CVE-2021-25220: DNS forwarders - cache poisoning vulnerability

[Everett B. Fulton]

CVE: CVE-2021-25220
Document version: 2.0
Posting date: 16 March 2022
Program impacted: BIND
Versions affected:
    BIND
      9.11.0 -> 9.11.36
      9.12.0 -> 9.16.26
      9.17.0 -> 9.18.0
    BIND Supported Preview Editions:
      9.11.4-S1 -> 9.11.36-S1
      9.16.8-S1 -> 9.16.26-S1
    Versions of BIND 9 earlier than those shown - back to 9.1.0, including
    Supported Preview Editions - are also believed to be affected but have
    not been tested as they are EOL.
Severity: Medium
Exploitable: Remotely

Description:

    When using forwarders, bogus NS records supplied by, or via, those
    forwarders may be cached and used by named if it needs to recurse for
    any reason, causing it to obtain and pass on potentially incorrect
    answers.

    Some examples of configurations that will be vulnerable are:

    * Resolvers using per zone or global forwarding with `forward first`
      (`forward first` is the default).
    * Resolvers not using global forwarding, but with per-zone forwarding
      with either `forward first` (the default) or `forward only`.
    * Resolvers configured with global forwarding along with zone statements
      that disable forwarding for part of the DNS namespace.

    Authoritative-only BIND 9 servers are not vulnerable to this flaw.

Impact:

    The cache could become poisoned with incorrect records leading to
    queries being made to the wrong servers, which might also result in
    false information being returned to clients.

CVSS Score: 6.2

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:N/I:H/A:N/E:U/RL:U/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:N/AC:L/PR:H/UI:N/S:C/C:N/I:H/A:N/E:U/RL:U/RC:C&version=3.1

Workarounds:

    If applicable, modify your configuration to either remove all
    forwarding or all possibility of recursion. Depending on your use-case,
    it may be possible to use other zone types to replace forward zones.

Active exploits:

    We are not aware of any active exploits.

Solution:

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

      BIND 9.11.37
      BIND 9.16.27
      BIND 9.18.1

    BIND Supported Preview Edition is a special feature preview branch of
    BIND provided to eligible ISC support customers.

      BIND 9.11.37-S1
      BIND 9.16.27-S1

Acknowledgments:

    ISC would like to thank Xiang Li, Baojun Liu, and Chaoyi Lu from
    Network and Information Security Lab, Tsinghua University and Changgen
    Zou from Qi An Xin Group Corp. for discovering and reporting this issue.

Document revision history:

    1.0 Early Notification, 9 March 2022
    1.1 Correction to official document link, 16 March 2022
    2.0 Public disclosure, 16 March 2022

Related documents:

See our BIND 9 Security Vulnerability Matrix for a complete listing of
security vulnerabilities and versions affected:
https://kb.isc.org/docs/aa-00913

Do you still have questions? Questions regarding this advisory should go
to security-officer at isc.org. To report a new issue, please encrypt your
message using security-officer at 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-2021-25220 is the
complete and official security advisory document.

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