Laravel Excel writes exports outside the configured filesystem disk when given a caller-controlled path

Summary

Laravel Excel provides supercharged Excel exports and imports in Laravel. From 3.1.8 until 3.1.70, in src/Files/Disk.php the Maatwebsite\Excel\Files\Disk::copy() method resolves the caller-controlled $destination supplied through Excel::store(), $export->store(), or storeExcel() against the process working directory with realpath() instead of the configured filesystem disk. If the path names an existing writable file, Disk::copy() opens it with fopen() in rb+ mode and uses stream_copy_to_stream(), bypassing Flysystem path confinement and allowing an attacker whose application input controls the export path to overwrite arbitrary existing files with export content. The rb+ behavior creates a non-truncating overwrite and trailing bytes when the new export is shorter, and overwriting an executable PHP file can lead to remote code execution. This issue is fixed in version 3.1.70.