Security Headers Analyzer — TypeScript source

// Pure HTTP security-header analysis logic - no React, no DOM, deterministic.
// Parses a raw response-header block (the "key: value" lines copied from
// DevTools' Network panel), grades each of 8 security headers A/B/C/F, and
// rolls a weighted overall score. Header names match case-insensitively,
// folded (indented) continuation lines join their parent value, and duplicate
// names join with ", ". Malformed lines and the HTTP status line are ignored.
// Never throws.

/** Per-header grade: A best practice, B present but weak, C missing or dangerous, F actively harmful. */
export type HeaderGrade = 'A' | 'B' | 'C' | 'F';

/** One security header as analyzed: the parsed value plus its grade and guidance. */
export interface ParsedHeader {
  name: string; // canonical name, e.g. 'Strict-Transport-Security'
  value: string | null; // raw value as parsed; null when the header is absent
  grade: HeaderGrade;
  explanation: string;
  recommendation: string;
}

/** The full analysis result: per-header rows, the overall grade, and fix list. */
export interface HeaderAnalysis {
  headers: ParsedHeader[];
  grade: HeaderGrade;
  score: number; // 0-100 weighted average of per-header grades
  recommendations: string[];
}

/** Catalog entry: canonical name, why the header matters, and its score weight. */
export interface HeaderSpec {
  name: string;
  description: string;
  weight: number;
}

/** The 8 security headers this analyzer grades, in display order. */
export const SECURITY_HEADERS = [
  {
    name: 'Strict-Transport-Security',
    description:
      "HSTS tells the browser 'only ever reach this site over HTTPS' for max-age seconds. It defeats SSL-strip attacks, which downgrade the very first plain-HTTP visit. includeSubDomains extends the guarantee to every subdomain; preload lets the site join the browser-built HSTS preload list so even the first visit is HTTPS.",
    weight: 1.5,
  },
  {
    name: 'Content-Security-Policy',
    description:
      "CSP is the strongest anti-XSS control available in a header: it declares which sources scripts, styles, and other resources may load from. 'unsafe-inline' and 'unsafe-eval' punch holes straight through it - they re-allow inline injection and string-to-code evaluation. A default-src directive is the baseline so resource types you forgot to list inherit a restrictive fallback instead of the browser's permissive one.",
    weight: 2,
  },
  {
    name: 'X-Frame-Options',
    description:
      'Stops other sites from embedding this page in an iframe, which is the basis of clickjacking: an attacker overlays invisible UI on your framed page and harvests the victim\'s clicks. DENY blocks all framing; SAMEORIGIN allows only your own origin. The modern replacement is the CSP frame-ancestors directive.',
    weight: 1,
  },
  {
    name: 'X-Content-Type-Options',
    description:
      "With the value nosniff, the browser must respect the declared Content-Type instead of 'helpfully' sniffing the payload. Sniffing turns an uploaded text file into executable JavaScript when its bytes look script-shaped - the classic content-type-confusion attack.",
    weight: 1,
  },
  {
    name: 'Referrer-Policy',
    description:
      'Controls how much of the URL the browser leaks in the Referer header when the user navigates to another site. Full URLs can carry IDs, tokens, or search queries to third parties. strict-origin-when-cross-origin sends only the origin cross-origin (the browser default since 2020); no-referrer and same-origin leak even less. unsafe-url sends the full URL everywhere, including on plain-HTTP requests.',
    weight: 1,
  },
  {
    name: 'Permissions-Policy',
    description:
      "Declares which browser features (camera, microphone, geolocation, payment, USB...) the page and its embedded iframes may use. An empty allowlist - feature=() - switches the feature off entirely; an =* wildcard re-allows it everywhere, defeating the point of declaring it.",
    weight: 1,
  },
  {
    name: 'X-XSS-Protection',
    description:
      "The 2010-era XSS Auditor: browsers inspected reflected input in the page and blocked obviously injected scripts. Chrome removed it in 2019 (its heuristics introduced cross-site leaks of their own) and every modern browser now ignores it. The header is deprecated - CSP is the real defense. Sites either omit it or send '0' to switch old browsers off.",
    weight: 0.5,
  },
  {
    name: 'Cross-Origin-Opener-Policy',
    description:
      'Isolates the page\'s window handle from other origins. Without COOP, a site that opened your page in a popup (or was opened by it) keeps a JS reference to it, enabling some cross-window attacks and Spectre-class side channels. same-origin cuts that shared handle; unsafe-none restores the old shared browsing context.',
    weight: 1,
  },
] as const satisfies readonly HeaderSpec[];

/** Union of the canonical header names in the catalog. */
export type SecurityHeaderName = (typeof SECURITY_HEADERS)[number]['name'];

/** Grade points used for the weighted overall score. */
const GRADE_POINTS: Record<HeaderGrade, number> = { A: 100, B: 70, C: 40, F: 0 };

// One year in seconds - the minimum HSTS max-age worth calling strong.
const ONE_YEAR = 31536000;

/**
 * Parse a raw header block into a lowercased-name map. Lines are `name: value`;
 * the `HTTP/1.1 200 OK` status line and any line without a colon are ignored.
 * A line starting with space/tab continues the previous header (RFC 7230
 * obs-fold); a repeated name joins its values with ", ".
 */
export function parseHeaders(raw: string): Map<string, string> {
  const headers = new Map<string, string>();
  let lastName: string | null = null;
  for (const line of (raw ?? '').split(/\r?\n/)) {
    if (/^\s/.test(line)) {
      // Folded continuation - append to the previous header, if any.
      if (lastName !== null) {
        const prev = headers.get(lastName) ?? '';
        headers.set(lastName, `${prev} ${line.trim()}`.trim());
      }
      continue;
    }
    const colon = line.indexOf(':');
    if (colon === -1) continue; // status line, blank line, or junk
    const name = line.slice(0, colon).trim().toLowerCase();
    if (name === '') continue;
    const value = line.slice(colon + 1).trim();
    const prev = headers.get(name);
    headers.set(name, prev !== undefined ? `${prev}, ${value}` : value);
    lastName = name;
  }
  return headers;
}

// --- Per-header graders: value (null when absent) -> grade + guidance ---

/** The verdict fields every grader returns; folded into ParsedHeader. */
interface Grading {
  grade: HeaderGrade;
  explanation: string;
  recommendation: string;
}

function gradeHsts(value: string | null): Grading {
  if (value === null) {
    return {
      grade: 'C',
      explanation: 'Missing - the browser accepts plain HTTP, so the first visit (and SSL-strip attacks) can downgrade the connection.',
      recommendation: "Add: strict-transport-security: max-age=31536000; includeSubDomains; preload",
    };
  }
  const maxAgeMatch = /max-age\s*=\s*(\d+)/i.exec(value);
  if (maxAgeMatch === null) {
    return {
      grade: 'B',
      explanation: `Present but malformed - no readable max-age directive (${value}).`,
      recommendation: "Use: strict-transport-security: max-age=31536000; includeSubDomains",
    };
  }
  const maxAge = Number(maxAgeMatch[1]);
  if (maxAge === 0) {
    return {
      grade: 'F',
      explanation: 'max-age=0 actively deletes the HSTS policy - the site opts out of HTTPS-only enforcement.',
      recommendation: 'Raise max-age to at least 31536000 (one year).',
    };
  }
  const hasSubs = /;\s*includesubdomains/i.test(value);
  if (maxAge < ONE_YEAR) {
    return {
      grade: 'B',
      explanation: `Present with max-age=${maxAge}s (< 1 year)${hasSubs ? ' and includeSubDomains' : ''} - a shorter window weakens the guarantee.`,
      recommendation: 'Raise max-age to 31536000 (one year) or more.',
    };
  }
  if (!hasSubs) {
    return {
      grade: 'B',
      explanation: 'Strong max-age (>= 1 year), but includeSubDomains is missing - subdomains stay downgradable.',
      recommendation: "Append ; includeSubDomains once every subdomain serves HTTPS.",
    };
  }
  const hasPreload = /;\s*preload/i.test(value);
  return {
    grade: 'A',
    explanation: `Strong: max-age >= 1 year + includeSubDomains${hasPreload ? ' + preload - eligible for the browser preload list.' : '. Add preload to join the browser preload list.'}`,
    recommendation: 'Best practice met - nothing to change.',
  };
}

function gradeCsp(value: string | null): Grading {
  if (value === null) {
    return {
      grade: 'C',
      explanation: 'Missing - no restriction on where scripts and resources load from, so injected markup runs at full power.',
      recommendation: "Add a policy, e.g.: content-security-policy: default-src 'none'; script-src 'self'; ...",
    };
  }
  const inline = /'unsafe-inline'/i.test(value);
  const evaluates = /'unsafe-eval'/i.test(value);
  if (inline && evaluates) {
    return {
      grade: 'F',
      explanation: "Actively harmful - 'unsafe-inline' + 'unsafe-eval' together re-allow inline injection and string evaluation, gutting the policy while it still looks present.",
      recommendation: "Remove 'unsafe-inline' and 'unsafe-eval'; use nonces or hashes for inline scripts.",
    };
  }
  if (inline || evaluates) {
    const culprit = inline ? "'unsafe-inline'" : "'unsafe-eval'";
    return {
      grade: 'B',
      explanation: `Present but weak - ${culprit} punches a hole through the anti-XSS guarantee.`,
      recommendation: `Remove ${culprit}; use nonces or hashes for legitimate inline code.`,
    };
  }
  if (!/(^|\s)default-src\s/i.test(value)) {
    return {
      grade: 'B',
      explanation: 'Present with no default-src - resource types not listed explicitly fall back to the browser permissive default.',
      recommendation: "Start the policy with default-src 'none' or 'self', then allow what you need.",
    };
  }
  return {
    grade: 'A',
    explanation: 'Strong - a default-src fallback with no unsafe-inline / unsafe-eval escape hatches.',
    recommendation: 'Best practice met - nothing to change.',
  };
}

function gradeFrameOptions(value: string | null): Grading {
  if (value === null) {
    return {
      grade: 'C',
      explanation: 'Missing - any site can embed this page in an iframe and overlay it (clickjacking).',
      recommendation: 'Add: x-frame-options: DENY (or SAMEORIGIN if you embed yourself)',
    };
  }
  const v = value.trim().toUpperCase();
  if (v === 'DENY' || v === 'SAMEORIGIN') {
    return {
      grade: 'A',
      explanation: `Strong - ${v} blocks framing by other origins.`,
      recommendation: 'Best practice met - nothing to change.',
    };
  }
  if (v.startsWith('ALLOW-FROM')) {
    return {
      grade: 'B',
      explanation: 'ALLOW-FROM is deprecated and modern browsers ignore it - the page is effectively frameable.',
      recommendation: "Replace with a CSP frame-ancestors directive (x-frame-options: DENY as fallback).",
    };
  }
  return {
    grade: 'B',
    explanation: `Unrecognized value (${value}) - browsers ignore it, leaving the page frameable.`,
    recommendation: 'Use DENY or SAMEORIGIN.',
  };
}

function gradeContentTypeOptions(value: string | null): Grading {
  if (value === null) {
    return {
      grade: 'C',
      explanation: 'Missing - browsers may sniff uploaded content into executable types.',
      recommendation: 'Add: x-content-type-options: nosniff',
    };
  }
  if (value.trim().toLowerCase() === 'nosniff') {
    return {
      grade: 'A',
      explanation: 'Strong - MIME sniffing is off; the declared Content-Type is final.',
      recommendation: 'Best practice met - nothing to change.',
    };
  }
  return {
    grade: 'B',
    explanation: `Present but the value (${value}) is not nosniff, so browsers ignore the header.`,
    recommendation: 'Send exactly: x-content-type-options: nosniff',
  };
}

const REFERRER_STRICT = ['no-referrer', 'same-origin', 'strict-origin', 'strict-origin-when-cross-origin'];
const REFERRER_WEAK = ['origin', 'origin-when-cross-origin', 'no-referrer-when-downgrade'];

function gradeReferrerPolicy(value: string | null): Grading {
  if (value === null) {
    return {
      grade: 'C',
      explanation: 'Missing - older browsers default to leaking the full URL to whatever site the user navigates to.',
      recommendation: 'Add: referrer-policy: strict-origin-when-cross-origin (or no-referrer)',
    };
  }
  const v = value.trim().toLowerCase();
  if (v === 'unsafe-url') {
    return {
      grade: 'F',
      explanation: 'Actively harmful - unsafe-url sends the full URL (including IDs and tokens in query strings) to every destination, even on plain HTTP.',
      recommendation: 'Use strict-origin-when-cross-origin or no-referrer instead.',
    };
  }
  if (REFERRER_STRICT.includes(v)) {
    return {
      grade: 'A',
      explanation: `Strong - ${v} leaks at most the origin cross-origin.`,
      recommendation: 'Best practice met - nothing to change.',
    };
  }
  if (REFERRER_WEAK.includes(v)) {
    return {
      grade: 'B',
      explanation: `Present but leaky - ${v} still shares the full URL in some same- or cross-origin cases.`,
      recommendation: 'Tighten to strict-origin-when-cross-origin, strict-origin, same-origin, or no-referrer.',
    };
  }
  return {
    grade: 'B',
    explanation: `Unrecognized value (${value}) - browsers fall back to their default policy.`,
    recommendation: 'Use one of the standard directives, e.g. strict-origin-when-cross-origin.',
  };
}

function gradePermissionsPolicy(value: string | null): Grading {
  if (value === null) {
    return {
      grade: 'C',
      explanation: 'Missing - powerful features (camera, geolocation, payment...) default to the browser policy, not yours.',
      recommendation: 'Add: permissions-policy: camera=(), microphone=(), geolocation=()',
    };
  }
  const v = value.replace(/\s+/g, '').toLowerCase();
  if (v === '') {
    return {
      grade: 'B',
      explanation: 'Present but empty - no feature is restricted, so it declares nothing.',
      recommendation: 'List the features to switch off, e.g. camera=(), microphone=().',
    };
  }
  if (v === '*' || v.includes('=*')) {
    return {
      grade: 'B',
      explanation: 'Present but permissive - a =* wildcard re-allows the listed feature(s) everywhere.',
      recommendation: 'Use an empty allowlist () or a specific origin instead of *.',
    };
  }
  return {
    grade: 'A',
    explanation: 'Strong - the policy restricts at least one powerful feature.',
    recommendation: 'Best practice met - nothing to change.',
  };
}

function gradeXssProtection(value: string | null): Grading {
  if (value === null) {
    return {
      grade: 'A',
      explanation: 'Correctly omitted - the header is deprecated; modern browsers removed the XSS Auditor it drove.',
      recommendation: 'Nothing to change - rely on Content-Security-Policy instead.',
    };
  }
  const v = value.trim();
  if (v === '0' || v.startsWith('0;')) {
    return {
      grade: 'A',
      explanation: "Explicitly disabled ('0') - the right call for a deprecated header old browsers still honor.",
      recommendation: 'Nothing to change - or remove the header entirely.',
    };
  }
  if (v.startsWith('1')) {
    return {
      grade: 'B',
      explanation: "Deprecated - '1; mode=block' is ignored by modern browsers and the old Auditor had cross-site leaks of its own.",
      recommendation: "Send '0' or drop the header; use Content-Security-Policy for XSS defense.",
    };
  }
  return {
    grade: 'B',
    explanation: `Present with an unrecognized value (${value}) - remove the deprecated header.`,
    recommendation: "Send '0' or drop the header entirely.",
  };
}

function gradeCoop(value: string | null): Grading {
  if (value === null) {
    return {
      grade: 'C',
      explanation: 'Missing - pages that open or are opened by other origins share a browsing context with them.',
      recommendation: 'Add: cross-origin-opener-policy: same-origin',
    };
  }
  const v = value.trim().toLowerCase();
  if (v === 'same-origin') {
    return {
      grade: 'A',
      explanation: 'Strong - the window handle is isolated from cross-origin openers.',
      recommendation: 'Best practice met - nothing to change.',
    };
  }
  if (v === 'same-origin-allow-popups') {
    return {
      grade: 'B',
      explanation: 'Partially isolated - popups you open keep a reference to this window.',
      recommendation: 'Use same-origin unless you genuinely need cross-origin popup handles.',
    };
  }
  if (v === 'unsafe-none') {
    return {
      grade: 'B',
      explanation: 'Explicit opt-out - unsafe-none restores the shared browsing context COOP exists to cut.',
      recommendation: 'Use same-origin unless a documented integration requires shared handles.',
    };
  }
  return {
    grade: 'B',
    explanation: `Unrecognized value (${value}) - browsers treat it as no COOP.`,
    recommendation: 'Use same-origin.',
  };
}

// Compile-time-complete grader table: every catalog name has a grader.
const GRADERS: Record<SecurityHeaderName, (value: string | null) => Grading> = {
  'Strict-Transport-Security': gradeHsts,
  'Content-Security-Policy': gradeCsp,
  'X-Frame-Options': gradeFrameOptions,
  'X-Content-Type-Options': gradeContentTypeOptions,
  'Referrer-Policy': gradeReferrerPolicy,
  'Permissions-Policy': gradePermissionsPolicy,
  'X-XSS-Protection': gradeXssProtection,
  'Cross-Origin-Opener-Policy': gradeCoop,
};

/** Map a weighted 0-100 score to the overall A/B/C/F grade. */
function overallGrade(score: number): HeaderGrade {
  if (score >= 90) return 'A';
  if (score >= 70) return 'B';
  if (score >= 45) return 'C';
  return 'F';
}

/**
 * Analyze a raw HTTP response-header block. Every catalog header gets a
 * ParsedHeader row (in catalog order); the overall score is the weight-adjusted
 * average of per-header grade points, and recommendations list one fix per
 * header that is not at grade A.
 */
export function analyzeHeaders(raw: string): HeaderAnalysis {
  const parsed = parseHeaders(raw);
  const headers: ParsedHeader[] = SECURITY_HEADERS.map((spec) => {
    const value = parsed.get(spec.name.toLowerCase()) ?? null;
    const { grade, explanation, recommendation } = GRADERS[spec.name](value);
    return { name: spec.name, value, grade, explanation, recommendation };
  });
  const totalWeight = SECURITY_HEADERS.reduce((sum, spec) => sum + spec.weight, 0);
  const score = Math.round(
    headers.reduce((sum, h, i) => sum + GRADE_POINTS[h.grade] * SECURITY_HEADERS[i].weight, 0) / totalWeight,
  );
  const recommendations = headers.filter((h) => h.grade !== 'A').map((h) => h.recommendation);
  return { headers, grade: overallGrade(score), score, recommendations };
}