A webmail client is a browser-based email interface that lets users read, send, search, and manage mail without a separate desktop program. It renders message bodies, attachments, folders, and account settings inside the browser, usually after the user authenticates to the mail server.
In cyber security, webmail clients are important because they sit at a trust boundary: they display untrusted content while holding access to sensitive mailbox data and active sessions. If message rendering is weak, attackers can use malicious HTML or script to trigger stored cross-site scripting, steal session tokens, change forwarding rules, or harvest recent messages. Defenders reduce this risk by patching promptly, sanitizing rendered content, limiting risky previews, enforcing strong authentication, and monitoring for unusual mailbox access or bulk downloads. Because the client itself processes hostile input, even opening a message can become an attack path.