FOSSBilling is a free, open-source billing and client management system. In versions 0.5.3 through 0.7.2, the Guest "serviceapikey/get_info" API endpoint is accessible without authentication. Any caller with a valid API key can retrieve all custom configuration parameters ("custom_*" fields) stored in the key's database record. These custom fields are populated by billing administrators and can contain business-sensitive data such as pricing tiers, feature flags, rate limits, expiry overrides, or access scope data. Version 0.8.0 patches the issue. Some workarounds are available. Administrators can avoid storing sensitive data in "custom_*" API key configuration fields, monitor API logs for suspicious calls to "/api/guest/serviceapikey/get_info", and/or disable the Serviceapikey module if not in active use.