CVE-2026-55717: In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'res

Routine

MEDIUM 5.9REMOTEUPGRADE

CVE-2026-55717 is a medium-severity flaw (CVSS 5.9) affecting unbound. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'response-ip: <net> redirect' /'response-ip-data: <net> CNAME <target>' rule (or the RPZ 'rpz-cname-override' equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets 'alias_rrset'

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.9 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 86%)
CISA exploited list	No
Weakness	CWE-476
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: >= 1.10.0 and < 1.25.2

Fixed in: 1.25.2

Safe to downgrade to: a release before 1.10.0

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 1.10.0 (Debian / Ubuntu)

apt-cache madison unbound sudo apt install unbound=<VERSION_BEFORE_1.10.0> 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_1.10.0> 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.

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-2026-55717?

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

Is CVE-2026-55717 being exploited?

CVE-2026-55717 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-2026-55717 affect?

CVE-2026-55717 affects unbound. Severity is medium (CVSS 5.9). In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'response-ip: <net> redirect' /'response-ip-data: <net> CNAME <target>' rule (or the RPZ 'rpz-cname-override' equivalent), a remote cl

How urgent is CVE-2026-55717?

Priority 29/100. Remotely exploitable, no login, no user action

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References

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