diff --git a/components/email/__tests__/composer-island-sanitize.test.ts b/components/email/__tests__/composer-island-sanitize.test.ts
new file mode 100644
index 000000000..e297be0af
--- /dev/null
+++ b/components/email/__tests__/composer-island-sanitize.test.ts
@@ -0,0 +1,64 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { Editor } from '@tiptap/core';
+import Document from '@tiptap/extension-document';
+import Paragraph from '@tiptap/extension-paragraph';
+import Text from '@tiptap/extension-text';
+import type { Editor as ReactEditor } from '@tiptap/react';
+
+import { QuotedHtml, serializeEditorContent } from '../quoted-html';
+import { SignatureBlock, SIGNATURE_BLOCK_MARKER } from '../signature-block';
+
+// A re-opened draft hands its stored HTML to the editor as is, and the quote
+// and signature islands copy their inner HTML verbatim into the app document.
+// A draft written by another client, or by another member of a shared
+// mailbox, could carry event handlers that then ran in the webmail origin.
+
+const PAYLOAD = '<img src="x" onerror="window.__xss = 1"><a href="javascript:alert(1)">l</a><script>window.__xss = 2</script><p>Quoted original</p>';
+
+function mount(html: string): Editor {
+  return new Editor({ extensions: [Document, Paragraph, Text, QuotedHtml, SignatureBlock], content: html });
+}
+
+function islandHtml(editor: Editor, selector: string): string {
+  const host = editor.view.dom.querySelector(selector) as HTMLElement;
+  return host.shadowRoot!.innerHTML;
+}
+
+function expectInert(html: string) {
+  expect(html).not.toMatch(/onerror/i);
+  expect(html).not.toMatch(/javascript:/i);
+  expect(html).not.toMatch(/<script/i);
+}
+
+let editor: Editor | null = null;
+afterEach(() => {
+  editor?.destroy();
+  editor = null;
+});
+
+describe('composer islands sanitize what they parse', () => {
+  it('strips script vectors from a quoted original in a re-opened draft', () => {
+    editor = mount(`<p>Hi</p><div data-quoted-html>${PAYLOAD}</div>`);
+    const rendered = islandHtml(editor, '.quoted-html-island');
+    expectInert(rendered);
+    expect(rendered).toContain('Quoted original');
+    expectInert(serializeEditorContent(editor as unknown as ReactEditor));
+  });
+
+  it('strips script vectors from a signature in a re-opened draft', () => {
+    editor = mount(`<p>Hi</p><div ${SIGNATURE_BLOCK_MARKER}>${PAYLOAD}</div>`);
+    const rendered = islandHtml(editor, `[${SIGNATURE_BLOCK_MARKER}]`);
+    expectInert(rendered);
+    expect(rendered).toContain('Quoted original');
+    expectInert(serializeEditorContent(editor as unknown as ReactEditor));
+  });
+
+  it('keeps the composer markers the islands depend on', () => {
+    editor = mount('<div data-quoted-html><img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-cid="logo@x"></div>'
+      + `<div ${SIGNATURE_BLOCK_MARKER}><div data-signature-block="start"></div>Jane<div data-signature-block="end"></div></div>`);
+    const out = serializeEditorContent(editor as unknown as ReactEditor);
+    expect(out).toContain('data-cid="logo@x"');
+    expect(out).toContain('data-signature-block="start"');
+    expect(out).toContain('data-signature-block="end"');
+  });
+});
diff --git a/components/email/quoted-html.ts b/components/email/quoted-html.ts
index 1e0fcd80c..71ab1f72d 100644
--- a/components/email/quoted-html.ts
+++ b/components/email/quoted-html.ts
@@ -7,7 +7,7 @@ import type { Editor } from "@tiptap/react";
 
 import { addTrailingParagraphBreaks } from "@/components/email/paragraph-breaks";
 import { buildSignatureBlock } from "@/components/email/signature-block";
-import { parkRemoteResources, restoreRemoteResources } from "@/lib/email-sanitization";
+import { parkRemoteResources, restoreRemoteResources, sanitizeComposerIslandHtml } from "@/lib/email-sanitization";
 import { useSettingsStore } from "@/stores/settings-store";
 
 // Marker attribute that identifies the quoted-original wrapper in serialized
@@ -65,8 +65,9 @@ export const QuotedHtml = TiptapNode.create({
         default: "",
         // Capture the verbatim inner HTML when parsing. Because the node is
         // an atom, ProseMirror does NOT descend into the children, so they
-        // never hit the schema.
-        parseHTML: (el) => el.innerHTML,
+        // never hit the schema - nor its attribute allowlist, so sanitize
+        // here: a re-opened draft hands this wrapper in unsanitized.
+        parseHTML: (el) => sanitizeComposerIslandHtml(el.innerHTML),
         // Not rendered as an attribute - the real content round-trips via the
         // custom serializer. renderHTML below only needs the wrapper.
         renderHTML: () => ({}),
diff --git a/components/email/signature-block.ts b/components/email/signature-block.ts
index 0150a80c6..c2e9b7676 100644
--- a/components/email/signature-block.ts
+++ b/components/email/signature-block.ts
@@ -3,7 +3,7 @@
 import { Node as TiptapNode, mergeAttributes, type Editor } from "@tiptap/core";
 import { TextSelection } from "@tiptap/pm/state";
 import { DOMParser as ProseMirrorDOMParser } from "@tiptap/pm/model";
-import { parseHtmlSafely } from "@/lib/email-sanitization";
+import { parseHtmlSafely, sanitizeComposerIslandHtml } from "@/lib/email-sanitization";
 
 // Marker attribute that identifies the signature wrapper in serialized HTML,
 // so parseHTML can recognise it on the way back in (initial content, drafts).
@@ -80,7 +80,8 @@ export const SignatureBlock = TiptapNode.create({
         // Capture the verbatim inner HTML when parsing. Because the node is an
         // atom, ProseMirror does NOT descend into the children, so the rich
         // signature markup never hits (and is never mangled by) the schema.
-        parseHTML: (el) => el.innerHTML,
+        // Sanitized here, since a re-opened draft hands it in unsanitized.
+        parseHTML: (el) => sanitizeComposerIslandHtml(el.innerHTML),
         // The real content round-trips via the custom serializer
         // (serializeEditorContent); renderHTML below only needs the wrapper.
         renderHTML: () => ({}),
diff --git a/lib/email-sanitization.ts b/lib/email-sanitization.ts
index 3600ef6c3..a2a74e500 100644
--- a/lib/email-sanitization.ts
+++ b/lib/email-sanitization.ts
@@ -57,6 +57,25 @@ export function sanitizeEmailHtml(html: string): string {
   return sanitizeWithDataUriGuard(html, EMAIL_SANITIZE_CONFIG);
 }
 
+const COMPOSER_ISLAND_SANITIZE_CONFIG = {
+  ...EMAIL_SANITIZE_CONFIG,
+  // The composer's own markers (data-cid on inline images, the signature's
+  // data-signature-block brackets) must survive; data-* is inert.
+  ALLOW_DATA_ATTR: true,
+};
+
+/**
+ * Sanitize the verbatim HTML a composer island (quoted original, signature)
+ * picks up when the editor parses its wrapper. The islands render into the
+ * app document, not the sandboxed viewer frame, and their wrapper reaches
+ * the editor from places that never ran a sanitizer: a re-opened draft
+ * written by another client or another member of a shared mailbox, a paste.
+ */
+export function sanitizeComposerIslandHtml(html: string): string {
+  if (!html) return '';
+  return sanitizeWithDataUriGuard(html, COMPOSER_ISLAND_SANITIZE_CONFIG);
+}
+
 /**
  * Sanitize config for emails rendered inside a sandboxed iframe.
  * Allows <style> tags because CSS is scoped to the iframe document and