A DHCP security update has been released for Ubuntu Linux 18.04 LTS, 20.04 LTS, 20.10, and 21.04.

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Ubuntu Security Notice USN-4969-1
May 27, 2021

isc-dhcp vulnerability
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A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 21.04
- Ubuntu 20.10
- Ubuntu 20.04 LTS
- Ubuntu 18.04 LTS

Summary:

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

Software Description:
- isc-dhcp: DHCP server and client

Details:

Jon Franklin and Pawel Wieczorkiewicz discovered that DHCP incorrectly
handled lease file parsing. A remote attacker could possibly use this issue
to cause DHCP 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 21.04:
isc-dhcp-client 4.4.1-2.2ubuntu6.1
isc-dhcp-server 4.4.1-2.2ubuntu6.1

Ubuntu 20.10:
isc-dhcp-client 4.4.1-2.1ubuntu10.1
isc-dhcp-server 4.4.1-2.1ubuntu10.1

Ubuntu 20.04 LTS:
isc-dhcp-client 4.4.1-2.1ubuntu5.20.04.2
isc-dhcp-server 4.4.1-2.1ubuntu5.20.04.2

Ubuntu 18.04 LTS:
isc-dhcp-client 4.3.5-3ubuntu7.3
isc-dhcp-server 4.3.5-3ubuntu7.3

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

References:
  https://ubuntu.com/security/notices/USN-4969-1
CVE-2021-25217

Package Information:
  https://launchpad.net/ubuntu/+source/isc-dhcp/4.4.1-2.2ubuntu6.1
  https://launchpad.net/ubuntu/+source/isc-dhcp/4.4.1-2.1ubuntu10.1
  https://launchpad.net/ubuntu/+source/isc-dhcp/4.4.1-2.1ubuntu5.20.04.2
  https://launchpad.net/ubuntu/+source/isc-dhcp/4.3.5-3ubuntu7.3