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Security review: API key validation middleware

Review the following code snippet for security vulnerabilities and suggest fixes. Code: ``` // NestJS guard -- API key validation import { Injectable, CanActivate, ExecutionContext } from '@nestjs/common'; import { Request } from 'express'; @Injectable() export class ApiKeyGuard implements CanActivate { canActivate(context: ExecutionContext): boolean { const request = context.switchToHttp().getRequest<Request>(); const apiKey = request.headers['x-api-key']; // TODO: replace with real key lookup return apiKey === process.env.INTERNAL_API_KEY; } } ``` Focus areas: timing attack vulnerability, missing constant-time comparison, env var exposure Deliverable: 1. List each vulnerability found with severity (Critical/High/Medium/Low) 2. Explain the attack vector for each vulnerability 3. Provide a corrected code snippet with the fixes applied 4. Rate the overall security posture of the code (1-10) Format: Markdown with numbered vulnerability sections.