SiYuan is an open-source personal knowledge management system. Prior to 3.7.0, CSS snippet body containing </style> breaks out of its surrounding <style> tag when renderSnippet() interpolates it via insertAdjacentHTML. A payload like runs arbitrary JavaScript in the renderer. On Electron desktop builds the renderer runs with nodeIntegration:true, so require('child_process') is reachable from the injected handler and the XSS chains to host RCE. Snippets sync via the workspace repository, so an attacker with write access to any synced workspace plants the payload once and it fires on every device that pulls. The bug also bypasses the user's enabledCSS / enabledJS separation. A user who turned enabledJS off was making a deliberate call not to run untrusted JavaScript; the CSS path runs it anyway. This vulnerability is fixed in 3.7.0.

Affected: Go: github.com/siyuan-note/siyuan/kernel

### Summary

A CSS snippet body containing `</style>` breaks out of its surrounding `<style>` tag when `renderSnippet()` interpolates it via `insertAdjacentHTML`. A payload like `</style><img src=x onerror="...">` runs arbitrary JavaScript in the renderer. On Electron desktop builds the renderer runs with `nodeIntegration:true`, so `require('child_process')` is reachable from the injected handler and the XSS chains to host RCE. Snippets sync via the workspace repository, so an attacker with write access to any synced workspace plants the payload once and it fires on every device that pulls.

The bug also bypasses the user's `enabledCSS` / `enabledJS` separation. A user who turned `enabledJS` off was making a deliberate call not to run untrusted JavaScript; the CSS path runs it anyway.

### Details

Affected:

- HEAD `96dfe0b` (v3.6.5, 2026-04-21) - Sink: `app/src/config/util/snippets.ts:32` - Source: `/api/snippet/getSnippet`, backed by `data/snippets/conf.json` - Default config: `EnabledCSS: true`, `EnabledJS: true` at `kernel/conf/snippet.go:26-27` - Electron config: `nodeIntegration:true`, `contextIsolation:false`, `webSecurity:false` on every `BrowserWindow` in `app/electron/main.js:307,408-411,1107-1110,1150-1153,1322`

The write path stores raw content. `kernel/api/snippet.go:107-130` copies `Content` from the request straight into the snippet record with no HTML escape, no `</style>` check, no type-specific validation:

```go snippet := &conf.Snippet{ ID: m["…