CVE-2025-40777Network attack vector

Unauthenticated resolver crash via stale CNAME handling

Published Jul 16, 2025 · Updated Nov 4, 2025

Denial of service in ISC BIND 9 allows remote attackers to crash named by querying a resolver configured to serve stale answers. ISC has not identified the failing assertion or function; named aborts while resolving a CNAME chain containing a specific mix of cached or authoritative records. The flaw requires serve-stale-enable set to yes and stale-answer-client-timeout set to 0, and it interrupts resolver service by exiting the daemon.

CVSS severity7.5
High
EPSS probability0.92%
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 crash named by querying a resolver configured to serve stale answers. ISC has not identified the failing assertion or function; named aborts while resolving a CNAME chain containing a specific mix of cached or authoritative records. The flaw requires serve-stale-enable set to yes and stale-answer-client-timeout set to 0, and it interrupts resolver service by exiting the daemon.

The record

CVE
CVE-2025-40777
Published
Jul 16, 2025
Updated
Nov 4, 2025
Vendor
Internet Systems Consortium
Product
BIND 9
Classifications
CWE-617, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 16, 2025CVE publishedPublication date reported by the CVE source.
  2. Nov 4, 2025Record 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=9.20.0 <=9.20.10
  2. Affected versionversion=9.20.9-S1 <=9.20.10-S1
  3. Affected versionversion=9.21.0 <=9.21.9

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Internet Systems Consortium · BIND 9version=9.20.0 <=9.20.10; version=9.20.9-S1 <=9.20.10-S1; version=9.21.0 <=9.21.9

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

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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