From fcc4c21ad2aadd2d63553c6afe127f981a8ea73f Mon Sep 17 00:00:00 2001
From: Bruno Windels <brunow@matrix.org>
Date: Tue, 27 Oct 2020 14:35:33 +0100
Subject: [PATCH] filter mime types for blob urls

---
 .../session/room/timeline/tiles/ImageTile.js  |  1 -
 src/platform/web/dom/BufferURL.js             | 59 +++++++++++++++++++
 2 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/src/domain/session/room/timeline/tiles/ImageTile.js b/src/domain/session/room/timeline/tiles/ImageTile.js
index ea876b51..27e68250 100644
--- a/src/domain/session/room/timeline/tiles/ImageTile.js
+++ b/src/domain/session/room/timeline/tiles/ImageTile.js
@@ -32,7 +32,6 @@ export class ImageTile extends MessageTile {
         if (this.isDisposed) {
             return;
         }
-        // TODO: fix XSS bug here by not checking mimetype
         return this.track(this.platform.createBufferURL(buffer, file.mimetype));
     }
 
diff --git a/src/platform/web/dom/BufferURL.js b/src/platform/web/dom/BufferURL.js
index 144acf35..28730022 100644
--- a/src/platform/web/dom/BufferURL.js
+++ b/src/platform/web/dom/BufferURL.js
@@ -14,8 +14,67 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+
+// WARNING: We have to be very careful about what mime-types we allow into blobs.
+//
+// This means that the content is rendered using the origin of the script which
+// called createObjectURL(), and so if the content contains any scripting then it
+// will pose a XSS vulnerability when the browser renders it.  This is particularly
+// bad if the user right-clicks the URI and pastes it into a new window or tab,
+// as the blob will then execute with access to Element's full JS environment(!)
+//
+// See https://github.com/matrix-org/matrix-react-sdk/pull/1820#issuecomment-385210647
+// for details.
+//
+// We mitigate this by only allowing mime-types into blobs which we know don't
+// contain any scripting, and instantiate all others as application/octet-stream
+// regardless of what mime-type the event claimed.  Even if the payload itself
+// is some malicious HTML, the fact we instantiate it with a media mimetype or
+// application/octet-stream means the browser doesn't try to render it as such.
+//
+// One interesting edge case is image/svg+xml, which empirically *is* rendered
+// correctly if the blob is set to the src attribute of an img tag (for thumbnails)
+// *even if the mimetype is application/octet-stream*.  However, empirically JS
+// in the SVG isn't executed in this scenario, so we seem to be okay.
+//
+// Tested on Chrome 65 and Firefox 60
+//
+// The list below is taken mainly from
+// https://developer.mozilla.org/en-US/docs/Web/HTML/Supported_media_formats
+// N.B. Matrix doesn't currently specify which mimetypes are valid in given
+// events, so we pick the ones which HTML5 browsers should be able to display
+//
+// For the record, mime-types which must NEVER enter this list below include:
+//   text/html, text/xhtml, image/svg, image/svg+xml, image/pdf, and similar.
+
+const ALLOWED_BLOB_MIMETYPES = {
+    'image/jpeg': true,
+    'image/gif': true,
+    'image/png': true,
+
+    'video/mp4': true,
+    'video/webm': true,
+    'video/ogg': true,
+
+    'audio/mp4': true,
+    'audio/webm': true,
+    'audio/aac': true,
+    'audio/mpeg': true,
+    'audio/ogg': true,
+    'audio/wave': true,
+    'audio/wav': true,
+    'audio/x-wav': true,
+    'audio/x-pn-wav': true,
+    'audio/flac': true,
+    'audio/x-flac': true,
+};
+
 export class BufferURL {
     constructor(buffer, mimetype) {
+        mimetype = mimetype ? mimetype.split(";")[0].trim() : '';
+        if (!ALLOWED_BLOB_MIMETYPES[mimetype]) {
+            mimetype = 'application/octet-stream';
+        }
         const blob = new Blob([buffer], {type: mimetype});
         this.url = URL.createObjectURL(blob);
     }