Is CVE-2021-40906 in the CISA KEV catalog?

No. CVE-2021-40906 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2021-40906?

CVE-2021-40906 has a CVSS v3 base score of 6.1 (MEDIUM).

What is the EPSS score for CVE-2021-40906?

CVE-2021-40906 has an EPSS score of 0.99% (58th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2021-40906?

Upgrade check-mk on ubuntu 18.04 to 1.2.8p16-1ubuntu0.2 or later. Fixes are available for 4 affected products.

CVE & CISA-KEV Catalog

CVE-2021-40906

MEDIUM

6.1

CVSS v3

NVD

Description

CheckMK Raw Edition software (versions 1.5.0 to 1.6.0) does not sanitise the input of a web service parameter that is in an unauthenticated zone. This Reflected XSS allows an attacker to open a backdoor on the device with HTML content and interpreted by the browser (such as JavaScript or other client-side scripts) or to steal the session cookies of a user who has previously authenticated via a man in the middle. Successful exploitation requires access to the web service resource without authentication.

CWE-79Cross-Site Scripting (XSS)

How to fix

Remediation Available

check-mkUbuntu

Fixed in:1.2.8p16-1ubuntu0.2USN-5527-1

check-mk-livestatusUbuntu

Fixed in:1.2.8p16-1ubuntu0.2USN-5527-1

check-mk-multisiteUbuntu

Fixed in:1.2.8p16-1ubuntu0.2USN-5527-1

check-mk-serverUbuntu

Fixed in:1.2.8p16-1ubuntu0.2USN-5527-1

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionRequired

ScopeChanged

Impact

ConfidentialityLow

IntegrityLow

AvailabilityNone

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Exploit Intelligence

0.99%probability of exploitation in 30 days

58thpercentile

Moderate risk: more likely to be exploited than 58% of all known CVEs.

References

Third-Party Advisory1

github.com

Product1

checkmk.com

Related Vulnerabilities

Other CWE-79 (Cross-Site Scripting (XSS)) vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2020-11022	Medium	6.9	99%	-	Fix
CVE-2019-3929	Critical	9.8	99%	KEV	Fix
CVE-2020-9496	Medium	6.1	99%	-	-
CVE-2023-28341	Medium	6.1	99%	-	Fix
CVE-2018-12998	Medium	6.1	98%	-	-
CVE-2025-4123	High	7.6	98%	-	Fix

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-07-05.