CVE-2021-25219Network attack vector

Remote denial of service via unbounded lame cache

Published Oct 27, 2021 · Updated Sep 16, 2024

Denial of service in affected ISC BIND 9 branches allows remote attackers to delay recursive DNS queries via broken-authority responses. The response-processing path inserts broken-authority results into a lame cache whose internal structures can grow almost without bound. Lame caching is enabled by default, but authoritative-only servers are unaffected; exposed resolvers can spend most CPU time managing the cache and cause client timeouts.

CVSS severity5.3
Medium
EPSS probability10.58%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in affected ISC BIND 9 branches allows remote attackers to delay recursive DNS queries via broken-authority responses. The response-processing path inserts broken-authority results into a lame cache whose internal structures can grow almost without bound. Lame caching is enabled by default, but authoritative-only servers are unaffected; exposed resolvers can spend most CPU time managing the cache and cause client timeouts.

The record

CVE
CVE-2021-25219
Published
Oct 27, 2021
Updated
Sep 16, 2024
Vendor
Internet Systems Consortium
Product
BIND 9
Classifications
T1499.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 27, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 16, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=Development Branch 9.17 9.17.0 through versions before 9.17.19
  2. Affected versionversion=Open Source Branches 9.12 through 9.16 9.12.0 through versions before 9.16.22
  3. Affected versionversion=Open Source Branches 9.3 through 9.11 9.3.0 through versions before 9.11.36
  4. Affected versionversion=Supported Preview Branch 9.16-S 9.16.8-S1 through versions before 9.16.22-S1
  5. Affected versionversion=Supported Preview Branches 9.9-S through 9.11-S 9.9.3-S1 through versions before 9.11.36-S1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Internet Systems Consortium · BIND 9version=Development Branch 9.17 9.17.0 through versions before 9.17.19; version=Open Source Branches 9.12 through 9.16 9.12.0 through versions before 9.16.22; version=Open Source Branches 9.3 through 9.11 9.3.0 through versions before 9.11.36; version=Supported Preview Branch 9.16-S 9.16.8-S1 through versions before 9.16.22-S1; version=Supported Preview Branches 9.9-S through 9.11-S 9.9.3-S1 through versions before 9.11.36-S1

Published CVSS scores

5.0National Institute of Standards and TechnologyAV:N/AC:L/Au:N/C:N/I:N/A:P
4.9Internet Systems ConsortiumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L/E:F/RL:O/RC:C
5.3Internet Systems ConsortiumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1499.002Service Exhaustion Flood

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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