Updated libx11 packages has been released for Ubuntu Linux:

USN-3758-1: libx11 vulnerabilities
USN-3758-2: libx11 vulnerabilities

USN-3758-1: libx11 vulnerabilities

==========================================================================
Ubuntu Security Notice USN-3758-1
August 30, 2018

libx11 vulnerabilities
==========================================================================

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

- Ubuntu 18.04 LTS
- Ubuntu 16.04 LTS
- Ubuntu 14.04 LTS

Summary:

Several security issues were fixed in libx11.

Software Description:
- libx11: X11 client-side library

Details:

Tobias Stoeckmann discovered that libx11 incorrectly handled certain
images. An attacker could possibly use this issue to access sensitive
information (CVE-2016-7942)

Tobias Stoeckmann discovered that libx11 incorrectly handled certain
inputs. An attacker could possibly use this issue to access sensitive
information. (CVE-2016-7943)

It was discovered that libx11 incorrectly handled certain inputs.
An attacker could possibly use this issue to cause a denial of service.
(CVE-2018-14598, CVE-2018-14599, CVE-2018-14600)

Update instructions:

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

Ubuntu 18.04 LTS:
  libx11-6 2:1.6.4-3ubuntu0.1
  libx11-dev 2:1.6.4-3ubuntu0.1

Ubuntu 16.04 LTS:
  libx11-6 2:1.6.3-1ubuntu2.1
  libx11-dev 2:1.6.3-1ubuntu2.1

Ubuntu 14.04 LTS:
  libx11-6 2:1.6.2-1ubuntu2.1
  libx11-dev 2:1.6.2-1ubuntu2.1

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

References:
  https://usn.ubuntu.com/usn/usn-3758-1
  CVE-2016-7942, CVE-2016-7943, CVE-2018-14598, CVE-2018-14599,
  CVE-2018-14600

Package Information:
  https://launchpad.net/ubuntu/+source/libx11/2:1.6.4-3ubuntu0.1
  https://launchpad.net/ubuntu/+source/libx11/2:1.6.3-1ubuntu2.1
  https://launchpad.net/ubuntu/+source/libx11/2:1.6.2-1ubuntu2.1

USN-3758-2: libx11 vulnerabilities

==========================================================================
Ubuntu Security Notice USN-3758-2
August 30, 2018

libx11 vulnerabilities
==========================================================================

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

- Ubuntu 12.04 ESM

Summary:

Several security issues were fixed in libx11.

Software Description:
- libx11: X11 client-side library

Details:

USN-3758-1 fixed several vulnerabilities in libx11. This update
provides the corresponding update for Ubuntu 12.04 ESM.

Original advisory details:

 Tobias Stoeckmann discovered that libx11 incorrectly handled certain
 images. An attacker could possibly use this issue to access sensitive
 information (CVE-2016-7942)

 Tobias Stoeckmann discovered that libx11 incorrectly handled certain
 inputs. An attacker could possibly use this issue to access sensitive
 information. (CVE-2016-7943)

 It was discovered that libx11 incorrectly handled certain inputs.
 An attacker could possibly use this issue to cause a denial of
 service. (CVE-2018-14598, CVE-2018-14599, CVE-2018-14600)

Update instructions:

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

Ubuntu 12.04 ESM:
  libx11-6 2:1.4.99.1-0ubuntu2.4
  libx11-dev 2:1.4.99.1-0ubuntu2.4

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

References:
  https://usn.ubuntu.com/usn/usn-3758-2
  https://usn.ubuntu.com/usn/usn-3758-1
  CVE-2016-7942, CVE-2016-7943, CVE-2018-14598, CVE-2018-14599,
  CVE-2018-14600