Safely parsing email files in the browser

An email file is not just text plus a few attachments. It can contain HTML, nested MIME parts, misleading filenames, inline resources, remote tracking pixels, malformed encodings, and enough data to exhaust a browser tab. Moving parsing into the browser removes an upload from the architecture, but it does not automatically make the viewer safe. It changes the security job: untrusted content is now being interpreted next to the user’s active web session. This is the checklist I use for a local EML and winmail.dat/TNEF reader. Treat every parsed field as untrusted The sender, subject, recipient, filename, MIME type, and message body all came from a file. Render headers and filenames as text, never by concatenating HTML. The same applies to errors. A parser exception can include a filename or fragment of malformed input. Showing that message verbatim may leak data into logs or turn it into markup. Map parser failures to stable error categories, then display a controlled explanation. Normalize into one internal model EML and TNEF have different container structures, but the UI should not contain two independent security implementations. Both parsers can produce a common message model: subject, sender, to, cc, date, plain body, sanitized HTML candidate, attachments[] { safe filename, MIME type, bytes, inline flag, content ID, content location } The normalization layer is the right place to enforce per-source limits and reject unsupported structures. The viewer and download code then work against the same constrained data regardless of input format. Sanitize HTML as hostile input Email HTML was designed for mail clients, not for direct insertion into an application DOM. A conservative policy removes: scripts and event handlers; forms and interactive controls; iframe , object , and embed elements; styles and CSS URLs; unsafe protocols; executable or unexpected embedded content. Use a maintained sanitizer with a pinned version, but do not stop at its default configuration.