Copier Template Trust Bypass via Path Traversal

Compliance Impact

The vulnerability allows arbitrary command execution by bypassing the trust-prefix mechanism in the Copier tool, which could lead to unauthorized code execution and potential compromise of systems using the tool.

Such unauthorized execution risks could impact compliance with standards and regulations like GDPR and HIPAA, which require strict controls over data integrity, confidentiality, and system security.

If exploited, this vulnerability might lead to unauthorized access or modification of sensitive data or system configurations, thereby violating regulatory requirements for protecting personal or health information.

However, the provided information does not explicitly discuss compliance impacts or specific regulatory considerations.

Useful 0 Not Useful 0

Executive Summary

CVE-2026-53951 is a vulnerability in the Copier tool (versions 9.5.0 to 9.15.1) that allows an attacker to bypass the trust-prefix mechanism via path traversal, enabling arbitrary command execution without user confirmation.

The issue arises because the trust check compares the raw template URL against a trusted prefix using a simple string prefix match without path normalization, while the actual template fetching normalizes paths.

This allows a malicious URL like `https://github.com/trusted-org/../attacker-org/repo.git` to appear trusted (since it starts with the trusted prefix) but resolve to a different, attacker-controlled template.

The attacker can then execute tasks, migrations, or Jinja extensions embedded in the template without the `--trust` prompt, leading to arbitrary command execution.

Useful 0 Not Useful 0

Impact Analysis

This vulnerability can lead to arbitrary command execution on your system without user confirmation if an attacker can influence the template URL used by Copier.

An attacker-controlled template can run tasks, migrations, or Jinja extensions with high privileges, potentially compromising the security and integrity of your environment.

The risk is especially high in scenarios where the template URL can be manipulated, such as through a project's `.copier-answers.yml` file during a copier update operation.

Useful 0 Not Useful 0

Detection Guidance

This vulnerability can be detected by inspecting the template URLs used in Copier configurations or projects, especially looking for URLs that start with a trusted prefix but contain path traversal sequences like ".." which could bypass the trust check.

Since the vulnerability involves path traversal in template URLs, you can search for such patterns in your project files, for example in the .copier-answers.yml file or any configuration that specifies template URLs.

• Use grep or similar tools to find suspicious template URLs with path traversal sequences, e.g.: grep -r "https://github.com/.*/\.\./" .
• Check Copier versions in use to ensure they are not between 9.5.0 and 9.15.1, as these are vulnerable.

There are no specific built-in commands or network detection tools mentioned for detecting exploitation attempts of this vulnerability.

Useful 0 Not Useful 0

Mitigation Strategies

The immediate step to mitigate this vulnerability is to upgrade Copier to version 9.15.2 or later, where the issue has been fixed by normalizing paths before trust comparison.

Additionally, review and sanitize any template URLs used in your projects to ensure they do not contain path traversal sequences that could bypass trust checks.

Avoid using untrusted or user-influenced template URLs until the update is applied.

Useful 0 Not Useful 0