An IDN, or internationalized domain name, is a domain name that can include Unicode characters instead of only basic ASCII letters and numbers. This lets websites use local scripts and names, but it also creates a security challenge: some Unicode characters look almost identical to other letters. Attackers exploit these confusables to register lookalike domains that visually mimic trusted brands, a technique often called homoglyph spoofing. In a browser address bar, that can make a malicious site appear legitimate at a quick glance.
IDNs matter in cyber security because display logic is part of trust. Browsers may show an IDN in Unicode or convert it to punycode, the ASCII form, to reduce confusion. Security controls, warning systems, and password managers often rely on this behavior to detect suspicious hostnames and prevent phishing. Defenders watch for mixed-script names, Unicode confusables, and unexpected IDN registrations, while users should treat a familiar-looking URL as untrusted until the exact domain is verified.