security: headers and safe loading

security checks the HTTP response headers your pages are served with and the markup that decides what those pages load. A missing Strict-Transport-Security or clickjacking header is invisible in the page and easy to lose in a hosting migration; an http:// script or a CDN file without integrity is one careless edit away. It is not in the default set.

Needs

Nothing to install. It reads the build (dist/) and starts no server. The header checks need a source of headers, see below; the HTML checks always run.

The HTML checks always read the shipped files, also with render.mode on: they are about what the server sends.

Run it

npx vidimus security                                  # headers from dist/_headers
npx vidimus security --origin https://preview.example.com  # headers from a live site

Where headers come from

Static hosts set headers outside the HTML, so vidimus reads them from one of two places:

1. A live origin, when --origin (or the origin config key) is set. vidimus sends a HEAD request for every built page to the origin plus the page path, and retries with GET if the server answers 405. Redirects on the same origin (such as /about to /about/) are followed, up to 5; a redirect to another origin is not, and its own headers are checked. A request that fails or takes longer than 20 seconds is an error finding, could not fetch headers, with the reason as detail.
2. A headers file in the build, dist/_headers by default (security.file), in the Netlify and Cloudflare Pages format. Put it in the folder your framework copies to the build as is (public/ in most of them).

With neither, the log says header checks skipped: no _headers in the build and no --origin to fetch headers from, the summary says headers from nowhere (skipped), and only the HTML checks run. The server.headers of the built-in static server are not used by this audit.

The _headers format

A line starting at column 0 is a URL pattern; the indented lines below it are Name: value headers for the paths it matches. Lines starting with # are comments.

# every page
/*
  X-Content-Type-Options: nosniff

# one segment: /blog/hello/, not /blog/2024/hello/
/blog/:slug
  Cache-Control: public, max-age=600

# full URLs work too; only the path is used
https://example.com/admin/*
  ! X-Frame-Options
  Content-Security-Policy: frame-ancestors 'self' https://cms.example.com

• * matches anything, including /. :name matches one path segment.
• A trailing slash is optional: /about also matches /about/.
• Every rule that matches a path applies, in file order. When two rules set the same header, the values are joined with , .
• An indented ! Name removes a header that an earlier rule set for that path.

GitHub Pages and other hosts without custom headers

GitHub Pages cannot set custom response headers, and a <meta http-equiv> cannot replace HSTS, X-Content-Type-Options or frame-ancestors. Put the site behind a CDN that can (Cloudflare, for example) and audit it with --origin, or turn the header checks off and keep the HTML ones:

export default defineConfig({
  security: {
    require: {
      'strict-transport-security': false,
      'x-content-type-options': false,
      'referrer-policy': false,
      'permissions-policy': false,
    },
    clickjacking: false,
  },
});

What it checks

Headers (when a source exists)

Required headers (security.require) is a map of header name to a regular expression the value must match (case-insensitive). A missing header fails with missing <name> header; a value that does not match fails with <name> header does not match <regex> and shows the value. '' requires only presence, false turns a header off. For the four defaults the fix quotes a recommended value:

Header	Must match	Recommended
strict-transport-security	max-age of at least 31000000 seconds (about a year)	max-age=31536000; includeSubDomains
x-content-type-options	exactly nosniff	nosniff
referrer-policy	anything except unsafe-url and no-referrer-when-downgrade	strict-origin-when-cross-origin
permissions-policy	any value	camera=(), microphone=(), geolocation=()

The other header checks:

• Clickjacking (security.clickjacking): a Content-Security-Policy header with a frame-ancestors directive, or X-Frame-Options: DENY or SAMEORIGIN. Without either: no clickjacking protection: add CSP frame-ancestors or X-Frame-Options. A <meta> CSP does not count, since browsers ignore frame-ancestors there.
• Stack leaks (live origin only): an X-Powered-By header (x-powered-by header leaks the stack) and a Server header containing a version number (server header leaks a version) are warnings.

HTML (always)

• Unsafe CSP (security.unsafeInline): the script-src of every CSP, from headers and <meta http-equiv="content-security-policy">, falling back to default-src. 'unsafe-eval' is a warning. 'unsafe-inline' is a warning unless the same list has a nonce or a sha256-/sha384-/sha512- hash, in which case browsers ignore it. The csp audit checks that those hashes match your inline scripts.
• Mixed content (security.mixedContent): any http:// URL in <script src>, <img src|srcset>, <source src|srcset>, <iframe src>, <video src|poster>, <audio src>, <object data>, <embed src>, <form action>, and <link href> with relstylesheet, icon, preload, modulepreload or manifest. Each URL is an error, mixed content: http://…. Plain <a href="http://…"> links are not mixed content.
• Subresource integrity (security.sri): a <script src> or <link rel="stylesheet"> on another origin (not siteUrl) without an integrity attribute is a warning, cross-origin <script> without integrity: https://…. Protocol-relative //cdn… URLs count.

security.txt (security.securityTxt)

RFC 9116 asks every site to publish /.well-known/security.txt so researchers know where to report a vulnerability. vidimus reads it from the build:

• Without .well-known/security.txt or a root security.txt: no /.well-known/security.txt (warning).
• Only a root security.txt: security.txt is not under /.well-known/ (warning). The root location is a legacy fallback.
• No Contact field, or one that is not a URI (security@example.com without mailto:, or an http:// URL): error.
• No Expires field, more than one, one that is not a date, or one in the past (security.txt expired on …): error. An Expires more than a year away is a warning, since the RFC recommends reviewing the file at least yearly.

A PGP-signed file is read as is; the signature block is ignored. When siteUrl has a path (https://example.github.io/project/), the check is skipped with a log line: the file belongs at the origin root, which the build does not own.

Example output

─── security ────────────────────────────────────────────────────

✖ missing strict-transport-security header
    on: / /about/ /blog/ +37 more
    → Add "Strict-Transport-Security: max-age=31536000; includeSubDomains" to /* in _headers, or set security.require["strict-transport-security"] to false to skip.

✖ no clickjacking protection: add CSP frame-ancestors or X-Frame-Options
    on: / /about/ /blog/ +37 more
    → Send "Content-Security-Policy: frame-ancestors 'self'" (or X-Frame-Options: DENY) as a header; a <meta> CSP can't set frame-ancestors.

⚠ CSP allows 'unsafe-inline' scripts
    on: /contact/
    → Drop 'unsafe-inline' from script-src and allow inline scripts by 'sha256-…' hash or nonce, or move them to files.

⚠ cross-origin <script> without integrity: https://cdn.jsdelivr.net/npm/alpinejs@3/dist/cdn.min.js
    on: /contact/
    → Add integrity="sha384-…" and crossorigin="anonymous", or self-host the file.

✖ security: 40 pages, headers from _headers, 4 problem(s) (0.1s)

With --origin, the fixes say "your host's header config" instead of /* in _headers.

Options

Key	Default	Description
security.file	'_headers'	headers file, relative to the build output
security.require	the four headers above	header name → regex its value must match; '' requires presence, false skips
security.clickjacking	true	require frame-ancestors or X-Frame-Options
security.unsafeInline	true	warn on 'unsafe-inline' without hashes or nonces, and on 'unsafe-eval'
security.mixedContent	true	fail on resources loaded over http://
security.sri	true	warn on cross-origin scripts and stylesheets without integrity
security.securityTxt	true	check /.well-known/security.txt; warn when it is missing
security.exclude	[]	URL path patterns to skip

Top-level keys used: origin, siteUrl, exclude. Objects merge deeply, so adding a key to security.require keeps the defaults:

import { defineConfig } from 'vidimus';

export default defineConfig({
  siteUrl: 'https://example.com',
  audits: ['i18n', 'csp', 'a11y', 'links', 'r12s', 'security'],
  security: {
    require: {
      'cross-origin-opener-policy': '^same-origin$',
      'permissions-policy': 'camera=\\(\\)',
    },
    exclude: ['^/embed/'],
  },
});

Common fixes

A recommended dist/_headers that passes every default check. Adjust the CSP to what your pages load:

/*
  Strict-Transport-Security: max-age=31536000; includeSubDomains
  X-Content-Type-Options: nosniff
  Referrer-Policy: strict-origin-when-cross-origin
  Permissions-Policy: camera=(), microphone=(), geolocation=()
  Content-Security-Policy: frame-ancestors 'self'
  X-Frame-Options: DENY

X-Frame-Options is redundant next to frame-ancestors in current browsers; keep it for old ones or drop it. If a page must be embeddable, detach and replace the header for it:

/embed/*
  ! X-Frame-Options
  ! Content-Security-Policy
  Content-Security-Policy: frame-ancestors https://partner.example.com

Add integrity to a CDN script, or better, self-host it:

<script src="https://cdn.jsdelivr.net/npm/alpinejs@3.14.1/dist/cdn.min.js"
  integrity="sha384-…" crossorigin="anonymous" defer></script>

An integrity hash only works with a pinned version: the file behind @3 changes with every release. Generate it with curl -s <url> | openssl dgst -sha384 -binary | openssl base64 -A.

For a live server, remove X-Powered-By in the framework (app.disable('x-powered-by') in Express) and hide the version with server_tokens off; in nginx or ServerTokens Prod in Apache. See CI for auditing a preview deployment with --origin.

A security.txt that passes, in public/.well-known/security.txt (renew Expires before it lapses):

Contact: mailto:security@example.com
Expires: 2027-06-30T00:00:00Z
Preferred-Languages: en
Canonical: https://example.com/.well-known/security.txt