Widely-used Checkmk monitoring, high-severity stored XSS affecting multiple orgs.
CVE-2026-20915
Stored XSS in Checkmk Pending Changes UI.
Is CVE-2026-20915 being exploited?
Not confirmed. CVE-2026-20915 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.15% probability of exploitation in the next 30 days.
How severe is CVE-2026-20915?
CVE-2026-20915 is rated High with a CVSS score of 8.5. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-20915?
Yes. A fix has been recorded for CVE-2026-20915. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-20915 affect?
CVE-2026-20915 affects Checkmk 2.5.0 beta (pre-2.5.0b2). Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-20915?
Upgrade to 2.5.0b2 or newer
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Authenticated user can inject JS into Pending Changes UI; runs in other viewers' browsers, risking session/data exposure.
Patch when possible
- affected>= 2.5.0b1 and < 2.5.0b2
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.