A LibVNCServer security update has been released for Ubuntu Linux 20.04 LTS, 18.04 LTS, and 16.04 LTS to address several security issues in LibVNCServer.

USN-4434-1: LibVNCServer vulnerabilities

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Ubuntu Security Notice USN-4434-1
July 23, 2020

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

- Ubuntu 20.04 LTS
- Ubuntu 18.04 LTS
- Ubuntu 16.04 LTS

Summary:

Several security issues were fixed in LibVNCServer.

Software Description:
- libvncserver: vnc server library

Details:

Ramin Farajpour Cami discovered that LibVNCServer incorrectly handled
certain malformed unix socket names. A remote attacker could exploit this
with a crafted socket name, leading to a denial of service, or possibly
execute arbitrary code. (CVE-2019-20839)

It was discovered that LibVNCServer did not properly access byte-aligned
data. A remote attacker could possibly use this issue to cause
LibVNCServer to crash, resulting in a denial of service. This issue only
affected Ubuntu 18.04 LTS and Ubuntu 16.04 LTS. (CVE-2019-20840)

Christian Beier discovered that LibVNCServer incorrectly handled anonymous
TLS connections. A remote attacker could possibly use this issue to cause
LibVNCServer to crash, resulting in a denial of service. This issue only
affected Ubuntu 20.04 LTS. (CVE-2020-14396)

It was discovered that LibVNCServer incorrectly handled region clipping. A
remote attacker could possibly use this issue to cause LibVNCServer to
crash, resulting in a denial of service. (CVE-2020-14397)

It was discovered that LibVNCServer did not properly reset incorrectly
terminated TCP connections. A remote attacker could possibly use this
issue to cause an infinite loop, resulting in a denial of service.
(CVE-2020-14398)

It was discovered that LibVNCServer did not properly access byte-aligned
data. A remote attacker could possibly use this issue to cause
LibVNCServer to crash, resulting in a denial of service. (CVE-2020-14399,
CVE-2020-14400)

It was discovered that LibVNCServer incorrectly handled screen scaling on
the server side. A remote attacker could use this issue to cause
LibVNCServer to crash, resulting in a denial of service, or possibly
execute arbitrary code. (CVE-2020-14401)

It was discovered that LibVNCServer incorrectly handled encodings. A
remote attacker could use this issue to cause LibVNCServer to crash,
resulting in a denial of service, or possibly execute arbitrary code.
(CVE-2020-14402, CVE-2020-14403, CVE-2020-14404)

It was discovered that LibVNCServer incorrectly handled TextChat messages.
A remote attacker could possibly use this issue to cause LibVNCServer to
crash, resulting in a denial of service. (CVE-2020-14405)

Update instructions:

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

Ubuntu 20.04 LTS:
libvncclient1 0.9.12+dfsg-9ubuntu0.2
libvncserver1 0.9.12+dfsg-9ubuntu0.2

Ubuntu 18.04 LTS:
libvncclient1 0.9.11+dfsg-1ubuntu1.3
libvncserver1 0.9.11+dfsg-1ubuntu1.3

Ubuntu 16.04 LTS:
libvncclient1 0.9.10+dfsg-3ubuntu0.16.04.5
libvncserver1 0.9.10+dfsg-3ubuntu0.16.04.5

After a standard system update you need to restart LibVNCServer
applications to make all the necessary changes.

References:
  https://usn.ubuntu.com/4434-1
CVE-2019-20839, CVE-2019-20840, CVE-2020-14396, CVE-2020-14397,
CVE-2020-14398, CVE-2020-14399, CVE-2020-14400, CVE-2020-14401,
CVE-2020-14402, CVE-2020-14403, CVE-2020-14404, CVE-2020-14405

Package Information:
  https://launchpad.net/ubuntu/+source/libvncserver/0.9.12+dfsg-9ubuntu0.2
  https://launchpad.net/ubuntu/+source/libvncserver/0.9.11+dfsg-1ubuntu1.3
  https://launchpad.net/ubuntu/+source/libvncserver/0.9.10+dfsg-3ubuntu0.16.04.5