A c-ares security update has been released for Ubuntu Linux 16.04 ESM.

USN-5034-2: c-ares vulnerability

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Ubuntu Security Notice USN-5034-2
August 10, 2021

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

- Ubuntu 16.04 ESM

Summary:

c-ares could be made to return wrong domains.

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

Details:

USN-5034-1 fixed a vulnerability in c-ares. This update provides
the corresponding update for Ubuntu 16.04 ESM.

Original advisory 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 16.04 ESM:
libc-ares2 1.10.0-3ubuntu0.2+esm1

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

References:
  https://ubuntu.com/security/notices/USN-5034-2
  https://ubuntu.com/security/notices/USN-5034-1
CVE-2021-3672