diff --git a/src/app/api/files/upload-complete/route.js b/src/app/api/files/upload-complete/route.js
index 7dc940b..0f41110 100644
--- a/src/app/api/files/upload-complete/route.js
+++ b/src/app/api/files/upload-complete/route.js
@@ -70,6 +70,15 @@ export async function POST(request, { params } = {}) {
 			return NextResponse.json({ error: 'Invalid encrypted metadata' }, { status: 400 });
 		}
 
+		// Only complete the object created by upload-url for this authenticated
+		// user, conversation, and file id. Without this check a caller could
+		// register an unrelated object path against an authorized message.
+		const expectedStoragePath = `${user.id}/${conversationId}/${fileId}`;
+		if (storagePath !== expectedStoragePath) {
+			console.error('UPLOAD-COMPLETE: Storage path does not match upload request');
+			return NextResponse.json({ error: 'Storage path does not match upload request' }, { status: 403 });
+		}
+
 		// Get the internal user ID from the users table using auth_user_id
 		const { data: internalUser, error: userError } = await supabase
 			.from('users')
diff --git a/src/app/api/files/upload-complete/route.test.js b/src/app/api/files/upload-complete/route.test.js
index e545f12..228c548 100644
--- a/src/app/api/files/upload-complete/route.test.js
+++ b/src/app/api/files/upload-complete/route.test.js
@@ -131,4 +131,27 @@ describe('POST /api/files/upload-complete validation', () => {
 		expect(mocks.from).not.toHaveBeenCalled();
 		expect(mocks.broadcastToRoom).not.toHaveBeenCalled();
 	});
+
+	it('rejects a storage path that is not bound to the authenticated upload', async () => {
+		const { POST } = await import('./route.js');
+		const request = {
+			json: vi.fn().mockResolvedValue({
+				storagePath: 'another-user/conversation-1/file-1',
+				fileId: 'file-1',
+				metadata: {
+					messageId: 'message-1',
+					conversationId: 'conversation-1',
+					encryptedMetadata: { 'user-1': 'encrypted-metadata' }
+				}
+			})
+		};
+
+		const response = await POST(request);
+		const body = await response.json();
+
+		expect(response.status).toBe(403);
+		expect(body.error).toBe('Storage path does not match upload request');
+		expect(mocks.from).not.toHaveBeenCalled();
+		expect(mocks.broadcastToRoom).not.toHaveBeenCalled();
+	});
 });