diff --git a/backend/shared/middleware/__tests__/securityHeaders.test.ts b/backend/shared/middleware/__tests__/securityHeaders.test.ts
index e3d235ed..054b0616 100644
--- a/backend/shared/middleware/__tests__/securityHeaders.test.ts
+++ b/backend/shared/middleware/__tests__/securityHeaders.test.ts
@@ -104,4 +104,61 @@ describe('applySecurityHeadersToResponse', () => {
     applySecurityHeadersToResponse(res, { hstsMaxAge: 100 });
     expect(headersOut['Strict-Transport-Security']).toBe('max-age=100; includeSubDomains; preload');
   });
+
+  it('skips all headers when response is already sent', () => {
+    const setHeader = jest.fn();
+    const res = { headersSent: true, setHeader } as any;
+
+    applySecurityHeadersToResponse(res);
+    expect(setHeader).not.toHaveBeenCalled();
+  });
+});
+
+describe('Security Header Audit (#1773)', () => {
+  const REQUIRED_HEADERS = [
+    'Strict-Transport-Security',
+    'Permissions-Policy',
+    'X-Content-Type-Options',
+    'X-Frame-Options',
+    'Referrer-Policy',
+  ] as const;
+
+  it('emits every required security header with default options', () => {
+    const headers = buildSecurityHeaders({});
+    for (const name of REQUIRED_HEADERS) {
+      expect(headers).toHaveProperty(name);
+      expect(headers[name]).toBeTruthy();
+    }
+  });
+
+  it('X-Content-Type-Options is always nosniff regardless of custom options', () => {
+    const headers = buildSecurityHeaders({
+      hstsMaxAge: 3600,
+      hstsIncludeSubDomains: false,
+      hstsPreload: false,
+      permissionsPolicy: { geolocation: ["'self'"] },
+    });
+    expect(headers['X-Content-Type-Options']).toBe('nosniff');
+  });
+
+  it('Referrer-Policy is always strict-origin-when-cross-origin', () => {
+    expect(buildSecurityHeaders({})['Referrer-Policy']).toBe('strict-origin-when-cross-origin');
+    expect(buildSecurityHeaders({ hstsMaxAge: 0 })['Referrer-Policy']).toBe('strict-origin-when-cross-origin');
+  });
+
+  it('X-Frame-Options is always SAMEORIGIN', () => {
+    expect(buildSecurityHeaders({})['X-Frame-Options']).toBe('SAMEORIGIN');
+  });
+
+  it('HSTS max-age is at least 1 year (31536000 s) by default for preload eligibility', () => {
+    const hsts = buildSecurityHeaders({})['Strict-Transport-Security'];
+    const match = /max-age=(\d+)/.exec(hsts);
+    expect(match).not.toBeNull();
+    expect(Number(match![1])).toBeGreaterThanOrEqual(31_536_000);
+  });
+
+  it('payment feature-policy is none by default (no inadvertent Payment Request API exposure)', () => {
+    const policy = buildSecurityHeaders({})['Permissions-Policy'];
+    expect(policy).toContain("payment='none'");
+  });
 });
diff --git a/backend/shared/middleware/securityHeaders.ts b/backend/shared/middleware/securityHeaders.ts
index ce8bda56..6681b82d 100644
--- a/backend/shared/middleware/securityHeaders.ts
+++ b/backend/shared/middleware/securityHeaders.ts
@@ -38,7 +38,7 @@ const DEFAULT_PERMISSIONS_POLICY: Record<string, (string | 'none')[]> = {
   xrSpatialTracking: ["'none'"],
 };
 
-function buildHsts(options: SecurityHeadersOptions): string {
+export function buildHsts(options: SecurityHeadersOptions): string {
   const maxAge = options.hstsMaxAge ?? 63_072_000;
   const includeSubDomains = options.hstsIncludeSubDomains ?? true;
   const preload = options.hstsPreload ?? true;
@@ -48,7 +48,7 @@ function buildHsts(options: SecurityHeadersOptions): string {
   return parts.join('; ');
 }
 
-function buildPermissionsPolicy(
+export function buildPermissionsPolicy(
   options: SecurityHeadersOptions,
 ): string {
   const policy = { ...DEFAULT_PERMISSIONS_POLICY, ...(options.permissionsPolicy ?? {}) };