CVE-2026-40691: In Unbound 1.9.0 up to and including 1.25.1, when a DNSCrypt query is received over TCP, the routine that encr

Routine

HIGH 7.5REMOTEUPGRADE

CVE-2026-40691 is a high-severity flaw (CVSS 7.5) affecting unbound. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

In Unbound 1.9.0 up to and including 1.25.1, when a DNSCrypt query is received over TCP, the routine that encrypts the reply in place fails to bound the reply length against the destination buffer size. The size clamp that protects the UDP path is not applied on the TCP path, so a reply larger than 65504 bytes is shifted forward by 48 bytes inside a buffer of capacity equal to 'msg-buffer-size', writing past the end of the heap allocation. A sing

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	0.3% (top 79%)
CISA exploited list	No
Weakness	CWE-122, CWE-787
Published	2026-07-22
Last modified	2026-10-24

Publisher (CNA): NLnet Labs · Vendor: nlnetlabs

Last updated 2026-10-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-22 14:17 UTCPublished CVE record published
2026-10-24 13:10 UTCUpdated Record last modified

nlnetlabs unbound UPGRADE

Affected: >= 1.9.0 and < 1.25.2

Fixed in: 1.25.2

Safe to downgrade to: a release before 1.9.0

1. Check installed version

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

2. UPGRADE to 1.25.2 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade unbound

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 1.9.0 (Debian / Ubuntu)

apt-cache madison unbound sudo apt install unbound=<VERSION_BEFORE_1.9.0> sudo apt-mark hold unbound

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

dnf --showduplicates list unbound sudo dnf downgrade unbound-<VERSION_BEFORE_1.9.0> sudo dnf versionlock add unbound

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.

Exact fixes by distribution

Fedora 46 · sugar-toolkit-gtk3 · FIXED · 0.122-2.fc46 · FEDORA-2026-d11eb2faf0 · source

sudo dnf upgrade --advisory=FEDORA-2026-d11eb2faf0

Fedora ELN · ntpd-rs · FIXED · 1.7.2-3.eln159 · FEDORA-2026-57e4a99310 · source

sudo dnf upgrade --advisory=FEDORA-2026-57e4a99310

Ubuntu 20.04 · unbound · FIXED · 1.9.4-2ubuntu1.11+esm3

sudo apt update && sudo apt install --only-upgrade unbound dpkg --compare-versions "$(dpkg-query -W -f='${Version}' unbound)" ge "1.9.4-2ubuntu1.11+esm3" && echo PATCHED || echo STILL-VULNERABLE

Ubuntu 22.04 · unbound · FIXED · 1.13.1-1ubuntu5.17

sudo apt update && sudo apt install --only-upgrade unbound dpkg --compare-versions "$(dpkg-query -W -f='${Version}' unbound)" ge "1.13.1-1ubuntu5.17" && echo PATCHED || echo STILL-VULNERABLE

Ubuntu 24.04 · unbound · FIXED · 1.19.2-1ubuntu3.12

sudo apt update && sudo apt install --only-upgrade unbound dpkg --compare-versions "$(dpkg-query -W -f='${Version}' unbound)" ge "1.19.2-1ubuntu3.12" && echo PATCHED || echo STILL-VULNERABLE

Debian 12 · unbound · NO FIX YET

Debian 13 · unbound · FIXED · 1.26.1-0+deb13u1

sudo apt update && sudo apt install --only-upgrade unbound dpkg --compare-versions "$(dpkg-query -W -f='${Version}' unbound)" ge "1.26.1-0+deb13u1" && echo PATCHED || echo STILL-VULNERABLE

RHEL 10 · unbound · NOT AFFECTED

RHEL 6 · unbound · NOT AFFECTED

RHEL 7 · unbound · NOT AFFECTED

RHEL 8 · unbound · NOT AFFECTED

RHEL 9 · unbound · NOT AFFECTED

Alpine 3.22 · unbound · NOT AFFECTED

Alpine 3.23 · unbound · NOT AFFECTED

Alpine 3.24 · unbound · NOT AFFECTED

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-2026-40691?

unbound: upgrade to 1.25.2. Exact commands for each OS are on this page.

Is CVE-2026-40691 being exploited?

CVE-2026-40691 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-2026-40691 affect?

CVE-2026-40691 affects unbound. Severity is high (CVSS 7.5). In Unbound 1.9.0 up to and including 1.25.1, when a DNSCrypt query is received over TCP, the routine that encrypts the reply in place fails to bound the reply length against the destination buffer size. The size clamp that protects the UDP

How urgent is CVE-2026-40691?

Priority 36/100. High severity; Remotely exploitable, no login, no user action

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References

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