CVE-2021-25219: Lame cache can be abused to severely degrade resolver performance

[Everett B. Fulton]

CVE: CVE-2021-25219

Document version: 2.0
Posting date: 27 October 2021
Program impacted: BIND
Versions affected: BIND 9.3.0 -> 9.11.35, 9.12.0 -> 9.16.21, and 
versions 9.9.3-S1 -> 9.11.35-S1 and 9.16.8-S1 -> 9.16.21-S1 of BIND 
Supported Preview Edition, as well as release versions 9.17.0 -> 9.17.18 
of the BIND 9.17 development branch
Severity: Medium
Exploitable: Remotely

Description:

Exploitation of broken authoritative servers using a flaw in response 
processing can cause degradation in BIND resolver performance. The way 
the lame cache is currently designed makes it possible for its internal 
data structures to grow almost infinitely, which may cause significant 
delays in client query processing.

The purpose of a resolver's lame cache is to ensure that if an 
authoritative server responds to a resolver's query in a specific broken 
way, subsequent client queries for the same <QNAME, QTYPE> tuple do not 
trigger further queries to the same server for a configurable amount of 
time. The lame cache is enabled by setting the lame-ttl option in 
named.conf to a value greater than 0. That option is set to lame-ttl 
600; in the default configuration, which means the lame cache is enabled 
by default.

Impact:

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

A successful attack exploiting this flaw causes a named resolver to 
spend most of its CPU time on managing and checking the lame cache. This 
results in client queries being responded to with large delays, and 
increased likelihood of DNS timeouts on client hosts.

CVSS Score: 4.9

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

Workarounds:

Setting lame-ttl 0; disables the lame cache and prevents the performance 
issue. Our research and testing indicate that in the current Internet 
there is almost no downside to disabling the lame cache.

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 9.11.36
     BIND 9.16.22
     BIND 9.17.19

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

     BIND 9.11.36-S1
     BIND 9.16.22-S1

Acknowledgments: ISC would like to thank Kishore Kumar Kothapalli of 
Infoblox for bringing this vulnerability to our attention.

Document revision history:

1.0 Advance notification to customers, 20 October 2021
1.1 Revised Versions Affected (now includes all older versions of BIND 
Supported Preview Edition), 26 October 2021
2.0 Public disclosure, 27 October 2021

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 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-2021-25219 is the 
complete and official security advisory document.

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