CVE-2021-25217: A buffer overrun in lease file parsing code can be used to exploit a common vulnerability shared by dhcpd and dhclient
  • 26 May 2021
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CVE-2021-25217: A buffer overrun in lease file parsing code can be used to exploit a common vulnerability shared by dhcpd and dhclient

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Article summary

CVE: CVE-2021-25217

Document version: 2.0

Posting date: 26 May 2021

Program impacted: ISC DHCP. Due to shared code, multiple components are affected; please continue reading for details.

Versions affected: ISC DHCP 4.1-ESV-R1 -> 4.1-ESV-R16, ISC DHCP 4.4.0 -> 4.4.2.
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 no longer supported by ISC. From inspection it is clear that the defect is also present in releases from those series, but they have not been officially tested for the vulnerability.

Severity: High

Exploitable: Remotely


Program code used by the ISC DHCP package to read and parse stored leases has a defect that can be exploited by an attacker to cause one of several undesirable outcomes, depending on the component attacked and the way in which it was compiled.

Because of a discrepancy between the code which handles encapsulated option information in leases transmitted "on the wire" and the code which reads and parses lease information after it has been written to disk storage, it is potentially possible for an attacker to deliberately cause a situation where:

  • dhcpd, while running in DHCPv4 or DHCPv6 mode, or
  • dhclient, the ISC DHCP client implementation

will attempt to read a stored lease that contains option information which will trigger a bug in the option parsing code.


The outcome of encountering the defect while reading a lease that will trigger it varies, according to:

  • the component being affected (i.e., dhclient or dhcpd)
  • whether the package was built as a 32-bit or 64-bit binary
  • whether the compiler flag -fstack-protection-strong was used when compiling

In dhclient, ISC has not successfully reproduced the error on a 64-bit system. However, on a 32-bit system it is possible to cause dhclient to crash when reading an improper lease, which could cause network connectivity problems for an affected system due to the absence of a running DHCP client process.

In dhcpd, when run in DHCPv4 or DHCPv6 mode:

  • if the dhcpd server binary was built for a 32-bit architecture AND the -fstack-protection-strong flag was specified to the compiler, dhcpd may exit while parsing a lease file containing an objectionable lease, resulting in lack of service to clients. Additionally, the offending lease and the lease immediately following it in the lease database may be improperly deleted.
  • if the dhcpd server binary was built for a 64-bit architecture OR if the -fstack-protection-strong compiler flag was NOT specified, the crash will not occur, but it is possible for the offending lease and the lease which immediately followed it to be improperly deleted.

CVSS Scores:

Against dhclient, the ISC DHCP client: 7.4
Against dhcpd, the ISC DHCP server: 6.5

CVSS Vectors:

dhclient: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:P/RL:O/RC:C

dhcpd: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:P/RL:O/RC:C

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


None known.

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:

  • ISC DHCP 4.1-ESV-R16-P1
  • ISC DHCP 4.4.2-P1


ISC would like to thank Jon Franklin from Dell and Pawel Wieczorkiewicz from Amazon Web Services for (independently) reporting this vulnerability.

Document revision history:

1.0 Early Notification, 05 May 2021
1.1 Advance Security Notification, 19 May 2021
2.0 Public Disclosure, 26 May 2021

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