Composer is a dependency manager for the PHP programming language. Integrators using Composer code to call VcsDriver::getFileContent can have a code injection vulnerability if the user can control the $file or $identifier argument. This leads to a vulnerability on packagist.org for example where the composer.json's readme field can be used as a vector for injecting parameters into hg/Mercurial via the $file argument, or git via the $identifier argument if you allow arbitrary data there (Packagist does not, but maybe other integrators do).

Composer itself should not be affected by the vulnerability as it does not call getFileContent with arbitrary data into $file/$identifier. To the best of our knowledge this was not abused, and the vulnerability has been patched on packagist.org and Private Packagist within a day of the vulnerability report.