CVE-2026-13400: Unknown Simply Schedule Appointments

Medium severity, CVSS 6.1. EPSS: 0.3% chance of exploitation in the next 30 days.

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.

Affected products

• Unknown Simply Schedule Appointments: before 1.6.12.4 (fixed in 1.6.12.4)

Published 2026-07-27. Last modified 2026-07-27.