fix(mcp): improve SseAuthGuard timing-safe comparison

apps/mcp-server/src/mcp/sse-auth.guard.spec.ts

@@ -1,4 +1,5 @@
 import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { createHmac } from 'crypto';
 import { ExecutionContext, UnauthorizedException } from '@nestjs/common';
 import { SseAuthGuard } from './sse-auth.guard';
 
@@ -92,5 +93,22 @@ describe('SseAuthGuard', () => {
 
       expect(() => guard.canActivate(context)).toThrow(UnauthorizedException);
     });
+
+    it('should reject tokens of different lengths without timing leak', () => {
+      const context = createMockContext('Bearer short');
+
+      expect(() => guard.canActivate(context)).toThrow(UnauthorizedException);
+    });
+
+    it('should use HMAC-based comparison (no length early-return)', () => {
+      // Verify HMAC produces fixed-length digests regardless of input length
+      const key = Buffer.from('codingbuddy-sse-auth');
+      const expectedHash = createHmac('sha256', key).update('my-secret-token').digest();
+      const providedHash = createHmac('sha256', key).update('wrong').digest();
+
+      // Both hashes should be same length (32 bytes) regardless of input length
+      expect(expectedHash.length).toBe(providedHash.length);
+      expect(expectedHash.length).toBe(32);
+    });
   });
 });

apps/mcp-server/src/mcp/sse-auth.guard.ts

@@ -6,7 +6,7 @@ import {
   UnauthorizedException,
 } from '@nestjs/common';
 import { Request } from 'express';
-import { timingSafeEqual } from 'crypto';
+import { createHmac, timingSafeEqual } from 'crypto';
 
 /**
  * Guard for SSE endpoints that validates Bearer token against MCP_SSE_TOKEN env var.
@@ -53,17 +53,13 @@ export class SseAuthGuard implements CanActivate {
   }
 
   /**
-   * Timing-safe token comparison to prevent timing attacks.
-   * Handles tokens of different lengths safely.
+   * Timing-safe token comparison using HMAC to prevent timing attacks.
+   * HMAC produces fixed-length digests, eliminating length-based timing leaks.
    */
   private tokensMatch(expected: string, provided: string): boolean {
-    const expectedBuf = Buffer.from(expected, 'utf-8');
-    const providedBuf = Buffer.from(provided, 'utf-8');
-
-    if (expectedBuf.length !== providedBuf.length) {
-      return false;
-    }
-
-    return timingSafeEqual(expectedBuf, providedBuf);
+    const key = Buffer.from('codingbuddy-sse-auth');
+    const expectedHash = createHmac('sha256', key).update(expected).digest();
+    const providedHash = createHmac('sha256', key).update(provided).digest();
+    return timingSafeEqual(expectedHash, providedHash);
   }
 }