microsoft · compulim · Sep 3, 2025 · Sep 3, 2025 · Sep 3, 2025 · Sep 3, 2025
@@ -39,3 +39,8 @@ rules:
       message: Use clock functions from ponyfill
     - name: setTimeout
       message: Use clock functions from ponyfill
+  no-restricted-properties:
+    - error
+    - object: URL
+      property: createObjectURL
+      message: Make sure content type is sanitized before presenting the URL in HTML
@@ -120,6 +120,7 @@ Notes: web developers are advised to use [`~` (tilde range)](https://github.com/
    - `@msinternal/botframework-webchat-debug-theme` package for enabling debugging scenarios
    - `@msinternal/botframework-webchat-react-hooks` for helpers for React hooks
 - Added link sanitization and ESLint rules, in PR [#5564](https://github.com/microsoft/BotFramework-WebChat/pull/5564), by [@compulim](https://github.com/compulim)
+- Added blob URL sanitization and ESLint rules, in PR [#5568](https://github.com/microsoft/BotFramework-WebChat/pull/5568), by [@compulim](https://github.com/compulim)
 
 ### Changed
 

@@ -0,0 +1,53 @@
+<!doctype html>
+<html lang="en-US">
+  <head>
+    <link href="/assets/index.css" rel="stylesheet" type="text/css" />
+    <script crossorigin="anonymous" src="/test-harness.js"></script>
+    <script crossorigin="anonymous" src="/test-page-object.js"></script>
+    <script crossorigin="anonymous" src="/__dist__/webchat-es5.js"></script>
+  </head>
+  <body>
+    <main id="webchat"></main>
+    <script>
+      run(async function () {
+        const {
+          testHelpers: { createDirectLineEmulator }
+        } = window;
+
+        const { directLine, store } = createDirectLineEmulator();
+
+        WebChat.renderWebChat({ directLine, store }, document.getElementById('webchat'));
+
+        await pageConditions.uiConnected();
+
+        await directLine.emulateIncomingActivity({
+          attachments: [
+            {
+              contentType: 'image/png',
+              contentUrl:
+                'data:application/octet-stream;base64,iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAK0lEQVQ4T2P8z8Dwn4GKgHHUQIpDczQMKQ5ChtEwHA1DMkJgNNmQEWhoWgBMAiftPRtHngAAAABJRU5ErkJggg=='
+            }
+          ],
+          type: 'message'
+        });
+
+        const imageElement = pageElements.activities()[0].querySelector('img');
+
+        const imageURL = imageElement.getAttribute('src');
+
+        // THEN: This test is only relevant it is a blob URL.
+        expect(imageURL.startsWith('blob:')).toEqual(true);
+
+        const res = await fetch(imageURL);
+
+        const contentType = res.headers.get('content-type');
+
+        // THEN: Content type should be "application/octet-stream".
+        expect(contentType).toBe('application/octet-stream');
+
+        // THEN: Should show the image properly.
+        await host.snapshot('local');
+      });
+    </script>
+  </body>
+</html>
@@ -0,0 +1,53 @@
+<!doctype html>
+<html lang="en-US">
+  <head>
+    <link href="/assets/index.css" rel="stylesheet" type="text/css" />
+    <script crossorigin="anonymous" src="/test-harness.js"></script>
+    <script crossorigin="anonymous" src="/test-page-object.js"></script>
+    <script crossorigin="anonymous" src="/__dist__/webchat-es5.js"></script>
+  </head>
+  <body>
+    <main id="webchat"></main>
+    <script>
+      run(async function () {
+        const {
+          testHelpers: { createDirectLineEmulator }
+        } = window;
+
+        const { directLine, store } = createDirectLineEmulator();
+
+        WebChat.renderWebChat({ directLine, store }, document.getElementById('webchat'));
+
+        await pageConditions.uiConnected();
+
+        await directLine.emulateIncomingActivity({
+          attachments: [
+            {
+              contentType: 'image/png',
+              contentUrl:
+                'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAK0lEQVQ4T2P8z8Dwn4GKgHHUQIpDczQMKQ5ChtEwHA1DMkJgNNmQEWhoWgBMAiftPRtHngAAAABJRU5ErkJggg=='
+            }
+          ],
+          type: 'message'
+        });
+
+        const imageElement = pageElements.activities()[0].querySelector('img');
+
+        const imageURL = imageElement.getAttribute('src');
+
+        // THEN: This test is only relevant it is a blob URL.
+        expect(imageURL.startsWith('blob:')).toEqual(true);
+
+        const res = await fetch(imageURL);
+
+        const contentType = res.headers.get('content-type');
+
+        // THEN: Content type should be "image/png".
+        expect(contentType).toBe('image/png');
+
+        // THEN: Should show the image properly.
+        await host.snapshot('local');
+      });
+    </script>
+  </body>
+</html>
@@ -0,0 +1,53 @@
+<!doctype html>
+<html lang="en-US">
+  <head>
+    <link href="/assets/index.css" rel="stylesheet" type="text/css" />
+    <script crossorigin="anonymous" src="/test-harness.js"></script>
+    <script crossorigin="anonymous" src="/test-page-object.js"></script>
+    <script crossorigin="anonymous" src="/__dist__/webchat-es5.js"></script>
+  </head>
+  <body>
+    <main id="webchat"></main>
+    <script>
+      run(async function () {
+        const {
+          testHelpers: { createDirectLineEmulator }
+        } = window;
+
+        const { directLine, store } = createDirectLineEmulator();
+
+        WebChat.renderWebChat({ directLine, store }, document.getElementById('webchat'));
+
+        await pageConditions.uiConnected();
+
+        await directLine.emulateIncomingActivity({
+          attachments: [
+            {
+              contentType: 'image/png',
+              contentUrl:
+                'data:text/plain;base64,iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAK0lEQVQ4T2P8z8Dwn4GKgHHUQIpDczQMKQ5ChtEwHA1DMkJgNNmQEWhoWgBMAiftPRtHngAAAABJRU5ErkJggg=='
+            }
+          ],
+          type: 'message'
+        });
+
+        const imageElement = pageElements.activities()[0].querySelector('img');
+
+        const imageURL = imageElement.getAttribute('src');
+
+        // THEN: This test is only relevant it is a blob URL.
+        expect(imageURL.startsWith('blob:')).toEqual(true);
+
+        const res = await fetch(imageURL);
+
+        const contentType = res.headers.get('content-type');
+
+        // THEN: Content type should be rectified to "application/octet-stream".
+        expect(contentType).toBe('application/octet-stream');
+
+        // THEN: Should show the image properly.
+        await host.snapshot('local');
+      });
+    </script>
+  </body>
+</html>
@@ -0,0 +1,59 @@
+import { validateProps } from '@msinternal/botframework-webchat-react-valibot';
+import React, { memo } from 'react';
+import { custom, object, optional, pipe, readonly, safeParse, string, union, type InferInput } from 'valibot';
+
+import readDataURIToBlob from '../Utils/readDataURIToBlob';
+import ImageContent from './ImageContent';
+import { type WebChatAttachment } from './private/types/WebChatAttachment';
+
+const imageAttachmentPropsSchema = pipe(
+  object({
+    attachment: custom<WebChatAttachment>(
+      value =>
+        safeParse(
+          union([
+            object({
+              contentUrl: string(),
+              name: optional(string()),
+              thumbnailUrl: optional(string())
+            }),
+            object({
+              contentUrl: optional(string()),
+              name: optional(string()),
+              thumbnailUrl: string()
+            })
+          ]),
+          value
+        ).success
+    )
+  }),
+  readonly()
+);
+
+type ImageAttachmentProps = InferInput<typeof imageAttachmentPropsSchema>;
+
+// React component is better with standard function than arrow function.
+// eslint-disable-next-line prefer-arrow-callback
+const ImageAttachment = memo(function ImageAttachment(props: ImageAttachmentProps) {
+  const { attachment } = validateProps(imageAttachmentPropsSchema, props);
+
+  let imageURL = attachment.thumbnailUrl || attachment.contentUrl;
+
+  // To support Content Security Policy, data URI cannot be used.
+  // We need to parse the data URI into a blob: URL.
+  const blob = readDataURIToBlob(imageURL);
+
+  if (blob) {
+    // Only allow image/* for image, otherwise, treat it as binary.
+    // eslint-disable-next-line no-restricted-properties
+    imageURL = URL.createObjectURL(
+      new Blob([blob], {
+        type: blob.type.startsWith('image/') ? blob.type : 'application/octet-stream'
+      })
+    );
+  }
+
+  return <ImageContent alt={attachment.name} src={imageURL} />;
+});
+
+export default ImageAttachment;
@@ -15,6 +15,8 @@ export default function createSpinnerAnimationStyle({
   spinnerAnimationPadding
 }: StrictStyleOptions) {
   defaultImageBlobURL ||
+    // Content is hardcoded.
+    // eslint-disable-next-line no-restricted-properties
     (defaultImageBlobURL = URL.createObjectURL(
       new Blob([new Uint8Array(toByteArray(DEFAULT_IMAGE_BASE64))], { type: DEFAULT_IMAGE_TYPE })
     ));

@@ -13,6 +13,8 @@ export default function createTypingAnimationStyle({
   typingAnimationWidth
 }: StrictStyleOptions) {
   defaultImageBlobURL ||
+    // Content is hardcoded.
+    // eslint-disable-next-line no-restricted-properties
     (defaultImageBlobURL = URL.createObjectURL(
       new Blob([new Uint8Array(toByteArray(DEFAULT_IMAGE_BASE64))], { type: DEFAULT_IMAGE_TYPE })
     ));

@@ -39,6 +39,8 @@ function createCanvas(width: number, height: number): HTMLCanvasElement {
 }
 
 function loadImageFromBlob(blob: Blob): Promise<HTMLImageElement> {
+  // Blob is only loaded in <img> created inline and not presented in HTML.
+  // eslint-disable-next-line no-restricted-properties
   const blobURL = URL.createObjectURL(blob);
 
   return new Promise<HTMLImageElement>((resolve, reject) => {

@@ -4,7 +4,13 @@ const PATTERN = /^data:([^,]*?)(;(base64)){0,1},([A-Za-z0-9+/=]+)/u;
 
 const DEFAULT_CONTENT_TYPE = 'text/plain;charset=US-ASCII';
 
-export function parse(dataURI) {
+export function parse(dataURI: string):
+  | {
+      base64: string;
+      contentType: string;
+      encoding: string;
+    }
+  | undefined {
   const match = PATTERN.exec(dataURI);
 
   if (!match) {
@@ -20,12 +26,12 @@ export function parse(dataURI) {
   return { base64, contentType: contentType || DEFAULT_CONTENT_TYPE, encoding };
 }
 
-export default function readDataURIToBlob(dataURI) {
+export default function readDataURIToBlob(dataURI: string): Blob | undefined {
   const parsed = parse(dataURI);
 
   if (!parsed) {
     return;
   }
 
-  return new Blob([toByteArray(parsed.base64)], { type: parsed.contentType });
+  return new Blob([toByteArray(parsed.base64).buffer as ArrayBuffer], { type: parsed.contentType });
 }
@@ -33,7 +33,10 @@ export default function useSendFiles(): (files: readonly File[]) => void {
                   name: file.name,
                   size: file.size,
                   thumbnail: thumbnailURL?.toString(),
-                  url: URL.createObjectURL(file)
+                  // The URL is passed to chat adapter and should be treated as binary.
+                  // Just in case the chat adapter naively echo back, it should show up as binary.
+                  // eslint-disable-next-line no-restricted-properties
+                  url: URL.createObjectURL(new Blob([file], { type: 'application/octet-stream' }))
                 }))
               )
             )

@@ -12,7 +12,10 @@ function* postActivityWithMessage({
       {
         attachments: attachments.map(({ blob, thumbnailURL }) => ({
           contentType: (blob instanceof File && blob.type) || 'application/octet-stream',
-          contentUrl: URL.createObjectURL(blob),
+          // Chat adapter should download the file as binary.
+          // In case the chat adapter naively echo back the URL, it will be treated as binary.
+          // eslint-disable-next-line no-restricted-properties
+          contentUrl: URL.createObjectURL(new Blob([blob], { type: 'application/octet-stream' })),
           name: blob instanceof File ? blob.name : undefined,
           thumbnailUrl: thumbnailURL?.toString()
         })),

@@ -15,6 +15,8 @@ const SLIDING_DOTS_SVG_STRING =
 
 const AssetComposer = memo(({ children }: AssetComposerProps) => {
   const slidingDotsURL = useMemo(
+    // Content is hardcoded.
+    // eslint-disable-next-line no-restricted-properties
     () => URL.createObjectURL(new Blob([SLIDING_DOTS_SVG_STRING], { type: 'image/svg+xml' })),
     []
   );