zrok copy writes attacker-controlled WebDAV paths outside the destination root

Summary

Alice runs zrok2 copy from a WebDAV or zrok drive controlled by Bob into a local filesystem target. Bob returns a DAV href such as /../outside.txt. The sync pipeline stores that path in the source inventory and passes it to FilesystemTarget.WriteStream, which joins it with the target root and creates the file outside Alice's selected directory.

Impact

Users given access to a zrok share may be able to traverse the directory tree arbitrarily with the sharing users credentials, allowing for sensitive information to be overwritten.