Security Fix Needed for OffMarketBusiness

My website www.offmarketbusinessforsale.com is experiencing security issues, specifically unauthorized access to user accounts. I need a professional to identify and fix the vulnerabilities. After the fix, I would like you to continue adding user posts and comments in order to test the security.

Key Requirements:
- Identify and rectify the security issue
- Ensure the integrity of user accounts
- Test the stability of the site post fix

Ideal Skills:
- Cybersecurity expertise
- Web development experience
- Familiarity with Two-factor authentication systems