CVE-2026-50551: SiYuan Personal Knowledge Management System: Stored XSS Vulnerability

SiYuan v3.6.5 and earlier versions contain a stored cross-site scripting (XSS) vulnerability in the Attribute View (database) asset cell renderer that escalates to remote code execution (RCE) in the Electron desktop client. This is a neighbor-bug of CVE-2026-44588: the fix for -44588 used `escapeAriaLabel()` (double-escapes `<`), but the AV asset renderers were left using the weaker `escapeAttr()` (escapes only quotes) or no escaping at all.

## Vulnerability Details

The Electron renderer is configured with `nodeIntegration: true` and `contextIsolation: false` (app/electron/main.js:307), allowing any JavaScript executing in the renderer to directly access Node.js APIs including `require('child_process')`.

Two XSS sinks exist.

### Sink 1 (Direct Stored XSS - triggers on page load)

`app/src/protyle/render/av/cell.ts:1008`:

text += `<span class="b3-chip av__celltext--url ariaLabel" aria-label="${escapeAttr(item.content)}" data-name="${escapeAttr(item.name)}"
data-url="${escapeAttr(item.content)}">${item.name || item.content}</span>`;

The `>${item.name || item.content}</span>` portion is raw user input with zero escaping.

`app/src/protyle/render/av/blockAttr.ts:93` (even worse - completely unescaped):

html += `<img loading="lazy" class="av__cellassetimg ariaLabel" aria-label="${item.content}" src="${getCompressURL(item.content)}">`;

Rendered via `action.ts:860`: `cellElement.innerHTML = renderCell(...)` results in immediate XSS on page load.

### Sink 2 (Hover-triggered XSS via aria-label round-trip)

- Same lines emit `aria-label="${escapeAttr(item.content)}"` on `.ariaLabel` elements.
- `escapeAttr()` (util/escape.ts:14) escapes only `"` and `'` — NOT `<` or `>`.
- `popover.ts:33` global mouseover handler reads `aria-label` via `getAttribute` (which attribute-decodes entities).
- Line 144: `showTooltip(decodeURIComponent(tip), ...)` then `tooltip.ts:41`: `messageElement.innerHTML = message` results in XSS on hover.

### Source

- `app/src/protyle/render/av/asset.ts:405`: `addAssetLink()` reads user input from a free-form `<textarea>` with no sanitization.
- Kernel stores `MAsset.Content` raw (kernel/av/value.go:53), no server-side sanitization.

## Attack Vector

1. Attacker creates a malicious note containing an Attribute View (database).
2. Attacker adds an asset cell with link content: `<img src=x onerror=require('child_process').exec('calc')>`
3. Victim opens the note for immediate RCE (Sink 1), or hovers over the cell for RCE (Sink 2).
4. In a sync/collaboration scenario, the malicious note propagates to all users.

## Proof of Concept

Payload (Direct XSS) — in an AV asset cell link field, enter:

<img src=x onerror=alert(document.domain)>

For RCE in Electron desktop:

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

### Steps to Reproduce

1. Open SiYuan desktop app (v3.6.5).
2. Create a new document.
3. Insert an Attribute View (database): `/` then select "Table".
4. Add a column of type "Asset".
5. Click the asset cell, then "Add Link".
6. In the "Link" textarea, paste: `<img src=x onerror=alert(1)>`
7. Leave "Title" empty or fill with benign text.
8. Click outside the dialog to save.
9. Observe: Alert fires immediately (Sink 1). Hovering over the cell also triggers (Sink 2).

## Impact

- Remote Code Execution on victim's system via malicious note sync/import.
- Data exfiltration: attacker can read all notes, access filesystem, steal credentials.
- Persistence: malicious payload stored in `.sy` files, executes on every open.

## Suggested Fix

1. Replace `escapeAttr()` with `escapeAriaLabel()` for all `aria-label` attributes in AV cell renderers.
2. Escape `item.name` and `item.content` with `escapeHtml()` before concatenating into element text content.

Affected files: `app/src/protyle/render/av/cell.ts`, `app/src/protyle/render/av/blockAttr.ts`, `app/src/protyle/render/av/asset.ts`.

## Additional Context

This vulnerability is a neighbor-bug of CVE-2026-44588. The fix for -44588 correctly used `escapeAriaLabel()` (which double-escapes `<` to survive the attribute -> `getAttribute` -> `innerHTML` round-trip), but the AV asset cell renderers were left using the weaker `escapeAttr()` or no escaping. This is part of a pattern of incomplete fixes in SiYuan (see also CVE-2026-33066, CVE-2026-29183). The long-term fix should set Electron`contextIsolation: true` and `nodeIntegration: false`.

## Report
Reporter (GitHub: Yunkaiwjs).

CWE-79 Cross-site Scripting (XSS)