CVE-2026-53951

# Copier: trust-prefix bypass via path traversal runs tasks unprompted ### Summary In copier `>= 9.5.0, <= 9.15.1`, the `trust` setting's prefix match (`copier/_settings.py`) compares the template URL against a trusted prefix with a raw `str.startswith` and **no path normalization**, while the URL *is* normalized when the template is actually fetched (`Path.resolve()` for local paths; libcurl dot-segment removal for `https`). A template reference that textually starts with a trusted prefix but contains `..` (e.g. `https://github.com/trusted-org/../attacker-org/repo.git`) is therefore granted trust yet resolves to a different, attacker-controlled template, whose `tasks` / `migrations` / `jinja_extensions` then run **without the `--trust` prompt** — arbitrary command execution. Likely **CWE-22 (Improper Limitation of a Pathname)** in the trust check leading to **CWE-94 (code execution)**. ### Details `trust` lets users mark template locations as trusted so copier skips the unsafe-feature gate. A trailing `/` makes an entry a **prefix** match (`docs/settings.md`: *"Locations ending with `/` will be matched as prefixes, trusting all templates from that location"*). `copier/_settings.py:141-146` (tag `v9.15.1`): ```python return any( repository.startswith(_normalize(t)) if t.endswith("/") else repository == _normalize(t) for t in trust ) ``` `_normalize` only expands `~`; it does **not** touch `..` or collapse segments — `copier/_settings.py:149-152` (tag `v9.15.1`): ```python def _normalize(url: str) -> str: if url.startswith("~"): # Only expand on str to avoid messing with URLs url = expanduser(url) # noqa: PTH111 return url ``` This decision gates code execution — `copier/_main.py:293` (tag `v9.15.1`): ```python if self.unsafe or is_trusted_repository(self.settings.trust, self.template.url): return # skip the unsafe-feature check entirely ``` The chain: the trust comparison sees