What it is

Without X-Content-Type-Options: nosniff, some browsers may try to guess a file's type instead of trusting what the server declares — that guessing is exactly how an uploaded file disguised as an image can end up executed as script in older browsers.

How to fix it

Add the X-Content-Type-Options: nosniff header. If you're not sure how, install the WordPress Plugin: its Security tab applies fixes like this one automatically.

In depth

Your WordPress site is missing a security instruction that tells browsers to trust only the actual file type you say something is, rather than guessing. Without this instruction, if someone uploads a malicious file that pretends to be a picture but is actually code, some older browsers might execute it as code anyway. This matters because executed code can steal information, deface your site, or create backdoors for hackers to return later. The fix is straightforward: you either need your hosting provider to add one line to your server settings, or you can install a WordPress security plugin like Wordfence or All in One WP Security which will automatically add this protective header for you. Most security plugins handle this in a single click without requiring any technical knowledge, so installing a reputable plugin is usually the easiest path for site owners who don't manage their server settings directly.