 to address several vulnerabilities.

DSA 4689-1: bind9 security update

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Debian Security Advisory DSA-4689-1 security@debian.org
  https://www.debian.org/security/ Salvatore Bonaccorso
May 19, 2020   https://www.debian.org/security/faq
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Package : bind9
CVE ID : CVE-2019-6477 CVE-2020-8616 CVE-2020-8617
Debian Bug : 945171

Several vulnerabilities were discovered in BIND, a DNS server
implementation.

CVE-2019-6477

It was discovered that TCP-pipelined queries can bypass tcp-client
limits resulting in denial of service.

CVE-2020-8616

It was discovered that BIND does not sufficiently limit the number
of fetches performed when processing referrals. An attacker can take
advantage of this flaw to cause a denial of service (performance
degradation) or use the recursing server in a reflection attack with
a high amplification factor.

CVE-2020-8617

It was discovered that a logic error in the code which checks TSIG
validity can be used to trigger an assertion failure, resulting in
denial of service.

For the oldstable distribution (stretch), these problems have been fixed
in version 1:9.10.3.dfsg.P4-12.3+deb9u6.

For the stable distribution (buster), these problems have been fixed in
version 1:9.11.5.P4+dfsg-5.1+deb10u1.

We recommend that you upgrade your bind9 packages.

For the detailed security status of bind9 please refer to its security
tracker page at:
  https://security-tracker.debian.org/tracker/bind9

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