Gmail Address Variants Bypass String Comparison and Enable Multiple Registrations
A single user created four separate accounts using valid Gmail address formats that all deliver to the same inbox: Ivan.Petrov@gmail.com, ivanpetrov@gmail.com, ivan.petrov+shop@gmail.com, and IVAN.PETROV@gmail.com. Simple string comparison in the registration code treated these as distinct entries, allowing circumvention of new-user promotions, free-tier limits, and one-application-per-person rules. The root cause lies in three Gmail-specific behaviors: case-insensitive local parts, plus-addressing that ignores everything after the + symbol, and dots that carry no semantic meaning in the local part. Additional risks arise from IDNA homograph domains that visually mimic legitimate addresses but encode to different Punycode strings. Proper mitigation requires storing both the original email for delivery and a domain-aware normalized form for uniqueness checks, combined with explicit IDNA validation on registration.
HabrSep 5, 2026Translated from Russian