Operational Notification for ISC DHCP 4.4.3

Posting date: 9 March 2022

Program impacted: ISC DHCP

Versions affected: ISC DHCP client - all versions prior to 4.4.3

Description:
DHCP messages sent from a client include a transaction ID (xid). This is used to allow DHCP servers to differentiate between requests, per RFC2131, Section 4.1.

Prior to release 4.4.3, the ISC DHCP client dhclient used various pieces of system information, including unixtime, to generate an xid.

Although the xid was never intended to be used for security purposes, some environments are known to use it in that way. For an example, please see https://github.com/irsl/gcp-dhcp-takeover-code-exec.

In light of this, the generation of the xid in ISC DHCP release 4.4.3 has been altered so that the randomness will be automatically seeded from a configurable source such as /dev/random (default).

Impact:
Possible misuse of DHCP

Workarounds:
Do not use the xid as proof of identity.

Solution:
Since the workaround listed above is not effective against deliberate exploitation, ISC recommends that operators running an affected release of dhclient install release 4.4.3.

Downloads are available via the ISC download page: https://www.isc.org/download#DHCP

and release notes for the updated releases can be viewed at this link:

4.4.3: https://downloads.isc.org/isc/dhcp/4.4.3/dhcp-4.4.3-RELNOTES

Do you still have questions? Questions regarding this advisory should go to security-officer@isc.org. To report a new issue, please encrypt your message using security-officer@isc.org's PGP key which can be found here: https://www.isc.org/pgpkey/. If you are unable to use encrypted email, you may also report new issues at: https://www.isc.org/reportbug/.

Note: ISC patches only currently supported versions. When possible we indicate EOL versions affected. (For current information on which versions are actively supported, please see https://www.isc.org/download/).

ISC Security Vulnerability Disclosure Policy: Details of our current security advisory policy and practice can be found here: ISC Software Defect and Security Vulnerability Disclosure Policy.

This Knowledgebase article is the complete and official operational notification document.

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