From drupal.org
By default, JSON:API works in a read-only mode which makes it impossible to exploit the vulnerability. Only sites that have the read_only set to FALSE under jsonapi.settings config are vulnerable.
Solution:
Install the latest version:
- If you are using Drupal 8.8.x, upgrade to Drupal 8.8.8.
- If you are using Drupal 8.9.x, upgrade to Drupal 8.9.1.
- If you are using Drupal 9.0.x, upgrade to Drupal 9.0.1.
Versions of Drupal 8 prior to 8.8.x are end-of-life and do not receive security coverage. Sites on 8.7.x or earlier should update to 8.8.8.