CVE-2026-3591: A use-after-return vulnerability exists in the `named` server when handling DNS queries signed with SIG(0)

Routine

MEDIUM 5.4UPGRADE

CVE-2026-3591 is a medium-severity flaw (CVSS 5.4) affecting bind. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A use-after-return vulnerability exists in the `named` server when handling DNS queries signed with SIG(0). Using a specially-crafted DNS request, an attacker may be able to cause an ACL to improperly (mis)match an IP address. In a default-allow ACL (denying only specific IP addresses), this may lead to unauthorized access. Default-deny ACLs should fail-secure. This issue affects BIND 9 versions 9.20.0 through 9.20.20, 9.21.0 through 9.21.19, and

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	5.4 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.4% (top 70%)
CISA exploited list	No
Weakness	CWE-305, CWE-562
Published	2026-03-25
Last modified	2026-06-17

Publisher (CNA): Internet Systems Consortium (ISC) · Vendor: isc

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-25 14:16 UTCPublished CVE record published
2026-06-17 10:43 UTCUpdated Record last modified

isc bind UPGRADE

Affected: >= 9.20.0 and < 9.20.21 · >= 9.21.0 and < 9.21.20

Fixed in: 9.20.21, 9.21.20

Safe to downgrade to: a release before 9.20.0

1. Check installed version

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

2. UPGRADE to 9.20.21 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade bind

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 9.20.0 (Debian / Ubuntu)

apt-cache madison bind sudo apt install bind=<VERSION_BEFORE_9.20.0> sudo apt-mark hold bind

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

dnf --showduplicates list bind sudo dnf downgrade bind-<VERSION_BEFORE_9.20.0> sudo dnf versionlock add bind

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-2026-3591?

bind: upgrade to 9.20.21, 9.21.20. Exact commands for each OS are on this page.

Is CVE-2026-3591 being exploited?

CVE-2026-3591 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-2026-3591 affect?

CVE-2026-3591 affects bind. Severity is medium (CVSS 5.4). A use-after-return vulnerability exists in the `named` server when handling DNS queries signed with SIG(0). Using a specially-crafted DNS request, an attacker may be able to cause an ACL to improperly (mis)match an IP address. In a default-

How urgent is CVE-2026-3591?

Priority 26/100. No elevated risk signals yet.

Software pages

BIND DNS vulnerabilities

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References

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