mirror of
https://github.com/element-hq/element-web.git
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79 lines
3 KiB
TypeScript
79 lines
3 KiB
TypeScript
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/*
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Copyright 2021 The Matrix.org Foundation C.I.C.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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// WARNING: We have to be very careful about what mime-types we allow into blobs,
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// as for performance reasons these are now rendered via URL.createObjectURL()
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// rather than by converting into data: URIs.
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//
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// This means that the content is rendered using the origin of the script which
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// called createObjectURL(), and so if the content contains any scripting then it
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// will pose a XSS vulnerability when the browser renders it. This is particularly
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// bad if the user right-clicks the URI and pastes it into a new window or tab,
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// as the blob will then execute with access to Element's full JS environment(!)
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//
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// See https://github.com/matrix-org/matrix-react-sdk/pull/1820#issuecomment-385210647
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// for details.
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//
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// We mitigate this by only allowing mime-types into blobs which we know don't
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// contain any scripting, and instantiate all others as application/octet-stream
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// regardless of what mime-type the event claimed. Even if the payload itself
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// is some malicious HTML, the fact we instantiate it with a media mimetype or
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// application/octet-stream means the browser doesn't try to render it as such.
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//
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// One interesting edge case is image/svg+xml, which empirically *is* rendered
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// correctly if the blob is set to the src attribute of an img tag (for thumbnails)
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// *even if the mimetype is application/octet-stream*. However, empirically JS
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// in the SVG isn't executed in this scenario, so we seem to be okay.
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//
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// Tested on Chrome 65 and Firefox 60
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//
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// The list below is taken mainly from
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// https://developer.mozilla.org/en-US/docs/Web/HTML/Supported_media_formats
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// N.B. Matrix doesn't currently specify which mimetypes are valid in given
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// events, so we pick the ones which HTML5 browsers should be able to display
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//
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// For the record, mime-types which must NEVER enter this list below include:
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// text/html, text/xhtml, image/svg, image/svg+xml, image/pdf, and similar.
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const ALLOWED_BLOB_MIMETYPES = [
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'image/jpeg',
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'image/gif',
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'image/png',
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'video/mp4',
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'video/webm',
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'video/ogg',
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'audio/mp4',
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'audio/webm',
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'audio/aac',
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'audio/mpeg',
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'audio/ogg',
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'audio/wave',
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'audio/wav',
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'audio/x-wav',
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'audio/x-pn-wav',
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'audio/flac',
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'audio/x-flac',
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];
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export function getBlobSafeMimeType(mimetype: string): string {
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if (!ALLOWED_BLOB_MIMETYPES.includes(mimetype)) {
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return 'application/octet-stream';
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}
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return mimetype;
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}
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