Developer Finds 4 Security Flaws in His Own JWT Authentication System

A self-taught backend developer discovered four critical security vulnerabilities while reviewing his own JWT-based authentication system built with refresh and access tokens. The flaws included SQL injection risks from unsanitized user input, which were resolved by switching to parameterized queries that treat all input as data rather than executable SQL. He also identified user enumeration exposure caused by distinct error messages for wrong email versus wrong password, fixed by returning a single generic 'Invalid credentials' response. A timing attack vulnerability in plain string password comparison was addressed by replacing it with bcrypt's constant-time comparison function. The findings highlight how common oversights in authentication logic can leave even self-built systems dangerously exposed in production environments.