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

• Severity: high (CVSS 8.8 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-08 · updated 2026-10-25
• Weakness: CWE-22

Description

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.

Affected products

• copier-org copier: from 9.5.0, < 9.15.2

References

• https://github.com/copier-org/copier/releases/tag/v9.15.2
• https://github.com/copier-org/copier/security/advisories/GHSA-9gmc-jqmh-3rvm
• https://github.com/copier-org/copier/security/advisories/GHSA-9gmc-jqmh-3rvm

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