A c-ares security update has been released for Ubuntu Linux 18.04 LTS, 20.04 LTS and 21.04.

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Ubuntu Security Notice USN-5034-1
August 10, 2021

c-ares vulnerability
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A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 21.04
- Ubuntu 20.04 LTS
- Ubuntu 18.04 LTS

Summary:

c-ares could be made to return wrong domains.

Software Description:
- c-ares: library for asynchronous name resolution

Details:

Philipp Jeitner and Haya Shulman discovered that c-ares incorrectly
validated certain hostnames returned by DNS servers. A remote attacker
could possibly use this issue to perform Domain Hijacking attacks.

Update instructions:

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

Ubuntu 21.04:
libc-ares2 1.17.1-1ubuntu0.1

Ubuntu 20.04 LTS:
libc-ares2 1.15.0-1ubuntu0.1

Ubuntu 18.04 LTS:
libc-ares2 1.14.0-1ubuntu0.1

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

References:
  https://ubuntu.com/security/notices/USN-5034-1
CVE-2021-3672

Package Information:
  https://launchpad.net/ubuntu/+source/c-ares/1.17.1-1ubuntu0.1
  https://launchpad.net/ubuntu/+source/c-ares/1.15.0-1ubuntu0.1
  https://launchpad.net/ubuntu/+source/c-ares/1.14.0-1ubuntu0.1