To the packagers and redistributors of ISC DHCP:

ISC would like to make you aware of an upcoming security disclosure,
scheduled for Wednesday May 26, 2021, covering one high-severity
ISC DHCP vulnerability.  Please consider this information confidential
and under embargo until ISC publicly announces the vulnerability
on the disclosure date.

The vulnerability is:

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

Although updated packages should not be released until ISC discloses
this vulnerability on the 26th, early access is being provided to
the security patch release so that packagers can have updated offerings
available quickly after public disclosure.

Maintainers who prefer to selectively apply the patch fix to a different
base version than the (until now) most recent stable releases in the
two supported ISC DHCP release branches (i.e. 4.1-ESV or 4.4)
can try their luck with the patch diffs they will find in the "patches"
subdirectory of the release directories listed below.  The patches
apply cleanly and are tested as effective in patching ISC DHCP
4.1-ESV-R16 and 4.4.2 (and are tested against those releases ONLY.
We expect that the patches will be useful against earlier base versions
but do not have resources to test.)

New releases of ISC DHCP which correct the vulnerability are available via:

  4.1-ESV-R16-P1: https://downloads.isc.org/isc/dhcp/private/faaa35f1cb9dc2e8
  4.4.2-P1: https://downloads.isc.org/isc/dhcp/private/3089a1ebd20a802e

Michael McNally
ISC Security Officer

---

CVE:                 CVE-2021-25217
Document version:    1.1
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 as well as 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

Description:

   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.

Impact:

   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:
https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:P/RL:O/RC:C&version=3.1.

Workarounds:

   None known.

Active exploits:

   We are not aware of any active exploits.

Solution:

   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

Document revision history:

   1.0 Early Notification, 05 May 2021
   1.1 Advance Security Notification, 19 May 2021

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 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.

Legal Disclaimer:

   Internet Systems Consortium (ISC) is providing this notice on
   an "AS IS" basis. No warranty or guarantee of any kind is expressed
   in this notice and none should be implied. ISC expressly excludes
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   time. A stand-alone copy or paraphrase of the text of this
   document that omits the document URL is an uncontrolled copy.
   Uncontrolled copies may lack important information, be out of
   date, or contain factual errors.



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