CVE-2021-25218Network attack vector

Remote service crash via fragmented UDP response assertion

Published Aug 18, 2021 · Updated Sep 17, 2024

Reachable assertion in ISC BIND 9 9.16.19, 9.17.16, and 9.16.19-S1 allows remote attackers to crash named via a DNS query. When RRL is active, named asserts while preparing a UDP response larger than the effective interface MTU, terminating the server process. Exposure requires an oversized UDP response caused by max-udp-size configuration or path MTU discovery; the result is a DNS service interruption.

CVSS severity7.5
High
EPSS probability3.59%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Reachable assertion in ISC BIND 9 9.16.19, 9.17.16, and 9.16.19-S1 allows remote attackers to crash named via a DNS query. When RRL is active, named asserts while preparing a UDP response larger than the effective interface MTU, terminating the server process. Exposure requires an oversized UDP response caused by max-udp-size configuration or path MTU discovery; the result is a DNS service interruption.

The record

CVE
CVE-2021-25218
Published
Aug 18, 2021
Updated
Sep 17, 2024
Vendor
Internet Systems Consortium
Product
BIND 9
Classifications
CWE-617, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 18, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 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 9.17.16
  2. Affected versionversion=Stable Branch 9.16.19
  3. Affected versionversion=Supported Preview Edition 9.16.19-S1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Internet Systems Consortium · BIND 9version=Development Branch 9.17.16; version=Stable Branch 9.16.19; version=Supported Preview Edition 9.16.19-S1

Published CVSS scores

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

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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 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.

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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