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CVE-2023-22735: CVE-2023-22735: Provide the Content-Disposition header from S3. · zulip/zulip@2f6c5a8
Zulip is an open-source team collaboration tool. In versions of zulip prior to commit 2f6c5a8 but after commit 04cf68b users could upload files with arbitrary Content-Type which would be served from the Zulip hostname with Content-Disposition: inline and no Content-Security-Policy header, allowing them to trick other users into executing arbitrary Javascript in the context of the Zulip application. Among other things, this enables session theft. Only deployments which use the S3 storage (not the local-disk storage) are affected, and only deployments which deployed commit 04cf68b45ebb5c03247a0d6453e35ffc175d55da, which has only been in main, not any numbered release. Users affected should upgrade from main again to deploy this fix. Switching from S3 storage to the local-disk storage would nominally mitigate this, but is likely more involved than upgrading to the latest main which addresses the issue.
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CVE-2023-22735: Provide the Content-Disposition header from S3.
The Content-Type of user-provided uploads was provided by the browser at initial upload time, and stored in S3; however, 04cf68b switched to determining the Content-Disposition merely from the filename. This makes uploads vulnerable to a stored XSS, wherein a file uploaded with a content-type of `text/html` and an extension of `.png` would be served to browsers as `Content-Disposition: inline`, which is unsafe.
The `Content-Security-Policy` headers in the previous commit mitigate this, but only for browsers which support them.
Revert parts of 04cf68b, specifically by allowing S3 to provide the Content-Disposition header, and using the `ResponseContentDisposition` argument when necessary to override it to `attachment`. Because we expect S3 responses to vary based on this argument, we include it in the cache key; since the query parameter has dashes in it, we can’t use use the helper `$arg_` variables, and must parse it from the query parameters manually.
Adding the disposition may decrease the cache hit rate somewhat, but downloads are infrequent enough that it is unlikely to have a noticeable effect. We take care to not adjust the cache key for requests which do not specify the disposition.
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