For WebDAV to be used as a storage backend for other programs, simply listing all content a user has read access to (in IMAP) can mistakenly lead the user (or client application) to start modifying the contents and attempt to write back the results.
An ACL extension to the WebDAV protocol is available through [[https://tools.ietf.org/html/rfc3744 | RFC 3744]] and supported by SabreDAV.