GHSA-3wgp-x9p5-c7cc

## Summary

Appium's base-driver mounts the built-in `/test/guinea-pig`, `/test/guinea-pig-scrollable` and `/test/guinea-pig-app-banner` routes **unconditionally** on every server. The handler reflects the `throwError` query param, the `comments` POST field, and the `User-Agent` request header into the returned HTML via `compileLodashTemplate`, which interpolates `<%= expr %>` as `String(expr)` with **no HTML/JS escaping**. This yields reflected XSS, and the `throwError` value is reflected **inside a `<script>` block**, giving arbitrary JavaScript execution on the server's origin. No authentication, no session, no driver and no plugin are required, and the default bind address is `0.0.0.0`.

## Details

### Affected - `@appium/base-driver` **10.6.0** (with Appium server **3.5.0**); tested live. - Template engine helper: `@appium/base-driver` `lib/utils.ts` `compileLodashTemplate`.

### Location (file:line) - Routes mounted unconditionally: `base-driver/lib/express/server.ts:216-219` (`app.all('/test/guinea-pig', guineaPig)` etc.). - Tainting: `base-driver/lib/express/static.ts:35-61` (`guineaPigTemplate`) — `throwError = String(req.params.throwError ?? req.query?.throwError)`, `params.comment = String(req.body.comments)`, `params.userAgent = req.headers['user-agent']`. - Unescaped render: `base-driver/lib/utils.ts:67-83` (`compileLodashTemplate`) emits `<%= expr %>` as `String(${expr})` via `new Function(...)`, no escaping. - Sinks (shipped templates): `base-driver/static/test/guinea-pig.html:11-12` (`throwError` inside `<script>`), `:50` (`comment`), `:87` (`userAgent`); same in `guinea-pig-scrollable.html` / `guinea-pig-app-banner.html`.

### PoC

Requests: ``` GET /test/guinea-pig?throwError=x%27%2balert(document.domain)%2b%27 POST /test/guinea-pig (body: comments=</span><img src=x onerror=alert(1)>) GET /test/guinea-pig (header: User-Agent: <script>alert(7)</script>) ```

<img width="973" height="276" alt="image" src="https://github.com/user-attachments/assets/f58e1dce-f3ad-43d3-b66e-1ff5efea3866" />

### Impact

An attacker who can get a victim to open a crafted link (or auto-submit a form) to the Appium server executes arbitrary JavaScript on the server's origin. With default CORS `*` + no authentication, that JS can drive the WebDriver REST API and plugin endpoints. The endpoints are debug/test fixtures that should not be reachable on a production listener at all.