Finding a "solid guide" for a Magento 1.9.0.0 exploit typically points to the Shoplift vulnerability (CVE-2015-1592)
Once an attacker created an admin account, they gained full control over the store, including access to customer data, payment information, and the ability to inject malicious scripts (like credit card skimmers). magento 1.9.0.0 exploit github
An unauthenticated SQL injection vulnerability affecting Magento Open Source <= 1.9.4.0 via the catalog/product_frontend_action/synchronize EDB-37811: Finding a "solid guide" for a Magento 1
For developers and administrators, it is essential to understand the code changes required to fix the vulnerability. The following code snippets demonstrate the fixes: Penetration testing on systems you own or have
This makes 1.9.0.0 the perfect target. It is widespread (millions of legacy installs) and completely defenseless.