Found a major website threat almost unintentionally, what should I do?

I recently applied for a rental. They seek lots of information as they conduct a background check/screening on each applicant. This includes all personal information, past/current residence, past/current employment, social security number, etc. I came across the flaw pretty much by accident. My significant other had to make their own application on their own account. I wanted to compare our applications to see if there were any discrepancies after I applied on my own, so I opened the link to …

accident check current cybersecurity employment etc flaw found information major personal personal information rental security social social security social security number threat website