zrok: WebDAV drive backend follows symlinks outside DriveRoot, enabling host filesystem read/write

Summary The zrok WebDAV drive backend (davServer.Dir) restricts path traversal through lexical normalization but does not prevent symlink following. When a symbolic link inside the shared DriveRoot points to a location outside that root, remote WebDAV consumers can read files and—on shares without OS-level permission restrictions—write or overwrite files anywhere on the host filesystem accessible to the zrok process.

• Attack Vector: Network — exploitation is performed entirely over the WebDAV endpoint; the attacker issues HTTP requests to the public zrok share URL.
• Attack Complexity: High — a precondition outside the attacker's direct control must hold: a symlink pointing outside DriveRoot must already exist within it (created locally, not via WebDAV).
• Privileges Required: None — zrok share public --backend-mode drive exposes the WebDAV endpoint with no authentication by default.
• User Interaction: None — once the symlink precondition is met, exploitation requires no user interaction.
• Scope: Changed — the vulnerability allows an attacker to escape the WebDAV root (the security boundary) and access the broader host filesystem.
• Confidentiality Impact: High — arbitrary files readable by the zrok process can be retrieved.
• Integrity Impact: High — the WebDAV PUT handler opens files with O_RDWR|O_CREATE|O_TRUNC, meaning symlink targets outside DriveRoot can be overwritten (e.g. ~/.ssh/authorized_keys).
• Availability Impact: None — no direct availability impact.

Affected Components

• drives/davServer/file.go — Dir.OpenFile (line 140), Dir.Stat (line 176), Dir.Mkdir (line 133), Dir.RemoveAll (line 151)
• endpoints/drive/backend.go — NewBackend (line 51–52)