Wheels 4.0.0 through 4.1.2 uses case-insensitive comparisons in path-containment checks on case-sensitive filesystems. Canonical-path checks for zip entries, package mappings, development assets/docs, guide images and image tags can accept paths through an existing symlink to a case-distinct sibling directory, allowing access outside the intended root, including limited file disclosure. Separately, the CLI database-dump output check can write outside the application root to an existing writable case-distinct sibling directory; that action requires the CLI POST, loopback and password gate. This is fixed in 4.2.0.