CVE-2019-6475: A flaw in mirror zone validity checking can allow zone data to be spoofed

[Cathy Almond]

CVE:                 CVE-2019-6475
Document version:    2.0
Posting date:        16 October 2019
Program impacted:    BIND
Versions affected:   BIND 9.14.0 -> 9.14.6. Also releases 9.15.0 ->
                     9.15.4 of the BIND 9.15 development branch.
Severity:            Medium
Exploitable:         Remotely

Description:

   Mirror zones are a BIND feature allowing recursive servers to
   pre-cache zone data provided by other servers. A mirror zone is
   similar to a zone of type secondary, except that its data is
   subject to DNSSEC validation before being used in answers, as
   if it had been looked up via traditional recursion, and when
   mirror zone data cannot be validated, BIND falls back to using
   traditional recursion instead of the mirror zone. However, an
   error in the validity checks for the incoming zone data can allow
   an on-path attacker to replace zone data that was validated with
   a configured trust anchor with forged data of the attacker's
   choosing.

   The mirror zone feature is most often used to serve a local copy
   of the root zone. If an attacker was able to insert themselves
   into the network path between a recursive server using a mirror
   zone and a root name server, this vulnerability could then be
   used to cause the recursive server to accept a copy of falsified
   root zone data.

Impact:

   An on-path attacker who manages to successfully exploit this
   vulnerability can replace the mirrored zone (usually the root)
   with data of their own choosing, effectively bypassing DNSSEC
   protection.

CVSS Score:          5.9
CVSS Vector:         CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N.

Workarounds:

   The vulnerability can be avoided by not using mirror zones.

Active exploits:

   None known.

Solution:

   Upgrade to the patched release most closely related to your
   current version of BIND:

   +  BIND 9.14.7
   +  BIND 9.15.5

Document revision history:

   1.0 Early Notification, 09 October 2019
   2.0 Public Disclosure, 16 October 2019

Related documents:

   See our BIND 9 Security Vulnerability Matrix for a complete
   listing of security vulnerabilities and versions affected.

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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
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