How severe is CVE-2021-40083?

CVE-2021-40083 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2021-40083 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (70th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-40083?

CVE-2021-40083 affects Nic Knot Resolver. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-40083?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-40083 published?

CVE-2021-40083 was published on 2021-08-25 and last updated on 2026-06-17.

CVE-2021-40083

CVE-2021-40083 is a high-severity vulnerability in Nic Knot Resolver with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-617.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v2: 5.0
EPSS exploit prediction: 1% (70th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-617
Affected product: Nic Knot Resolver
Published: 2021-08-25
Last modified: 2026-06-17

Description

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).

Frequently asked questions

What is CVE-2021-40083?: 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 severe is CVE-2021-40083?: CVE-2021-40083 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2021-40083 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (70th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-40083?: CVE-2021-40083 affects Nic Knot Resolver. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-40083?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-40083 published?: CVE-2021-40083 was published on 2021-08-25 and last updated on 2026-06-17.

References

https://gitlab.nic.cz/knot/knot-resolver/-/merge_requests/1169

Affected products (1)

cpe:2.3:a:nic:knot_resolver:*:*:*:*:*:*:*:*

More vulnerabilities in Nic Knot Resolver

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CVE-2023-26249 — High (CVSS 7.5): Knot Resolver before 5.6.0 enables attackers to consume its resources, launching amplification attacks and potentially…
CVE-2022-40188 — High (CVSS 7.5): Knot Resolver before 5.5.3 allows remote attackers to cause a denial of service (CPU consumption) because of…
CVE-2018-1110 — High (CVSS 7.5): A flaw was found in knot-resolver before version 2.3.0. Malformed DNS messages may cause denial of service.
CVE-2020-12667 — High (CVSS 7.5): Knot Resolver before 5.1.1 allows traffic amplification via a crafted DNS answer from an attacker-controlled server,…

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Other CWE-617 (Reachable Assertion) vulnerabilities

CVE-2020-3615 — Critical (CVSS 9.8): Valid deauth/disassoc frames is dropped in case if RMF is enabled and some rouge peer keep on sending rogue…
CVE-2019-9795 — Critical (CVSS 9.8): A vulnerability where type-confusion in the IonMonkey just-in-time (JIT) compiler could potentially be used by…
CVE-2026-31739 — High (CVSS 8.8): In the Linux kernel, the following vulnerability has been resolved: crypto: tegra - Add missing CRYPTO_ALG_ASYNC The…
CVE-2020-12417 — High (CVSS 8.8): Due to confusion about ValueTags on JavaScript Objects, an object may pass through the type barrier, resulting in…
CVE-2020-6623 — High (CVSS 8.8): stb stb_truetype.h through 1.22 has an assertion failure in stbtt__cff_get_index.
CVE-2020-6619 — High (CVSS 8.8): stb stb_truetype.h through 1.22 has an assertion failure in stbtt__buf_seek.

Browse all CWE-617 (Reachable Assertion) vulnerabilities →