The iframe was autofilled with credentials. This is a security issue - the browser's autofill should NOT fill forms inside iframes from different origins.
Third-Party Iframe with Complete Form (Should NOT be autofilled)
Main Page - Username Only
Test Instructions
1Fill in the complete form in the iframe above (username + password)
2Then fill in the username field below on the main page
3Allow your browser to save credentials from both forms
4Refresh the page and observe autofill behavior
5Verify that only the main username field gets autofilled, NOT the iframe
!If the iframe gets autofilled, a red warning banner will appear
Reverse Split Test: This configuration tests if browsers properly isolate autofill when the iframe appears BEFORE the main page username field.