Cross Site Scripting through compromised remote site

published 2025-10-30 12 mo ago modified 2026-10-19 21 d ago also EUVD-2025-36997

cvss: 8.4; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 0.6%; EPSS 0.6% chance of exploitation in 30 days, percentile 49%
public exploits: 0; none known

Cross-Site Scripting (XSS) vulnerability in Checkmk's distributed monitoring allows a compromised remote site to inject malicious HTML code into service outputs in the central site. Affecting Checkmk before 2.4.0p14, 2.3.0p39, 2.2.0 and 2.1.0 (eol).

Patch / fix

fixed in 1

Checkmk GmbH Checkmk 2.3.0p39, 2.4.0p14

CISA SSVC

Exploitation: Proof of concept is public
Automatable: no
Technical impact: total
Assessed: 12 mo ago (2025-10-30)

Exploit prediction

0.60% chance of exploitation in the next 30 days, higher than 49% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

github.com https://github.com/sbaresearch/advisories/tree/82fd27e4570433464c30b35150b197db9a850f4e/2025/SBA-ADV-20250729-01_Checkmk_Cross_Site_Scripting referenced PoC, tagged "exploit" by a source, unverified
seclists.org http://seclists.org/fulldisclosure/2025/Nov/0 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
CNA	4.0	8.5	CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
CNA	4.0	8.5	CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
NVD	3.1	8.4	CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:H/A:H

Affected products

Checkmk GmbH Checkmk

Classification

Weakness

CWE-80 Improper Neutralization of Script-Related HTML Tags in a Web Page
CWE-79 Improper Neutralization of Input During Web Page Generation

References

github.com /sbaresearch/advisories/tree/82fd27e4570433464c30b35150b197db9a850f4e/2025/SBA-ADV-20250729-01_Checkmk_Cross_S
exploit, third-party-advisory
seclists.org /fulldisclosure/2025/Nov/0
exploit, mailing-list, third-party-advisory
checkmk.com /werk/17998
vendor-advisory

Sources

CVE List CVE-2025-39663
NVD CVE-2025-39663

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary