packages/iios-service/src/media/media.controller.ts

@@ -5,6 +5,9 @@ import { SessionVerifier } from '../platform/session.verifier';
 import { PresignDownloadDto, PresignUploadDto } from './media.dto';
 import type { MessagePrincipal } from '../identity/actor.resolver';
 
+/** Types that can execute script if a browser renders them top-level — served as downloads only. */
+const SCRIPTABLE_MIMES = new Set(['text/html', 'application/xhtml+xml', 'image/svg+xml', 'text/xml', 'application/xml']);
+
 @Controller('v1/media')
 export class MediaController {
   constructor(
@@ -37,6 +40,12 @@ export class MediaController {
   async blob(@Param('token') token: string, @Res() res: Response) {
     const { data, mime } = await this.media.get(token);
     res.setHeader('Content-Type', mime);
+    // Never let the browser MIME-sniff an upload into something executable.
+    res.setHeader('X-Content-Type-Options', 'nosniff');
+    // Script-capable types must not render inline from our origin (stored-XSS) — force a download.
+    // Images/video/audio/pdf stay inline so the app can preview them. Note <img>/<video> still embed
+    // fine even with attachment disposition; only top-level navigation to the blob is affected.
+    if (SCRIPTABLE_MIMES.has(mime)) res.setHeader('Content-Disposition', 'attachment');
     res.setHeader('Cache-Control', 'private, max-age=3600');
     res.send(data);
   }

packages/iios-service/src/platform/dev-opa.port.spec.ts

@@ -49,6 +49,19 @@ describe('DevOpaPort (dev policy plane — membership rules)', () => {
     expect((await opa.decide({ action: 'iios.thread.participant.remove', callerRole: 'MEMBER' })).allow).toBe(true);
   });
 
+  it('media upload allows images + docs (incl. html/markdown/csv), denies unknown types and oversize', async () => {
+    const up = (mime: string, sizeBytes = 1024) => opa.decide({ action: 'iios.media.upload', mime, sizeBytes });
+    for (const mime of ['image/png', 'video/mp4', 'audio/mpeg', 'application/pdf', 'text/plain', 'text/markdown', 'text/html', 'text/csv']) {
+      expect((await up(mime)).allow).toBe(true);
+    }
+    const bad = await up('application/x-msdownload');
+    expect(bad.allow).toBe(false);
+    expect(bad.obligations[0]?.reason).toMatch(/not allowed/);
+    const big = await up('image/png', 30 * 1024 * 1024);
+    expect(big.allow).toBe(false);
+    expect(big.obligations[0]?.reason).toMatch(/too large/);
+  });
+
   it('self-join is governed only on membership threads', async () => {
     // generic / support thread (no membership attr) → open join, unchanged
     expect((await opa.decide({ action: 'iios.thread.join', alreadyMember: false })).allow).toBe(true);

packages/iios-service/src/platform/dev-opa.port.ts

@@ -22,7 +22,7 @@ export interface OpaInput {
 const MEDIA_MAX_BYTES = 25 * 1024 * 1024;
 const MEDIA_ALLOWED = /^(image|video|audio)\//;
 const MEDIA_ALLOWED_DOCS = new Set([
-  'application/pdf', 'text/plain', 'application/zip',
+  'application/pdf', 'text/plain', 'text/markdown', 'text/x-markdown', 'text/html', 'text/csv', 'application/zip',
   'application/msword', 'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
   'application/vnd.ms-excel', 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
   'application/vnd.ms-powerpoint', 'application/vnd.openxmlformats-officedocument.presentationml.presentation',