Copier vulnerabilities

Copier versions 9.5.0 through 9.16.0 contain a vulnerability that allows bypassing the trusted repository prefix. Percent-encoded path segments or separators in a template URL can match a configured trusted prefix before decoding by the HTTP server or Git transport, enabling unsafe template features from a repository outside the trusted prefix after user interaction.

A vulnerability in the Copier library (versions 9.5.0 through 9.15.1) allows bypassing the trust mechanism for templates. The prefix check uses raw `str.startswith` without path normalization, while template fetching normalizes the path, enabling `..` sequences to point to a different, attacker-controlled template. Consequently, code from `tasks`, `migrations`, or `jinja_extensions` executes without a trust prompt.

Copier before version 9.14.1 has a vulnerability in the _external_data feature that allows a template to load YAML files from template-controlled paths. A malicious template can read local YAML-parseable files accessible to the user and expose their contents in rendered output.

Copier before version 9.14.1 accepts parent-directory traversal (..) in the _subdirectory setting, allowing a template to escape its own directory and render files from the parent directory without the --UNSAFE flag.