CVE-2021-40083: Knot Resolver before 5.3.2 is prone to an assertion failure, triggerable by a remote attacker in an edge case

Q: How do I fix CVE-2021-40083?

knot resolver: upgrade to 5.3.2. Exact commands for each OS are on this page.

Q: Is CVE-2021-40083 being exploited?

CVE-2021-40083 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-40083 affect?

CVE-2021-40083 affects knot resolver. Severity is high (CVSS 7.5). Knot Resolver before 5.3.2 is prone to an assertion failure, triggerable by a remote attacker in an edge case (NSEC3 with too many iterations used for a positive wildcard proof).

Q: How urgent is CVE-2021-40083?

Priority 49/100. High severity; Remotely exploitable, no login, no user action

This month

HIGH 7.5REMOTEUPGRADE

CVE-2021-40083 is a high-severity flaw (CVSS 7.5) affecting knot resolver. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Knot Resolver before 5.3.2 is prone to an assertion failure, triggerable by a remote attacker in an edge case (NSEC3 with too many iterations used for a positive wildcard proof).

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	1.4% (top 27%)
CISA exploited list	No
Weakness	CWE-617
Published	2021-08-25
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: nic

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

Am I affected?

Timeline

2021-08-25 01:15 UTCPublished CVE record published
2026-06-17 04:06 UTCUpdated Record last modified

nic knot resolver UPGRADE

Affected: < 5.3.2

Fixed in: 5.3.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy knot-resolver || rpm -q knot-resolver || knot-resolver --version

2. UPGRADE to 5.3.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade knot-resolver

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade knot-resolver

Note

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

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-2021-40083?

knot resolver: upgrade to 5.3.2. Exact commands for each OS are on this page.

Is CVE-2021-40083 being exploited?

CVE-2021-40083 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-2021-40083 affect?

CVE-2021-40083 affects knot resolver. Severity is high (CVSS 7.5). Knot Resolver before 5.3.2 is prone to an assertion failure, triggerable by a remote attacker in an edge case (NSEC3 with too many iterations used for a positive wildcard proof).

How urgent is CVE-2021-40083?

Priority 49/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2023-46317 HIGH 7.5 UPGRADE Knot Resolver before 5.7.0 performs many TCP reconnections upon receiving certain nonsensical responses from s
CVE-2023-26249 HIGH 7.5 UPGRADE Knot Resolver before 5.6.0 enables attackers to consume its resources, launching amplification attacks and pot
CVE-2022-40188 HIGH 7.5 UPGRADE MITIGATE Knot Resolver before 5.5.3 allows remote attackers to cause a denial of service (CPU consumption) because of a
CVE-2022-32983 MEDIUM 5.3 UPGRADE Knot Resolver through 5.5.1 may allow DNS cache poisoning when there is an attempt to limit forwarding actions
CVE-2020-12667 HIGH 7.5 UPGRADE Knot Resolver before 5.1.1 allows traffic amplification via a crafted DNS answer from an attacker-controlled s

References

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