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             AUSCERT External Security Bulletin Redistribution

      CVE-2021-25220: DNS forwarders - cache poisoning vulnerability
                               17 March 2022


        AusCERT Security Bulletin Summary

Product:           Bind
Publisher:         ISC
Operating System:  UNIX variants (UNIX, Linux, OSX)
Resolution:        Patch/Upgrade
CVE Names:         CVE-2021-25220  

Original Bulletin: 

Comment: CVSS (Max):  6.2 CVE-2021-25220 (CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:N/I:H/A:N)
         CVSS Source: ISC
         Calculator:  https://www.first.org/cvss/calculator/3.1#CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:N/I:H/A:N

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CVE: CVE-2021-25220
Document version: 2.0
Posting date: 16 March 2022
Program impacted: BIND
Versions affected:
      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


    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

    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.


    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


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


    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.


    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


    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:

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/.


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

The Knowledgebase article https://kb.isc.org/docs/cve-2021-25220 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 and disclaims any
warranties regarding this notice or materials referred to in this 
notice, including, without limitation, any implied warranty of 
merchantability, fitness for a particular purpose, absence of hidden 
defects, or of non-infringement. Your use or reliance on this notice or 
materials referred to in this notice is at your own risk. ISC may change 
this notice at any 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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