diff --git a/demo/vite.config.ts b/demo/vite.config.ts
index 5a33944a..26b73ff5 100644
--- a/demo/vite.config.ts
+++ b/demo/vite.config.ts
@@ -1,7 +1,28 @@
-import { defineConfig } from 'vite'
-import react from '@vitejs/plugin-react'
+import { defineConfig } from 'vite';
+import react from '@vitejs/plugin-react';
+
+// Keep in sync with package/utils/is-allowed-embed-src.ts
+const EMBED_FRAME_SRC = [
+  "'self'",
+  'https://www.youtube.com',
+  'https://youtube.com',
+  'https://www.youtube-nocookie.com',
+  'https://youtube-nocookie.com',
+  'https://player.vimeo.com',
+  'https://w.soundcloud.com',
+].join(' ');
 
 // https://vitejs.dev/config/
 export default defineConfig({
   plugins: [react()],
-})
+  server: {
+    headers: {
+      'Content-Security-Policy': `frame-src ${EMBED_FRAME_SRC};`,
+    },
+  },
+  preview: {
+    headers: {
+      'Content-Security-Policy': `frame-src ${EMBED_FRAME_SRC};`,
+    },
+  },
+});
diff --git a/index.ts b/index.ts
index e5cbbc48..46efd9b6 100644
--- a/index.ts
+++ b/index.ts
@@ -19,6 +19,11 @@ export type {
   CssDiagnostic,
   CssValidationResult,
 } from './package/utils/sanitize-css';
+export {
+  isAllowedEmbedSrc,
+  recommendedEmbedFrameSrcCsp,
+  ALLOWED_EMBED_FRAME_ORIGINS,
+} from './package/utils/is-allowed-embed-src';
 export { buildVersionDiffSnapshot } from './package/components/tabs/utils/version-diff-snapshot';
 export type {
   DdocExportModalProps,
diff --git a/package/extensions/d-block/dblock-media-plugin.ts b/package/extensions/d-block/dblock-media-plugin.ts
index 8a4bb47b..5209611c 100644
--- a/package/extensions/d-block/dblock-media-plugin.ts
+++ b/package/extensions/d-block/dblock-media-plugin.ts
@@ -2,6 +2,7 @@ import type { Node as ProseMirrorNode } from '@tiptap/pm/model';
 import { Plugin, PluginKey } from '@tiptap/pm/state';
 import type { DBlockRuntimeState } from './dblock-runtime';
 import { getDBlockRuntimeState } from './dblock-runtime';
+import { isAllowedEmbedSrc } from '../../utils/is-allowed-embed-src';
 
 const DBLOCK_MEDIA_CONVERSION_META = 'dblock-media-conversion';
 
@@ -194,6 +195,13 @@ export const createDBlockMediaConversionPlugin = (
         candidates
           .sort((a, b) => b.from - a.from)
           .forEach((candidate) => {
+            if (
+              candidate.type === 'iframe' &&
+              !isAllowedEmbedSrc(candidate.src)
+            ) {
+              return;
+            }
+
             const node =
               candidate.type === 'img'
                 ? view.state.schema.nodes.resizableMedia?.create({
diff --git a/package/extensions/iframe/iframe.ts b/package/extensions/iframe/iframe.ts
index 6d1082c2..21473738 100644
--- a/package/extensions/iframe/iframe.ts
+++ b/package/extensions/iframe/iframe.ts
@@ -116,9 +116,21 @@ export const Iframe = Node.create<IframeOptions>({
   },
 
   renderHTML({ HTMLAttributes }) {
+    if (!isAllowedEmbedSrc(HTMLAttributes.src)) {
+      return [
+        'div',
+        { class: 'iframe-blocked' },
+        'Embed blocked: unsupported or disallowed source',
+      ];
+    }
+
     return [
       'iframe',
-      mergeAttributes(this.options.HTMLAttributes, HTMLAttributes),
+      mergeAttributes(this.options.HTMLAttributes, HTMLAttributes, {
+        sandbox:
+          'allow-scripts allow-same-origin allow-presentation allow-popups',
+        referrerpolicy: 'no-referrer',
+      }),
     ];
   },
 
diff --git a/package/extensions/resizable-media/resizable-media-node-view.tsx b/package/extensions/resizable-media/resizable-media-node-view.tsx
index e3d3fd5c..47e14f71 100644
--- a/package/extensions/resizable-media/resizable-media-node-view.tsx
+++ b/package/extensions/resizable-media/resizable-media-node-view.tsx
@@ -9,6 +9,8 @@ import ToolbarButton from '../../common/toolbar-button';
 import { SecureImage } from '../../components/secure-image.tsx';
 import { SecureImageV2 } from '../../components/secure-image-v2.tsx';
 import { IpfsImageFetchPayload } from '../../types.ts';
+import { isAllowedEmbedSrc } from '../../utils/is-allowed-embed-src';
+
 interface WidthAndHeight {
   width: number;
   height: number;
@@ -36,8 +38,11 @@ export const getResizableMediaNodeView =
 
     const isImageType = mediaType === 'img' || mediaType === 'secure-img';
 
+    const isSafeIframe =
+      mediaType === 'iframe' && isAllowedEmbedSrc(node.attrs.src);
+
     const isSoundcloudIframe =
-      mediaType === 'iframe' &&
+      isSafeIframe &&
       URL.canParse(node.attrs.src) &&
       new URL(node.attrs.src).hostname === 'w.soundcloud.com';
 
@@ -523,19 +528,31 @@ export const getResizableMediaNodeView =
               </video>
             )}
 
-            {mediaType === 'iframe' && (
-              <>
-                <iframe
-                  ref={resizableImgRef as LegacyRef<HTMLIFrameElement>}
-                  className={cn(
-                    'rounded-lg max-w-full',
-                    isMouseDown && 'pointer-events-none',
-                  )}
-                  src={node.attrs.src}
-                  width={node.attrs.width}
-                  height={node.attrs.height}
-                />
-              </>
+            {mediaType === 'iframe' && isSafeIframe && (
+              <iframe
+                ref={resizableImgRef as LegacyRef<HTMLIFrameElement>}
+                className={cn(
+                  'rounded-lg max-w-full',
+                  isMouseDown && 'pointer-events-none',
+                )}
+                src={node.attrs.src}
+                width={node.attrs.width}
+                height={node.attrs.height}
+                sandbox="allow-scripts allow-same-origin allow-presentation allow-popups"
+                referrerPolicy="no-referrer"
+              />
+            )}
+
+            {mediaType === 'iframe' && !isSafeIframe && (
+              <div
+                className="rounded-lg border color-border-default color-bg-secondary color-text-secondary text-sm p-4 max-w-full"
+                style={{
+                  width: node.attrs.width || 640,
+                  minHeight: node.attrs.height || 120,
+                }}
+              >
+                Embed blocked: unsupported or disallowed source
+              </div>
             )}
 
             {!isPreviewMode && !isImageType && (
diff --git a/package/utils/is-allowed-embed-src.test.ts b/package/utils/is-allowed-embed-src.test.ts
new file mode 100644
index 00000000..56e7aa67
--- /dev/null
+++ b/package/utils/is-allowed-embed-src.test.ts
@@ -0,0 +1,65 @@
+import { describe, expect, it } from 'vitest';
+import {
+  isAllowedEmbedSrc,
+  recommendedEmbedFrameSrcCsp,
+} from './is-allowed-embed-src';
+
+describe('isAllowedEmbedSrc', () => {
+  it('accepts allowlisted https embed hosts/paths', () => {
+    expect(
+      isAllowedEmbedSrc('https://www.youtube.com/embed/dQw4w9WgXcQ'),
+    ).toBe(true);
+    expect(
+      isAllowedEmbedSrc('https://www.youtube-nocookie.com/embed/dQw4w9WgXcQ'),
+    ).toBe(true);
+    expect(isAllowedEmbedSrc('https://player.vimeo.com/video/123456')).toBe(
+      true,
+    );
+    expect(
+      isAllowedEmbedSrc(
+        'https://w.soundcloud.com/player/?url=https%3A//api.soundcloud.com/tracks/1',
+      ),
+    ).toBe(true);
+  });
+
+  it('rejects arbitrary third-party frames', () => {
+    expect(isAllowedEmbedSrc('https://evil.example/payload')).toBe(false);
+    expect(isAllowedEmbedSrc('https://evil.example/embed/x')).toBe(false);
+    expect(isAllowedEmbedSrc('javascript:alert(1)')).toBe(false);
+    expect(isAllowedEmbedSrc('data:text/html,<script>alert(1)</script>')).toBe(
+      false,
+    );
+    expect(isAllowedEmbedSrc('https://www.youtube.com/watch?v=x')).toBe(false);
+    expect(isAllowedEmbedSrc('https://youtube.com/')).toBe(false);
+  });
+
+  it('rejects http even for allowlisted hosts', () => {
+    expect(isAllowedEmbedSrc('http://www.youtube.com/embed/dQw4w9WgXcQ')).toBe(
+      false,
+    );
+    expect(isAllowedEmbedSrc('http://player.vimeo.com/video/123')).toBe(false);
+  });
+
+  it('rejects non-strings and invalid URLs', () => {
+    expect(isAllowedEmbedSrc(null)).toBe(false);
+    expect(isAllowedEmbedSrc(undefined)).toBe(false);
+    expect(isAllowedEmbedSrc('')).toBe(false);
+    expect(isAllowedEmbedSrc('not-a-url')).toBe(false);
+  });
+});
+
+describe('recommendedEmbedFrameSrcCsp', () => {
+  it('includes self and allowlisted https origins', () => {
+    const value = recommendedEmbedFrameSrcCsp();
+    expect(value).toContain("'self'");
+    expect(value).toContain('https://www.youtube.com');
+    expect(value).toContain('https://player.vimeo.com');
+    expect(value).toContain('https://w.soundcloud.com');
+    expect(value).not.toContain('http://');
+  });
+
+  it('merges extra origins', () => {
+    const value = recommendedEmbedFrameSrcCsp(['https://cdn.example']);
+    expect(value).toContain('https://cdn.example');
+  });
+});
diff --git a/package/utils/is-allowed-embed-src.ts b/package/utils/is-allowed-embed-src.ts
index c6fb1ba9..8a303dc2 100644
--- a/package/utils/is-allowed-embed-src.ts
+++ b/package/utils/is-allowed-embed-src.ts
@@ -1,6 +1,6 @@
-// Allowlist for iframe `src` URLs. Only hosts/paths produced by the
-// in-app embed flows (YouTube, Vimeo, SoundCloud) are accepted. Everything
-// else is rejected to prevent loading arbitrary third-party frames.
+// Allowlist for iframe `src` URLs (HTTPS YouTube / Vimeo / SoundCloud only).
+// Host apps should also set CSP frame-src via recommendedEmbedFrameSrcCsp().
+
 const ALLOWED_EMBEDS: { host: string; pathPrefix: string }[] = [
   { host: 'www.youtube.com', pathPrefix: '/embed/' },
   { host: 'youtube.com', pathPrefix: '/embed/' },
@@ -10,10 +10,31 @@ const ALLOWED_EMBEDS: { host: string; pathPrefix: string }[] = [
   { host: 'w.soundcloud.com', pathPrefix: '/player' },
 ];
 
+export const ALLOWED_EMBED_FRAME_ORIGINS = [
+  'https://www.youtube.com',
+  'https://youtube.com',
+  'https://www.youtube-nocookie.com',
+  'https://youtube-nocookie.com',
+  'https://player.vimeo.com',
+  'https://w.soundcloud.com',
+] as const;
+
+/** CSP `frame-src` value for hosts embedding DdocEditor. */
+export function recommendedEmbedFrameSrcCsp(
+  extraOrigins: readonly string[] = [],
+): string {
+  const origins = new Set<string>([
+    "'self'",
+    ...ALLOWED_EMBED_FRAME_ORIGINS,
+    ...extraOrigins,
+  ]);
+  return Array.from(origins).join(' ');
+}
+
 export function isAllowedEmbedSrc(src: unknown): src is string {
   if (typeof src !== 'string' || !URL.canParse(src)) return false;
   const url = new URL(src);
-  if (url.protocol !== 'https:' && url.protocol !== 'http:') return false;
+  if (url.protocol !== 'https:') return false;
   return ALLOWED_EMBEDS.some(
     ({ host, pathPrefix }) =>
       url.hostname === host && url.pathname.startsWith(pathPrefix),