The following two updates has been released for Ubuntu Linux:

USN-3961-1: Dovecot vulnerabilities
USN-3962-1: libpng vulnerability

USN-3961-1: Dovecot vulnerabilities

==========================================================================
Ubuntu Security Notice USN-3961-1
April 30, 2019

dovecot vulnerabilities
==========================================================================

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

- Ubuntu 19.04
- Ubuntu 18.10

Summary:

Dovecot could be made to crash if it received specially crafted network
traffic.

Software Description:
- dovecot: IMAP and POP3 email server

Details:

It was discovered that the Dovecot Submission login service incorrectly
handled certain operations. A remote attacker could possibly use this issue
to cause Dovecot to crash, resulting in a denial of service.

Update instructions:

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

Ubuntu 19.04:
dovecot-core 1:2.3.4.1-1ubuntu2.2
dovecot-submissiond 1:2.3.4.1-1ubuntu2.2

Ubuntu 18.10:
dovecot-core 1:2.3.2.1-1ubuntu3.4
dovecot-submissiond 1:2.3.2.1-1ubuntu3.4

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

References:
https://usn.ubuntu.com/usn/usn-3961-1
CVE-2019-11494, CVE-2019-11499

Package Information:
https://launchpad.net/ubuntu/+source/dovecot/1:2.3.4.1-1ubuntu2.2
https://launchpad.net/ubuntu/+source/dovecot/1:2.3.2.1-1ubuntu3.4

USN-3962-1: libpng vulnerability

==========================================================================
Ubuntu Security Notice USN-3962-1
April 30, 2019

libpng1.6 vulnerability
==========================================================================

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

- Ubuntu 18.10
- Ubuntu 18.04 LTS

Summary:

libpng be made to crash or run programs if it opened a specially crafted
file.

Software Description:
- libpng1.6: PNG (Portable Network Graphics) file library

Details:

It was discovered that libpng incorrectly handled certain memory
operations. If a user or automated system were tricked into opening a
specially crafted PNG file, a remote attacker could use this issue to
cause libpng to crash, resulting in a denial of service, or possibly
execute arbitrary code.

Update instructions:

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

Ubuntu 18.10:
libpng16-16 1.6.34-2ubuntu0.1

Ubuntu 18.04 LTS:
libpng16-16 1.6.34-1ubuntu0.18.04.2

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

References:
https://usn.ubuntu.com/usn/usn-3962-1
CVE-2019-7317

Package Information:
https://launchpad.net/ubuntu/+source/libpng1.6/1.6.34-2ubuntu0.1
https://launchpad.net/ubuntu/+source/libpng1.6/1.6.34-1ubuntu0.18.04.2