Open this file via SSH or FTP. Look for the string SUPEE-5344 . If it is missing, your store is highly vulnerable.
Magento 1.9.0.0 is over 10 years old and highly insecure. magento 1900 exploit github link
Search for "Magento" in the GitHub Advisory Database to find CVE-mapped vulnerabilities and official security summaries. Open this file via SSH or FTP
Improper sanitization of parameters inside the core database abstraction layer. magento 1900 exploit github link