CVE-2026-13400: Simply Schedule Appointments is vulnerable to unauthenticated Stored Cross-Site Scripting in all versions up to and including 1.6.12.2. The root cause is a sanitization-ordering de
Simply Schedule Appointments is vulnerable to unauthenticated Stored Cross-Site Scripting in all versions up to and including 1.6.12.2. The root cause is a sanitization-ordering defect: the rendered notification content is decoded back into live HTML after it has already passed through the Simply Schedule Appointments WordPress plugin before 1.6.12.4's wp_kses_post() filter, so a double-encoded payload survives intake and is reintroduced as an executable element at render time.
Details
Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-13400
Exploitation outlook
EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.
- Low exploitation riskCVE-2026-134000.15% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 5% of all scored CVEs.
Referenced CVEs
| CVE | CSIRTS overview | External |
|---|---|---|
| CVE-2026-13400 | coverage & exploitation status | NVD · CVE.org |
Recent advisories for Simply Schedule Appointments
A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.
- unknownCVE-2026-16540: The Simply Schedule Appointments WordPress plugin before 1.6.12.6 does not correctly restrict …nvd · 2026-08-02
- mediumCVE-2026-59523: Missing Authorization vulnerability in NSquared Simply Schedule Appointments simply-schedule-a…nvd · 2026-07-13
- mediumCVE-2026-57812: Missing Authorization vulnerability in NSquared Simply Schedule Appointments simply-schedule-a…nvd · 2026-07-13
- highCVE-2026-57317: Unauthenticated Cross Site Scripting (XSS) in Simply Schedule Appointments <= 1.6.12.2 version…nvd · 2026-06-26
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- unknownCVE-2026-16540: The Simply Schedule Appointments WordPress plugin before 1.6.12.6 does not correctly restrict …2026-08-02