setup-php is a GitHub action to set up PHP with extensions, php.ini configuration, coverage drivers, and tools. From 2.25.0 prior to 2.37.1, shivammathur/setup-php resolves the PHP version from repository-controlled files such as .php-version, composer.lock through platform-overrides.php, and composer.json through config.platform.php, and insufficiently constrains those values before incorporating them into generated shell or PowerShell setup scripts, allowing command injection on a GitHub Actions runner when workflows such as pull_request_target check out attacker-controlled contents before invoking setup-php. This issue is fixed in version 2.37.1.

Affected: GitHub Actions: shivammathur/setup-php

### Summary

A command injection vulnerability was identified in `shivammathur/setup-php` when the action resolves the PHP version from repository-controlled files and uses that value while generating the platform setup script.

In affected versions, `setup-php` may read the PHP version from:

- `.php-version` - `composer.lock` via `platform-overrides.php` - `composer.json` via `config.platform.php`

If an attacker can influence one of these files and the workflow executes `setup-php` in a trusted context, they may be able to execute commands on the GitHub Actions runner.

### Impact

This issue is exploitable when `setup-php` is run after checking out attacker-controlled repository contents and resolves the PHP version from repository files.

The most significant example is a privileged workflow such as `pull_request_target` that checks out untrusted pull request code before invoking `setup-php`. Similar risk can also arise in other workflows that operate on attacker-controlled refs, branches, or repository contents in a trusted context.

This is not a separate security boundary when an attacker can already modify the workflow definition itself or directly control the `php-version` workflow input, since that level of access already permits arbitrary command execution in GitHub Actions.

### Technical details

In affected versions, repository-derived PHP version values were insufficiently constrained before being incorporated into the generated shell or PowerShell setup scri…