CVE-2021-25215Network attack vector

Unauthenticated DNS service crash via duplicate RRset insertion

Published Apr 29, 2021 · Updated Sep 16, 2024

Denial of service in ISC BIND 9 allows remote attackers to terminate named via a crafted query involving a DNAME record. The DNAME response path attempts to add the same RRset to the ANSWER section twice, causing an assertion check to fail. Both authoritative and recursive servers process the trigger; no authentication is required, and the reported effect is a DNS service crash.

CVSS severity7.5
High
EPSS probability11.39%
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 ISC BIND 9 allows remote attackers to terminate named via a crafted query involving a DNAME record. The DNAME response path attempts to add the same RRset to the ANSWER section twice, causing an assertion check to fail. Both authoritative and recursive servers process the trigger; no authentication is required, and the reported effect is a DNS service crash.

The record

CVE
CVE-2021-25215
Published
Apr 29, 2021
Updated
Sep 16, 2024
Vendor
Internet Systems Consortium
Product
BIND 9
Classifications
CWE-617, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 29, 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

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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 9.17.0 through versiosn before 9.17.12
  2. Affected versionversion=Open Source Branches 9.0 through 9.11 9.0.0 through versions before 9.11.30
  3. Affected versionversion=Open Source Branches 9.12 through 9.16 9.12.0 through versions before 9.16.14
  4. Affected versionversion=Supported Preview Branch 9.16-S 9.16.8-S1 through versions before 9.16.14-S1
  5. Affected versionversion=Supported Preview Branches 9.9-S through 9.11-S 9.9.3-S1 through versions before 9.11.30-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 versiosn before 9.17.12; version=Open Source Branches 9.0 through 9.11 9.0.0 through versions before 9.11.30; version=Open Source Branches 9.12 through 9.16 9.12.0 through versions before 9.16.14; version=Supported Preview Branch 9.16-S 9.16.8-S1 through versions before 9.16.14-S1; version=Supported Preview Branches 9.9-S through 9.11-S 9.9.3-S1 through versions before 9.11.30-S1

Published CVSS scores

5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5Internet Systems ConsortiumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-617Reachable Assertion
T1499.004Application or System Exploitation

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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