CVE-2026-19401Network attack vector

Remote service denial via repeated DNS Cookies

Published Aug 26, 2026 · Updated Aug 26, 2026

Reachable assertion in NLnet Labs NSD 4.3.7 through 4.15.0 allows remote attackers to crash a server child via crafted UDP DNS messages. DNS Cookie option parsing reaches an assertion when it processes a specially tuned repetition count, including 17 options with a 512-byte UDP payload. No authentication is required; repeated child crashes degrade service and deny all DNS responses when the sender is sufficiently close to sustain the traffic.

CVSS severity8.2
High
EPSS probability0.36%
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 NLnet Labs NSD 4.3.7 through 4.15.0 allows remote attackers to crash a server child via crafted UDP DNS messages. DNS Cookie option parsing reaches an assertion when it processes a specially tuned repetition count, including 17 options with a 512-byte UDP payload. No authentication is required; repeated child crashes degrade service and deny all DNS responses when the sender is sufficiently close to sustain the traffic.

The record

CVE
CVE-2026-19401
Published
Aug 26, 2026
Updated
Aug 26, 2026
Vendor
NLnet Labs
Product
NSD
Classifications
CWE-400, CWE-617, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 26, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 26, 2026Record 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=4.3.7 <4.15.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

NLnet Labs · NSDversion=4.3.7 <4.15.1

Published CVSS scores

8.2NLnet LabsCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA: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.

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

CWE-400Uncontrolled Resource Consumption
CWE-617Reachable Assertion
T1499Endpoint Denial of Service

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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