Skip to content
CVE Tracker
Back

CVE-2021-25219

Lame cache can be abused to severely degrade resolver performance

Medium5.3EPSS 11.2%
Published
Updated
Reserved
Assigned by
isc

Description

In 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, 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.

Scores

CVSS scores of CVE-2021-25219, by who scored it
Scored byCVSSScoreVector
iscCNA (assigner)3.1Medium5.3CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
NVDNVD · Modified3.1Medium5.3CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
EPSS (exploitation in 30 days)
11.2% · top 4.0% of all CVEs (3 Nov 2026)

Loading the EPSS history…

The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 5.3.

Weakness

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

Affected products

  • ISC · BIND9

    • affected Open Source Branches 9.3 through 9.11 9.3.0 through versions before 9.11.36
    • affected Open Source Branches 9.12 through 9.16 9.12.0 through versions before 9.16.22
    • affected Supported Preview Branches 9.9-S through 9.11-S 9.9.3-S1 through versions before 9.11.36-S1
    • affected Supported Preview Branch 9.16-S 9.16.8-S1 through versions before 9.16.22-S1
    • affected Development Branch 9.17 9.17.0 through versions before 9.17.19

What to do

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, or for BIND Supported Preview Edition (a special feature preview branch of BIND provided to eligible ISC support customers): BIND 9.11.36-S1, BIND 9.16.22-S1.

Workaround: 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.

Exploitation: We are not aware of any active exploits.

References (10)

Sources

The same CVE in the primary databases and aggregators, to check against.

Badge for a README or a ticket

CVE-2021-25219 badge
[![CVE-2021-25219](https://trackcve.app/badge/CVE-2021-25219.svg)](https://trackcve.app/cve/CVE-2021-25219)

Record last updated .