*Updated 2022-07-20 19:45 UTC to indicate that this only affects Apache web servers.*
Drupal core sanitizes filenames with dangerous extensions upon upload (reference: [SA-CORE-2020-012](https://www.drupal.org/sa-core-2020-012)) and strips leading and trailing dots from filenames to prevent uploading server configuration files (reference: [SA-CORE-2019-010](https://www.drupal.org/sa-core-2019-010)).
However, the protections for these two vulnerabilities previously did not work correctly together. As a result, if the site were configured to allow the upload of files with an `htaccess` extension, these files' filenames would not be properly sanitized. This could allow bypassing the protections provided by Drupal core's default `.htaccess` files and possible remote code execution on Apache web servers.
This issue is mitigated by the fact that it requires a field administrator to explicitly configure a file field to allow `htaccess` as an extension (a restricted permission), or a contributed module or custom code that overrides allowed file uploads.
