CVE-2026-19401: Any remote client can crash a (debugging/non-release build type) NSD serve child by sending it a special craft

This month

HIGH 7.5REMOTEUPGRADE

CVE-2026-19401 is a high-severity flaw (CVSS 7.5) affecting nsd. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Any remote client can crash a (debugging/non-release build type) NSD serve child by sending it a special crafted message with a specially tuned number of DNS Cookie options (17 when UDP payload size is 512). By continuously crashing the serve childs, the remote client can severely hamper or, when positioned sufficiently close, deny all DNS service.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 60%)
CISA exploited list	No
Weakness	CWE-400, CWE-617
Published	2026-08-26
Last modified	2026-09-08

Publisher (CNA): NLnet Labs · Vendor: nlnetlabs

Last updated 2026-09-08 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-08-26 09:16 UTCPublished CVE record published
2026-09-08 20:00 UTCUpdated Record last modified

nlnetlabs nsd UPGRADE

Affected: >= 4.3.7 and < 4.15.1

Fixed in: 4.15.1

Safe to downgrade to: a release before 4.3.7

1. Check installed version

apt-cache policy nsd || rpm -q nsd || nsd --version

2. UPGRADE to 4.15.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade nsd

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade nsd

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 4.3.7 (Debian / Ubuntu)

apt-cache madison nsd sudo apt install nsd=<VERSION_BEFORE_4.3.7> sudo apt-mark hold nsd

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list nsd sudo dnf downgrade nsd-<VERSION_BEFORE_4.3.7> sudo dnf versionlock add nsd

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-19401?

nsd: upgrade to 4.15.1. Exact commands for each OS are on this page.

Is CVE-2026-19401 being exploited?

CVE-2026-19401 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-19401 affect?

CVE-2026-19401 affects nsd. Severity is high (CVSS 7.5). Any remote client can crash a (debugging/non-release build type) NSD serve child by sending it a special crafted message with a specially tuned number of DNS Cookie options (17 when UDP payload size is 512). By continuously crashing the ser

How urgent is CVE-2026-19401?

Priority 41/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-12244 HIGH 8.8 UPGRADE If NSD is configured as secondary for a zone, the primary of that zone can crash NSD with an AXFR containing a
CVE-2026-12245 HIGH 7.5 UPGRADE NSD from version 4.13.0 has a heap use-after-free bug in logging errors on TLS connections, causing a crash of
CVE-2026-12246 HIGH 8.1 UPGRADE NSD version 4.14.0 introduced a bug where a specially crafted APL RR, with an adflength larger than permitted
CVE-2026-12490 HIGH 7.5 UPGRADE When a provide-xfr is given with a tls-auth-name, a secondary requesting a transfer should provide a client ce
CVE-2026-18664 CRITICAL 9.1 UPGRADE When ranges are used for access control (i.e

References

← Back to latest CVEs