DRUPAL-CONTRIB-2026-279

S3 File System (s3fs) provides an additional file system to your Drupal site that stores files in Amazon's Simple Storage Service (S3) storage.

The module doesn't sufficiently isolate upload context between separate object(file) operations in the same request resulting in the potential for objects to be uploaded with incorrect access control list (ACL) entries or corruption through incorrect operation metadata.

This vulnerability is mitigated by the fact that an attacker must be able to trigger multiple file operations on S3FS.

Some vectors are mitigated by:

• Not enabling private:// takeover ($settings['s3fs.use_s3_for_private'] = TRUE;) or only enabled with $settings['s3fs.upload_as_private'] = TRUE;
• The deployment does not utilize hook_s3fs_stream_open_params_alter(), hook_s3fs_upload_params_alter(), hook_s3fs_copy_params_alter(), or hook_s3fs_command_params_alter()

If both points above have always been true, all currently known vectors are mitigated.