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BREW-copier-CVE-2026-53951 — Copier has a trust-prefix bypass via path traversal that runs tasks unprompted

Affected: Homebrew: copier

# 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/_set…

CVE-2026-53951 — Copier: trust-prefix bypass via path traversal runs tasks unprompted

Affected: copier-org copier

Copier is a library and CLI app for rendering project templates. In versions 9.5.0 through 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 `..` 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. Version 9.15.2 patches the issue.