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filter mime types for blob urls

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This commit is contained in:
Bruno Windels 2020-10-27 14:35:33 +01:00
parent ade01f6cf7
commit fcc4c21ad2
2 changed files with 59 additions and 1 deletions
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1
src/domain/session/room/timeline/tiles/ImageTile.js
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@ -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));
}

59
src/platform/web/dom/BufferURL.js
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@ -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);
}