CVE-2021-25217: A buffer overrun in lease file parsing code can be used to exploit a common vulnerability shared by dhcpd and dhclient
mcnally at isc.org
Wed May 26 21:20:18 UTC 2021
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.
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
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
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.
Against dhclient (the ISC DHCP client): 7.4
Against dhcpd (the ISC DHCP server): 6.5
For more information on the Common Vulnerability Scoring System and
to obtain your specific environmental score, please visit:
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
Do you still have questions? Questions regarding this advisory
should go to security-officer at isc.org. To report a new issue, please
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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 https://kb.isc.org/docs/aa-00861.
The Knowledgebase article https://kb.isc.org/docs/cve-2021-25217
is the complete and official security advisory document.
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