CVE-2026-45318

Open WebUI is a self-hosted artificial intelligence platform designed to operate entirely offline. Prior to 0.9.3, his advisory tracks a regression of the original Excel-preview XSS (CVE-2026-44549). 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 This vulnerability is fixed in 0.9.3.

Open WebUI is a self-hosted artificial intelligence platform designed to operate entirely offline. Prior to 0.9.3, his advisory tracks a regression of the original Excel-preview XSS (CVE-2026-44549). 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 This vulnerability is fixed in 0.9.3.

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

| Metric | Value | |--------|-------| | Total {@html} usages | 23 | | With DOMPurify | 9 (39%) | | Without DOMPurify | 14 (61%) | | Confirmed exploitable (file preview) | 3 |

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…
   

Open WebUI is a self-hosted artificial intelligence platform designed to operate entirely offline. Prior to 0.9.3, his advisory tracks a regression of the original Excel-preview XSS (CVE-2026-44549). 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 This vulnerability is fixed in 0.9.3.

Open WebUI is a self-hosted artificial intelligence platform designed to operate entirely offline. Prior to 0.9.3, his advisory tracks a regression of the original Excel-preview XSS (CVE-2026-44549). 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 This vulnerability is fixed in 0.9.3.

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

| Metric | Value | |--------|-------| | Total {@html} usages | 23 | | With DOMPurify | 9 (39%) | | Without DOMPurify | 14 (61%) | | Confirmed exploitable (file preview) | 3 |

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…
   

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

| Metric | Value | |--------|-------| | Total {@html} usages | 23 | | With DOMPurify | 9 (39%) | | Without DOMPurify | 14 (61%) | | Confirmed exploitable (file preview) | 3 |

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…