CVE-2022-2928 An option refcount overflow exists in dhcpd
  • 05 Oct 2022
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CVE-2022-2928 An option refcount overflow exists in dhcpd

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Article summary

CVE: CVE-2022-2928

Document version: 2.0

Posting date: 5 October 2022

Program impacted: ISC DHCP

Versions affected:


  • 4.1-ESV-R1 -> 4.1-ESV-R16-P1
  • 4.4.0 -> 4.4.3.

Other branches of ISC DHCP (i.e., releases in the 4.0.x series or lower and releases in the 4.3.x series) are beyond their End-of-Life (EOL) and are no longer supported by ISC. From code inspection, it is probable, all versions after the introduction of lease query in ISC DHCP 3.0 are affected.

Severity: Medium


Vulnerable servers are those that are both network accessible to an attacker and configured to allow and process lease queries.


When the function "option_code_hash_lookup()" is called from "add_option()", it increases the option's "refcount" field. However, there is not a corresponding call to "option_dereference()" to decrement the "refcount" field. The function "add_option()" is only used in server responses to lease query packets. Each lease query response calls this function for several options, so eventually, the reference counters could overflow and cause the server to abort.


With a DHCP server configured with "allow leasequery;", a remote machine with access to the server can send lease queries for the same lease multiple times, leading to the "add_option()" function being repeatedly called. This could cause an option's "refcount" field to overflow and the server to abort. Internally, reference counters are integers and thus overflow at 2^31 references, so even at 1000 lease query responses per second, it would take more than three weeks to crash the server.

CVSS Score: 6.5


For more information on the Common Vulnerability Scoring System and to obtain your specific environmental score please visit:


Disable lease query on the server for DHCPv4 or restart the server periodically.

Active exploits:

We are not aware of any active exploits.


Upgrade to the patched release most closely related to your current version of ISC DHCP. These can all be downloaded from

  • 4.4.3-P1
  • 4.1-ESV-R16-P2


ISC would like to thank VictorV of Cyber Kunlun Lab for discovering and reporting this issue.

Document revision history:

1.0 Early Notification, 28 September 2022
2.0 Public Disclosure, 5 October 2022

Do you still have questions? Questions regarding this advisory should go to To report a new issue, please encrypt your message using's PGP key which can be found here: If you are unable to use encrypted email, you may also report new issues at:


ISC patches only currently supported versions. When possible we indicate EOL versions affected. (For current information on which versions are actively supported, please see

ISC Security Vulnerability Disclosure Policy:

Details of our current security advisory policy and practice can be found in the ISC Software Defect and Security Vulnerability Disclosure Policy at

The Knowledgebase article is the complete and official security advisory document.

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