diff --git a/app/api/upload/audio/[filename]/route.ts b/app/api/upload/audio/[filename]/route.ts
index 54edca7..065771c 100644
--- a/app/api/upload/audio/[filename]/route.ts
+++ b/app/api/upload/audio/[filename]/route.ts
@@ -1,3 +1,8 @@
+import { NextRequest } from 'next/server';
+import { auth, checkProjectAccess } from '@/lib/auth';
+import { db } from '@/lib/db';
+import { validateShareLinkAccess } from '@/lib/share-links';
+import { getShareSessionFromRequest } from '@/lib/share-session';
 import { apiErrors } from '@/lib/api-response';
 import { proxyR2MediaObject } from '@/lib/r2-media-proxy';
 import { logError } from '@/lib/logger';
@@ -20,7 +25,7 @@ function getContentType(filename: string): string {
 }
 
 export async function GET(
-  request: Request,
+  request: NextRequest,
   { params }: { params: Promise<{ filename: string }> }
 ) {
   try {
@@ -31,12 +36,70 @@ export async function GET(
       return apiErrors.badRequest('Invalid filename');
     }
 
+    // Parallelize the DB lookup and session check to narrow the timing delta
+    // between "asset not found" and "asset found, access denied" responses.
+    const voiceUrl = `/api/upload/audio/${filename}`;
+    const [comment, session] = await Promise.all([
+      db.comment.findFirst({
+        where: { voiceUrl },
+        select: {
+          version: {
+            select: {
+              video: {
+                select: {
+                  id: true,
+                  projectId: true,
+                  project: {
+                    select: {
+                      id: true,
+                      ownerId: true,
+                      workspaceId: true,
+                      visibility: true,
+                    },
+                  },
+                },
+              },
+            },
+          },
+        },
+      }),
+      auth(),
+    ]);
+
+    if (!comment) {
+      return apiErrors.forbidden('Access denied');
+    }
+
+    const { video } = comment.version;
+    const access = await checkProjectAccess(video.project, session?.user?.id);
+
+    if (!access.hasAccess) {
+      const shareSession = getShareSessionFromRequest(request, video.id);
+      const shareAccess = shareSession
+        ? await validateShareLinkAccess({
+          token: shareSession.token,
+          projectId: video.projectId,
+          videoId: video.id,
+          requiredPermission: 'VIEW',
+          passwordVerified: shareSession.passwordVerified,
+        })
+        : null;
+
+      if (!shareAccess?.hasAccess) {
+        return apiErrors.forbidden('Access denied');
+      }
+    }
+
     const key = `voice/${filename}`;
     return proxyR2MediaObject({
       request,
       key,
       fallbackContentType: getContentType(filename),
       cacheControl: 'private, no-store',
+      extraHeaders: {
+        'X-Content-Type-Options': 'nosniff',
+        'Content-Security-Policy': "default-src 'none'; sandbox",
+      },
       internalErrorMessage: 'Failed to retrieve audio',
     });
   } catch (error: unknown) {
diff --git a/app/api/upload/image/[filename]/route.ts b/app/api/upload/image/[filename]/route.ts
index 7b41bbd..272bfcf 100644
--- a/app/api/upload/image/[filename]/route.ts
+++ b/app/api/upload/image/[filename]/route.ts
@@ -1,3 +1,8 @@
+import { NextRequest } from 'next/server';
+import { auth, checkProjectAccess } from '@/lib/auth';
+import { db } from '@/lib/db';
+import { validateShareLinkAccess } from '@/lib/share-links';
+import { getShareSessionFromRequest } from '@/lib/share-session';
 import { apiErrors } from '@/lib/api-response';
 import { proxyR2MediaObject } from '@/lib/r2-media-proxy';
 import { logError } from '@/lib/logger';
@@ -18,7 +23,7 @@ function getContentType(filename: string): string {
 }
 
 export async function GET(
-    request: Request,
+    request: NextRequest,
     { params }: { params: Promise<{ filename: string }> }
 ) {
     try {
@@ -29,6 +34,60 @@ export async function GET(
             return apiErrors.badRequest('Invalid filename');
         }
 
+        // Parallelize the DB lookup and session check to narrow the timing delta
+        // between "asset not found" and "asset found, access denied" responses.
+        const imageUrl = `/api/upload/image/${filename}`;
+        const [comment, session] = await Promise.all([
+            db.comment.findFirst({
+                where: { imageUrl },
+                select: {
+                    version: {
+                        select: {
+                            video: {
+                                select: {
+                                    id: true,
+                                    projectId: true,
+                                    project: {
+                                        select: {
+                                            id: true,
+                                            ownerId: true,
+                                            workspaceId: true,
+                                            visibility: true,
+                                        },
+                                    },
+                                },
+                            },
+                        },
+                    },
+                },
+            }),
+            auth(),
+        ]);
+
+        if (!comment) {
+            return apiErrors.forbidden('Access denied');
+        }
+
+        const { video } = comment.version;
+        const access = await checkProjectAccess(video.project, session?.user?.id);
+
+        if (!access.hasAccess) {
+            const shareSession = getShareSessionFromRequest(request, video.id);
+            const shareAccess = shareSession
+                ? await validateShareLinkAccess({
+                    token: shareSession.token,
+                    projectId: video.projectId,
+                    videoId: video.id,
+                    requiredPermission: 'VIEW',
+                    passwordVerified: shareSession.passwordVerified,
+                })
+                : null;
+
+            if (!shareAccess?.hasAccess) {
+                return apiErrors.forbidden('Access denied');
+            }
+        }
+
         const key = `images/${filename}`;
         return proxyR2MediaObject({
             request,
diff --git a/prisma/schema.prisma b/prisma/schema.prisma
index 8008d32..0d57b4e 100644
--- a/prisma/schema.prisma
+++ b/prisma/schema.prisma
@@ -466,6 +466,8 @@ model Comment {
   @@index([guestIdentityId])
   @@index([timestamp])
   @@index([tagId])
+  @@unique([imageUrl])
+  @@unique([voiceUrl])
   @@index([versionId, isResolved, timestamp])
   @@index([versionId, parentId, createdAt])
   @@map("comments")