How to fix API Rate Limit Exhaustion
in ASP.NET Core
Executive Summary
API Rate Limit Exhaustion is a trivial vector for DoS and brute-force attacks. If your endpoint doesn't throttle, an attacker can saturate your thread pool or database connections with a simple loop. In ASP.NET Core, relying on 'hope' isn't a security posture. You need hard limits at the middleware level to drop malicious traffic before it hits your business logic.
The Vulnerable Pattern
[ApiController]
[Route("api/v1/auth")]
public class AuthController : ControllerBase {
[HttpPost("login")]
public async Task Login([FromBody] LoginRequest request) {
// VULNERABLE: No rate limiting.
// An attacker can send 10,000 requests/sec to brute force passwords
// or exhaust server resources.
var result = await _authService.VerifyAsync(request);
return Ok(result);
}
} The Secure Implementation
The secure implementation utilizes the native 'Microsoft.AspNetCore.RateLimiting' middleware introduced in .NET 7. The 'Fixed Window' policy is configured to allow only 5 requests per 60-second window. By setting 'QueueLimit' to 0, we ensure that any requests exceeding the limit are immediately rejected with a 429 (Too Many Requests) status instead of being buffered, which protects the server's memory and thread pool from exhaustion during a burst attack.
// In Program.cs builder.Services.AddRateLimiter(options => { options.RejectionStatusCode = StatusCodes.Status429TooManyRequests; options.AddFixedWindowLimiter("auth_policy", opt => { opt.Window = TimeSpan.FromSeconds(60); opt.PermitLimit = 5; opt.QueueLimit = 0; opt.QueueProcessingOrder = QueueProcessingOrder.OldestFirst; }); });
// In AuthController.cs [EnableRateLimiting(“auth_policy”)] [HttpPost(“login”)] public async TaskLogin([FromBody] LoginRequest request) { return Ok(await _authService.VerifyAsync(request)); }
Your ASP.NET Core API
might be exposed to API Rate Limit Exhaustion
74% of ASP.NET Core apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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