Browsers try to be helpful. If your server doesn’t clearly label a file, they may guess its type — a behaviour called content sniffing. That guesswork can turn a harmless-looking file into a vehicle for malware or cross-site scripting (XSS). The fix is simple: send the right HTTP security headers so browsers stop guessing and follow your rules.

What’s going on

When a page or file is missing the correct Content-Type, some browsers infer it. Attackers can exploit this by slipping executable code into uploads or mixed-content responses. Security headers tell the browser exactly how to treat your content — no improvisation.

Why this matters to your business

• Trust & reputation: Malicious behaviour erodes confidence and drives visitors away.
• Compliance & risk: Running unsafe scripts or framing can expose customer data.
• SEO & revenue: Security warnings hurt visibility, enquiries and sales.

The essential headers (plain English)

• X-Content-Type-Options: nosniff — Tells the browser to only use the declared Content-Type. Stops files being re-interpreted as HTML/JS.
• Content-Security-Policy (CSP) — A rulebook that whitelists where scripts, styles, images and frames may load from. Great for blocking injected scripts and reducing XSS risk.
• Strict-Transport-Security (HSTS) — Forces HTTPS so content and cookies aren’t exposed to downgrade attacks.
• X-Frame-Options / frame-ancestors — Controls who can frame your pages to prevent clickjacking.

Quick wins you can apply now

• Set accurate types everywhere: Ensure pages, APIs and assets send the correct Content-Type (e.g., text/html, text/css, application/javascript, image/png).
• Enable nosniff globally: Add once at the web server, reverse proxy or WAF layer so it covers your whole site.
• Introduce CSP safely: Start with Report-Only, review reports, then enforce. Avoid broad patterns like *, 'unsafe-inline' and 'unsafe-eval' where possible.
• Standard CSP baseline: default-src 'self'; script-src 'self' https://trusted-cdn.example; object-src 'none';
• Use a WAF: A web application firewall can add headers at the edge, provide virtual patching and support strong malware protection.

How to test and validate

• Browser check: In DevTools → Network → a request → Response Headers, confirm your security headers appear on HTML, APIs and assets.
• External scan: Use a tool like Sucuri SiteCheck to flag missing or misconfigured headers.
• Iterate: Review CSP report-only findings, then tighten rules without breaking legitimate services.

Common mistakes to avoid

• Partial coverage: Setting headers on pages but forgetting APIs, downloads or error pages.
• Over-permissive CSP: Policies that allow anything (*, 'unsafe-inline') defeat the point.
• Over-strict first attempt: Enforce gradually; start with Report-Only to prevent outages.
• Edge overrides: CDNs or WAFs can strip/override headers — align origin and edge configs.
• Set-and-forget: Revisit policies when you add third-party scripts or change hosting.

Prevention and ongoing protection

• Website maintenance: Keep WordPress, plugins and themes updated to reduce exploitable bugs.
• Security monitoring: Watch for unexpected file types, script loads or header regressions.
• Backups & recovery: Regular, off-site backups for fast rollback alongside malware removal readiness.
• WAF in front of WordPress: Consistent headers, bot mitigation and layered malware protection.

How matm can help

• Managed WordPress, plugin & theme updates
• Security monitoring and WAF setup
• Regular backups & fast site recovery
• Malware removal and emergency response

Want clear, practical help to deploy security headers and strengthen your WordPress security? Email [email protected] or call 01952 883 526.

Based on research by Sucuri — read the original analysis.