A libx11 security update has been released for Ubuntu Linux 14.04 ESM and 16.04 ESM.

USN-4966-2: libx11 vulnerability

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Ubuntu Security Notice USN-4966-2
May 25, 2021

libx11 vulnerability
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A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 16.04 ESM
- Ubuntu 14.04 ESM

Summary:

libx11 could allow unintended access to services.

Software Description:
- libx11: X11 client-side library

Details:

USN-4966-1 fixed a vulnerability in libx11. This update provides
the corresponding update for Ubuntu 14.04 ESM and Ubuntu 16.04 ESM.

Original advisory details:

It was discovered that libx11 incorrectly validated certain parameter
lengths. A remote attacker could possibly use this issue to trick libx11
into emitting extra X protocol requests.

Update instructions:

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

Ubuntu 16.04 ESM:
libx11-6 2:1.6.3-1ubuntu2.2+esm1

Ubuntu 14.04 ESM:
libx11-6 2:1.6.2-1ubuntu2.1+esm2

After a standard system update you need to reboot your computer to make
all the necessary changes.

References:
  https://ubuntu.com/security/notices/USN-4966-2
  https://ubuntu.com/security/notices/USN-4966-1
CVE-2021-31535