diff --git a/docs/SECURITY_HEADERS.md b/docs/SECURITY_HEADERS.md
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+++ b/docs/SECURITY_HEADERS.md
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+# 🛡️ HTTP Security Headers Specification
+
+This document details the HTTP security headers implemented in the NotifyChain API server (Issue #690).
+
+---
+
+## 1. Configured Security Headers
+
+| Header | Value | Purpose |
+|---|---|---|
+| `X-Content-Type-Options` | `nosniff` | Blocks MIME-type sniffing |
+| `X-Frame-Options` | `DENY` | Prevents clickjacking in iframes |
+| `Referrer-Policy` | `strict-origin-when-cross-origin` | Limits referrer leakage |
+| `X-XSS-Protection` | `0` | Disables legacy XSS auditor in favor of CSP |
+| `Permissions-Policy` | `camera=(), microphone=(), geolocation=()` | Restricts privileged browser APIs |
+| `Content-Security-Policy` | `default-src 'self'; frame-ancestors 'none';` | Restricts untrusted script/frame embedding |
+| `Strict-Transport-Security` | `max-age=31536000; includeSubDomains; preload` | Enforces HTTPS in production |
+
+---
+
+## 2. Integration & Verification
+
+The middleware `applySecurityHeaders` is automatically applied on all incoming HTTP requests in `listener/src/api/events-server.ts`.
+
+Automated tests are located at `listener/src/middleware/security-headers.test.ts`.
diff --git a/listener/src/middleware/security-headers.test.ts b/listener/src/middleware/security-headers.test.ts
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+++ b/listener/src/middleware/security-headers.test.ts
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+import { IncomingMessage, ServerResponse } from 'http';
+import { applySecurityHeaders, DEFAULT_SECURITY_HEADERS, HSTS_HEADER } from './security-headers';
+
+describe('Security Headers Middleware (Issue #690)', () => {
+  test('attaches core security headers to HTTP responses', () => {
+    const headersMap: Record<string, string> = {};
+    const req = {} as IncomingMessage;
+    const res = {
+      hasHeader: jest.fn((name: string) => name in headersMap),
+      setHeader: jest.fn((name: string, value: string) => {
+        headersMap[name] = value;
+      }),
+    } as unknown as ServerResponse;
+
+    applySecurityHeaders(req, res, { isProduction: false });
+
+    expect(res.setHeader).toHaveBeenCalledWith('X-Content-Type-Options', 'nosniff');
+    expect(res.setHeader).toHaveBeenCalledWith('X-Frame-Options', 'DENY');
+    expect(res.setHeader).toHaveBeenCalledWith('Referrer-Policy', 'strict-origin-when-cross-origin');
+    expect(res.setHeader).toHaveBeenCalledWith('X-XSS-Protection', '0');
+    expect(res.setHeader).toHaveBeenCalledWith('Content-Security-Policy', expect.stringContaining("default-src 'self'"));
+  });
+
+  test('enables HSTS in production environments', () => {
+    const headersMap: Record<string, string> = {};
+    const req = {} as IncomingMessage;
+    const res = {
+      hasHeader: jest.fn((name: string) => name in headersMap),
+      setHeader: jest.fn((name: string, value: string) => {
+        headersMap[name] = value;
+      }),
+    } as unknown as ServerResponse;
+
+    applySecurityHeaders(req, res, { isProduction: true, enableHsts: true });
+
+    expect(res.setHeader).toHaveBeenCalledWith('Strict-Transport-Security', HSTS_HEADER);
+  });
+
+  test('does not overwrite existing custom headers', () => {
+    const headersMap: Record<string, string> = {
+      'X-Frame-Options': 'SAMEORIGIN',
+    };
+    const req = {} as IncomingMessage;
+    const res = {
+      hasHeader: jest.fn((name: string) => name in headersMap),
+      setHeader: jest.fn((name: string, value: string) => {
+        headersMap[name] = value;
+      }),
+    } as unknown as ServerResponse;
+
+    applySecurityHeaders(req, res, { isProduction: false });
+
+    expect(res.setHeader).not.toHaveBeenCalledWith('X-Frame-Options', 'DENY');
+  });
+});
diff --git a/listener/src/middleware/security-headers.ts b/listener/src/middleware/security-headers.ts
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+++ b/listener/src/middleware/security-headers.ts
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+/**
+ * Security Headers Middleware (Issue #690)
+ *
+ * Attaches industry-standard HTTP security headers to all responses from the
+ * NotifyChain API server to mitigate clickjacking, MIME-sniffing, and XSS attacks.
+ */
+
+import { IncomingMessage, ServerResponse } from 'http';
+
+export interface SecurityHeadersConfig {
+  isProduction?: boolean;
+  enableHsts?: boolean;
+  customCsp?: string;
+}
+
+export const DEFAULT_SECURITY_HEADERS: Record<string, string> = {
+  'X-Content-Type-Options': 'nosniff',
+  'X-Frame-Options': 'DENY',
+  'Referrer-Policy': 'strict-origin-when-cross-origin',
+  'X-XSS-Protection': '0',
+  'Permissions-Policy': 'camera=(), microphone=(), geolocation=()',
+  'Content-Security-Policy': "default-src 'self'; frame-ancestors 'none';",
+};
+
+export const HSTS_HEADER = 'max-age=31536000; includeSubDomains; preload';
+
+/**
+ * Attaches configured security headers to an HTTP ServerResponse.
+ */
+export function applySecurityHeaders(
+  req: IncomingMessage,
+  res: ServerResponse,
+  config: SecurityHeadersConfig = {}
+): void {
+  const isProd = config.isProduction ?? process.env.NODE_ENV === 'production';
+  const enableHsts = config.enableHsts ?? isProd;
+
+  for (const [header, value] of Object.entries(DEFAULT_SECURITY_HEADERS)) {
+    if (!res.hasHeader(header)) {
+      res.setHeader(header, value);
+    }
+  }
+
+  if (enableHsts && !res.hasHeader('Strict-Transport-Security')) {
+    res.setHeader('Strict-Transport-Security', HSTS_HEADER);
+  }
+}