CVE-2026-18664: When ranges are used for access control (i.e

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CRITICAL 9.1REMOTEUPGRADE

CVE-2026-18664 is a critical-severity flaw (CVSS 9.1) affecting nsd. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

When ranges are used for access control (i.e. of the form 1.2.3.4-1.2.3.25), because NSD wrongly compares the IP address with the range on little endian systems, IPs that were meant to be allowed may be denied, and, IPs that were meant to be denied access could be allowed. An IPv4 address is compared with IPv4 ranges as unsigned 32 bit numbers directly with the endianness of the host, but the values to compare are in network byte order (big-endia

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.1 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 62%)
CISA exploited list	No
Weakness	CWE-284, CWE-697
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 20:02 UTCUpdated Record last modified

nlnetlabs nsd UPGRADE

Affected: >= 3.0.0 and < 4.15.1

Fixed in: 4.15.1

Safe to downgrade to: a release before 3.0.0

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 3.0.0 (Debian / Ubuntu)

apt-cache madison nsd sudo apt install nsd=<VERSION_BEFORE_3.0.0> 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_3.0.0> 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-2026-18664?

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

Is CVE-2026-18664 being exploited?

CVE-2026-18664 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-2026-18664 affect?

CVE-2026-18664 affects nsd. Severity is critical (CVSS 9.1). When ranges are used for access control (i.e. of the form 1.2.3.4-1.2.3.25), because NSD wrongly compares the IP address with the range on little endian systems, IPs that were meant to be allowed may be denied, and, IPs that were meant to b

How urgent is CVE-2026-18664?

Priority 46/100. Critical severity; Remotely exploitable, no login, no user action

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References

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