diff --git a/.npmrc b/.npmrc
new file mode 100644
index 00000000..bf2e7648
--- /dev/null
+++ b/.npmrc
@@ -0,0 +1 @@
+shamefully-hoist=true
diff --git a/src/deep-link/deep-link.controller.ts b/src/deep-link/deep-link.controller.ts
index 19fb91b3..4768ae68 100644
--- a/src/deep-link/deep-link.controller.ts
+++ b/src/deep-link/deep-link.controller.ts
@@ -1,8 +1,11 @@
 import { Controller, Get, Param, Res, Headers } from '@nestjs/common';
 import { Response } from 'express';
+import { DeepLinkService } from './deep-link.service';
 
 @Controller()
 export class DeepLinkController {
+  constructor(private readonly deepLinkService: DeepLinkService) {}
+
   @Get('.well-known/apple-app-site-association')
   getAppleAASA(@Res() res: Response) {
     const aasa = {
@@ -43,13 +46,9 @@ export class DeepLinkController {
     @Res() res: Response,
   ) {
     const isMobile = /Mobile|Android|iPhone|iPod|iPad/i.test(userAgent || '');
+    const type = isMobile ? 'app' : 'web';
+    const location = this.deepLinkService.buildDeepLink(type, 'course', id);
 
-    if (isMobile) {
-      // Redirect to custom URL scheme
-      return res.redirect(`teachlink://course/${id}`);
-    }
-
-    // Redirect to web URL
-    return res.redirect(`/course/${id}`);
+    return res.redirect(location);
   }
 }
diff --git a/src/deep-link/deep-link.module.ts b/src/deep-link/deep-link.module.ts
index 66c35693..8b41c4c3 100644
--- a/src/deep-link/deep-link.module.ts
+++ b/src/deep-link/deep-link.module.ts
@@ -1,7 +1,9 @@
 import { Module } from '@nestjs/common';
 import { DeepLinkController } from './deep-link.controller';
+import { DeepLinkService } from './deep-link.service';
 
 @Module({
   controllers: [DeepLinkController],
+  providers: [DeepLinkService],
 })
 export class DeepLinkModule {}
diff --git a/src/deep-link/deep-link.service.spec.ts b/src/deep-link/deep-link.service.spec.ts
new file mode 100644
index 00000000..801a8ef8
--- /dev/null
+++ b/src/deep-link/deep-link.service.spec.ts
@@ -0,0 +1,125 @@
+import { DeepLinkService } from './deep-link.service';
+
+describe('DeepLinkService', () => {
+  let service: DeepLinkService;
+
+  beforeEach(() => {
+    service = new DeepLinkService();
+  });
+
+  describe('validateRoute', () => {
+    it('should return true for allowlisted routes', () => {
+      expect(service.validateRoute('course')).toBe(true);
+      expect(service.validateRoute('/course')).toBe(true);
+    });
+
+    it('should return false for non-allowlisted routes', () => {
+      expect(service.validateRoute('admin')).toBe(false);
+      expect(service.validateRoute('/admin')).toBe(false);
+      expect(service.validateRoute('settings')).toBe(false);
+      expect(service.validateRoute('')).toBe(false);
+    });
+  });
+
+  describe('validateParam', () => {
+    it('should accept valid alphanumeric params', () => {
+      expect(service.validateParam('123')).toBe('123');
+      expect(service.validateParam('abc')).toBe('abc');
+      expect(service.validateParam('ABC')).toBe('ABC');
+      expect(service.validateParam('course-123')).toBe('course-123');
+      expect(service.validateParam('course_456')).toBe('course_456');
+    });
+
+    it('should trim whitespace from params', () => {
+      expect(service.validateParam('  123  ')).toBe('123');
+    });
+
+    it('should reject empty params', () => {
+      expect(() => service.validateParam('')).toThrow('Invalid parameter value');
+      expect(() => service.validateParam('   ')).toThrow('Parameter value cannot be empty');
+    });
+
+    it('should reject absolute URLs', () => {
+      expect(() => service.validateParam('http://evil.com')).toThrow('Absolute URLs are not allowed');
+      expect(() => service.validateParam('https://evil.com')).toThrow('Absolute URLs are not allowed');
+      expect(() => service.validateParam('ftp://evil.com')).toThrow('Absolute URLs are not allowed');
+      expect(() => service.validateParam('//evil.com')).toThrow('Absolute URLs are not allowed');
+    });
+
+    it('should reject external URL schemes', () => {
+      expect(() => service.validateParam('javascript:alert(1)')).toThrow('External URL schemes are not allowed');
+      expect(() => service.validateParam('data:text/html,<script>alert(1)</script>')).toThrow('External URL schemes are not allowed');
+      expect(() => service.validateParam('vbscript:msgbox(1)')).toThrow('External URL schemes are not allowed');
+    });
+
+    it('should reject path traversal attempts', () => {
+      expect(() => service.validateParam('../secret')).toThrow('Path traversal is not allowed');
+      expect(() => service.validateParam('..\\secret')).toThrow('Path traversal is not allowed');
+      expect(() => service.validateParam('../../etc/passwd')).toThrow('Path traversal is not allowed');
+      expect(() => service.validateParam('foo/../bar')).toThrow('Path traversal is not allowed');
+    });
+
+    it('should reject injection characters', () => {
+      expect(() => service.validateParam('<script>')).toThrow('Invalid characters in parameter');
+      expect(() => service.validateParam('>')).toThrow('Invalid characters in parameter');
+      expect(() => service.validateParam('"')).toThrow('Invalid characters in parameter');
+      expect(() => service.validateParam("'")).toThrow('Invalid characters in parameter');
+      expect(() => service.validateParam('`')).toThrow('Invalid characters in parameter');
+      expect(() => service.validateParam('{')).toThrow('Invalid characters in parameter');
+      expect(() => service.validateParam('}')).toThrow('Invalid characters in parameter');
+      expect(() => service.validateParam('\\')).toThrow('Invalid characters in parameter');
+    });
+
+    it('should reject invalid characters', () => {
+      expect(() => service.validateParam('hello world')).toThrow('Parameter contains invalid characters');
+      expect(() => service.validateParam('param@test')).toThrow('Parameter contains invalid characters');
+      expect(() => service.validateParam('param#test')).toThrow('Parameter contains invalid characters');
+      expect(() => service.validateParam('param?test')).toThrow('Parameter contains invalid characters');
+    });
+  });
+
+  describe('buildDeepLink', () => {
+    it('should build web deep link', () => {
+      const link = service.buildDeepLink('web', 'course', '123');
+      expect(link).toBe('/course/123');
+    });
+
+    it('should build app deep link', () => {
+      const link = service.buildDeepLink('app', 'course', '456');
+      expect(link).toBe('teachlink://course/456');
+    });
+
+    it('should pass valid params through unchanged', () => {
+      const link = service.buildDeepLink('web', 'course', 'abc-123_456');
+      expect(link).toBe('/course/abc-123_456');
+    });
+
+    it('should reject non-allowlisted routes', () => {
+      expect(() => service.buildDeepLink('web', 'admin', '123')).toThrow("Route 'admin' is not allowlisted");
+    });
+
+    it('should reject invalid params', () => {
+      expect(() => service.buildDeepLink('web', 'course', 'http://evil.com')).toThrow('Absolute URLs are not allowed');
+      expect(() => service.buildDeepLink('app', 'course', '../secret')).toThrow('Path traversal is not allowed');
+    });
+  });
+
+  describe('signLink and verifyLink', () => {
+    it('should sign and verify a link', () => {
+      const link = '/course/123';
+      const signed = service.signLink(link);
+      expect(signed).toMatch(/^\/course\/123\?sig=[a-f0-9]+$/);
+      expect(service.verifyLink(signed)).toBe(true);
+    });
+
+    it('should reject tampered signatures', () => {
+      const link = service.signLink('/course/123');
+      const tampered = link.replace('123', '999');
+      expect(service.verifyLink(tampered)).toBe(false);
+    });
+
+    it('should reject links without signature', () => {
+      expect(service.verifyLink('/course/123')).toBe(false);
+    });
+  });
+});
diff --git a/src/deep-link/deep-link.service.ts b/src/deep-link/deep-link.service.ts
new file mode 100644
index 00000000..9c690f85
--- /dev/null
+++ b/src/deep-link/deep-link.service.ts
@@ -0,0 +1,102 @@
+import { Injectable, BadRequestException } from '@nestjs/common';
+import { createHmac, timingSafeEqual } from 'crypto';
+
+export type DeepLinkType = 'web' | 'app';
+
+export interface DeepLinkRoute {
+  name: string;
+  path: string;
+  allowedTypes: DeepLinkType[];
+}
+
+@Injectable()
+export class DeepLinkService {
+  private readonly allowedRoutes: DeepLinkRoute[] = [
+    { name: 'course', path: '/course', allowedTypes: ['web', 'app'] },
+  ];
+
+  private readonly absoluteUrlPattern = /^(https?:\/\/|ftp:\/\/|\/\/)/i;
+  private readonly schemePattern = /^[a-zA-Z][a-zA-Z0-9+\-.]*:/;
+  private readonly pathTraversalPattern = /(\.\.[\/\\])/;
+  private readonly injectionPattern = /[<>\{\}\\"'`]/;
+  private readonly validParamPattern = /^[a-zA-Z0-9\-_.~]+$/;
+
+  private readonly signingKey = 'teachlink-deeplink-signing-key';
+
+  validateRoute(route: string): boolean {
+    return this.allowedRoutes.some(r => r.path === route || r.name === route);
+  }
+
+  validateParam(value: string): string {
+    if (!value || typeof value !== 'string') {
+      throw new BadRequestException('Invalid parameter value');
+    }
+
+    const sanitized = value.trim();
+
+    if (!sanitized) {
+      throw new BadRequestException('Parameter value cannot be empty');
+    }
+
+    if (this.absoluteUrlPattern.test(sanitized)) {
+      throw new BadRequestException('Absolute URLs are not allowed');
+    }
+
+    if (this.schemePattern.test(sanitized)) {
+      throw new BadRequestException('External URL schemes are not allowed');
+    }
+
+    if (this.pathTraversalPattern.test(sanitized)) {
+      throw new BadRequestException('Path traversal is not allowed');
+    }
+
+    if (this.injectionPattern.test(sanitized)) {
+      throw new BadRequestException('Invalid characters in parameter');
+    }
+
+    if (!this.validParamPattern.test(sanitized)) {
+      throw new BadRequestException('Parameter contains invalid characters');
+    }
+
+    return sanitized;
+  }
+
+  buildDeepLink(type: DeepLinkType, route: string, param: string): string {
+    if (!this.validateRoute(route)) {
+      throw new BadRequestException(`Route '${route}' is not allowlisted`);
+    }
+
+    const sanitizedParam = this.validateParam(param);
+    const encodedParam = encodeURIComponent(sanitizedParam);
+
+    if (type === 'app') {
+      return `teachlink://${route}/${encodedParam}`;
+    }
+
+    return `/${route}/${encodedParam}`;
+  }
+
+  signLink(link: string): string {
+    const hmac = createHmac('sha256', this.signingKey);
+    hmac.update(link);
+    const signature = hmac.digest('hex');
+    const separator = link.includes('?') ? '&' : '?';
+    return `${link}${separator}sig=${signature}`;
+  }
+
+  verifyLink(link: string): boolean {
+    const sigParamIndex = link.search(/[?&]sig=/);
+    if (sigParamIndex === -1) return false;
+
+    const signature = link.slice(sigParamIndex + 5);
+    const basePath = link.slice(0, sigParamIndex);
+
+    const hmac = createHmac('sha256', this.signingKey);
+    hmac.update(basePath);
+    const expected = hmac.digest('hex');
+
+    if (signature.length !== expected.length) return false;
+
+    return timingSafeEqual(Buffer.from(signature), Buffer.from(expected));
+  }
+}
diff --git a/test/deep-link.e2e-spec.ts b/test/deep-link.e2e-spec.ts
index e33aed11..90af86d0 100644
--- a/test/deep-link.e2e-spec.ts
+++ b/test/deep-link.e2e-spec.ts
@@ -54,4 +54,40 @@ describe('DeepLinkController (e2e)', () => {
 
     expect(response.header.location).toBe('teachlink://course/456');
   });
+
+  it('GET /deep-link/course/:id should reject absolute URLs in id param', async () => {
+    const response = await request(app.getHttpServer())
+      .get('/deep-link/course/http%3A%2F%2Fevil.com')
+      .set('user-agent', 'Mozilla/5.0 (Windows NT 10.0; Win64; x64)')
+      .expect(400);
+
+    expect(response.body.message).toContain('Absolute URLs are not allowed');
+  });
+
+  it('GET /deep-link/course/:id should reject external URL schemes in id param', async () => {
+    const response = await request(app.getHttpServer())
+      .get('/deep-link/course/javascript:alert(1)')
+      .set('user-agent', 'Mozilla/5.0 (Windows NT 10.0; Win64; x64)')
+      .expect(400);
+
+    expect(response.body.message).toContain('External URL schemes are not allowed');
+  });
+
+  it('GET /deep-link/course/:id should reject path traversal in id param', async () => {
+    const response = await request(app.getHttpServer())
+      .get('/deep-link/course/%2E%2E%2Fadmin')
+      .set('user-agent', 'Mozilla/5.0 (Windows NT 10.0; Win64; x64)')
+      .expect(400);
+
+    expect(response.body.message).toContain('Path traversal is not allowed');
+  });
+
+  it('GET /deep-link/course/:id should reject injection characters in id param', async () => {
+    const response = await request(app.getHttpServer())
+      .get('/deep-link/course/%3Cscript%3E')
+      .set('user-agent', 'Mozilla/5.0 (Windows NT 10.0; Win64; x64)')
+      .expect(400);
+
+    expect(response.body.message).toContain('Invalid characters in parameter');
+  });
 });