# CVE-2026-13400: Simply Schedule Appointments < 1.6.12.4 - Unauthenticated Stored XSS via Booking Customer Information

> Unauthenticated attackers can inject malicious scripts via booking information, leading to code execution or account takeover in Simply Schedule Appointments.

- **Severity:** Medium 6.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 27 Jul 2026
- **Updated:** 27 Jul 2026
- **Also known as:** GHSA-v365-26q9-f844

## What is CVE-2026-13400?

CVE-2026-13400 is a medium-severity cross-site scripting vulnerability in Unknown Simply Schedule Appointments, published on 27 Jul 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

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.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-13400?


## How do I fix CVE-2026-13400?

No fix has been published for CVE-2026-13400 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-13400 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-13400 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-13400 and fix it if it is affected.

What it is: CVE-2026-13400 is a medium-severity cross-site scripting vulnerability in Unknown Simply Schedule Appointments, published on 27 Jul 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-13400 exists.

Affected product: Unknown Simply Schedule Appointments. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-13400/ (Markdown: https://insights.sbomify.com/CVE-2026-13400/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-13400
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-13400
- OSV: https://osv.dev/vulnerability/CVE-2026-13400

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-13400/
Summaries are generated and may contain mistakes; the official description above is the record.