CVE-2021-25220Network attack vector

DNS cache poisoning via out-of-bailiwick NS records

Published Mar 23, 2022 · Updated Sep 16, 2024

DNS cache poisoning in BIND 9 9.11.0 through 9.18.0 allows remote attackers controlling a forwarder to return false DNS answers. The resolver accepts and caches bogus NS records outside the configured bailiwick from a forwarder, then uses them when recursion occurs. Exposure requires forwarding plus possible recursion; authoritative-only servers are unaffected, and clients can be directed to incorrect servers.

CVSS severity6.8
Medium
EPSS probability3.36%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

DNS cache poisoning in BIND 9 9.11.0 through 9.18.0 allows remote attackers controlling a forwarder to return false DNS answers. The resolver accepts and caches bogus NS records outside the configured bailiwick from a forwarder, then uses them when recursion occurs. Exposure requires forwarding plus possible recursion; authoritative-only servers are unaffected, and clients can be directed to incorrect servers.

The record

CVE
CVE-2021-25220
Published
Mar 23, 2022
Updated
Sep 16, 2024
Vendor
Internet Systems Consortium
Product
BIND 9
Classifications
CWE-444
Attack vector
network
Privileges
admin

Timeline

How it unfolded

  1. Mar 23, 2022CVE 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

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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 BIND 9.17 all version
  2. Affected versionversion=Open Source Branch 9.11 9.11.0 through versions before 9.11.37
  3. Affected versionversion=Open Source Branch 9.12-16 9.12.0 through versions before 9.16.27
  4. Affected versionversion=Open Source Branch 9.18 9.18.0
  5. Affected versionversion=Supported Preview Branch 9.11-S 9.11.0-S through versions before 9.11.37-S
  6. Affected versionversion=Supported Preview Branch 9.16-S 9.16.0-S through versions before 9.16.27-S

What conditions does exploitation require?

Attack vectornetwork
Required privilegesadmin

What is affected?

Internet Systems Consortium · BIND 9version=Development Branch 9.17 BIND 9.17 all version; version=Open Source Branch 9.11 9.11.0 through versions before 9.11.37; version=Open Source Branch 9.12-16 9.12.0 through versions before 9.16.27; version=Open Source Branch 9.18 9.18.0; version=Supported Preview Branch 9.11-S 9.11.0-S through versions before 9.11.37-S; version=Supported Preview Branch 9.16-S 9.16.0-S through versions before 9.16.27-S

Published CVSS scores

6.2Internet Systems ConsortiumCVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:N/I:H/A:N/E:U/RL:U/RC:C
4.0NIST National Vulnerability DatabaseAV:N/AC:L/Au:S/C:N/I:P/A:N
6.8Internet Systems ConsortiumCVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:N/I:H/A:N

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.

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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
Privilegesadmin
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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