Alamofire returns .Success on error HTTP status codes

From the Alamofire manual:

Validation

By default, Alamofire treats any completed request to be successful,
regardless of the content of the response. Calling validate before a
response handler causes an error to be generated if the response had
an unacceptable status code or MIME type.

You can manually validate the status code using the validate method, again, from the manual:

Alamofire.request(.GET, "https://httpbin.org/get", parameters: ["foo": "bar"])
     .validate(statusCode: 200..<300)
     .validate(contentType: ["application/json"])
     .response { response in
         print(response)
     }

Or you can semi-automatically validate the status code and content-type using the validate with no arguments:

Alamofire.request(.GET, "https://httpbin.org/get", parameters: ["foo": "bar"])
     .validate()
     .responseJSON { response in
         switch response.result {
         case .success:
             print("Validation Successful")
         case .failure(let error):
             print(error)
         }
     }