An Avahi security update has been released for Ubuntu Linux 18.04 LTS, 20.04 LTS, 20.10, and 21.04.

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Ubuntu Security Notice USN-5008-1
July 07, 2021

avahi vulnerabilities
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A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 21.04
- Ubuntu 20.10
- Ubuntu 20.04 LTS
- Ubuntu 18.04 LTS

Summary:

Several security issues were fixed in Avahi.

Software Description:
- avahi: IPv4LL network address configuration daemon

Details:

Thomas Kremer discovered that Avahi incorrectly handled termination signals
on the Unix socket. A local attacker could possibly use this issue to cause
Avahi to hang, resulting in a denial of service. This issue only affected
Ubuntu 18.04 LTS, Ubuntu 20.04 LTS, and Ubuntu 20.10. (CVE-2021-3468)

It was discovered that Avahi incorrectly handled certain hotnames. A local
attacker could possibly use this issue to cause Avahi to crash, resulting
in a denial of service. This issue only affected Ubuntu 20.10 and Ubuntu
21.04. (CVE-2021-3502)

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 21.04:
avahi-daemon 0.8-5ubuntu3.1
libavahi-core7 0.8-5ubuntu3.1

Ubuntu 20.10:
avahi-daemon 0.8-3ubuntu1.1
libavahi-core7 0.8-3ubuntu1.1

Ubuntu 20.04 LTS:
avahi-daemon 0.7-4ubuntu7.1
libavahi-core7 0.7-4ubuntu7.1

Ubuntu 18.04 LTS:
avahi-daemon 0.7-3.1ubuntu1.3
libavahi-core7 0.7-3.1ubuntu1.3

In general, a standard system update will make all the necessary changes.

References:
  https://ubuntu.com/security/notices/USN-5008-1
CVE-2021-3468, CVE-2021-3502

Package Information:
  https://launchpad.net/ubuntu/+source/avahi/0.8-5ubuntu3.1
  https://launchpad.net/ubuntu/+source/avahi/0.8-3ubuntu1.1
  https://launchpad.net/ubuntu/+source/avahi/0.7-4ubuntu7.1
  https://launchpad.net/ubuntu/+source/avahi/0.7-3.1ubuntu1.3