Missing input validation can lead to command execution in composer

The Composer method VcsDriver::getFileContent() with user-controlled $file or $identifier arguments is susceptible to an argument injection vulnerability. It can be leveraged to gain arbitrary command execution if the Mercurial or the Git driver are used.

This led to a vulnerability on Packagist.org and Private Packagist, i.e., using the composer.json readme field as a vector for injecting parameters into the $file argument for the Mercurial driver or via the $identifier argument for the Git and Mercurial drivers.

Composer itself can be attacked through branch names by anyone controlling a Git or Mercurial repository, which is explicitly listed by URL in a project's composer.json.

To the best of our knowledge, this was not actively exploited. The vulnerability has been patched on Packagist.org and Private Packagist within a day of the vulnerability report.