Each HTTP response message must contain a status code in its first line, indicating the result of the request.
The status codes fall into five groups, according to the code’s first digit:
1xx — Informational.
2xx — The request was successful.
3xx — The client is redirected to a different resource.
4xx — The request contains an error of some kind.
5xx — The server encountered an error fulfilling the request.
Here are the status codes you are most likely to encounter when attacking a web application
100 Continue is sent in some circumstances when a client submits a request containing a body. The response indicates that the request headers were received and that the client should continue sending the body. The server returns a second response when the request has been completed.
200 OK indicates that the request was successful and that the response body contains the result of the request.
201 Created is returned in response to a PUT request to indicate that the request was successful.
301 Moved Permanently redirects the browser permanently to a different URL, which is specified in the Location header. The client should use the new URL in the future rather than the original.
302 Found redirects the browser temporarily to a different URL, which is specified in the Location header. The client should revert to the original URL in subsequent requests.
304 Not Modified instructs the browser to use its cached copy of the requested resource. The server uses the If-Modified-Since and If-None- Match request headers to determine whether the client has the latest version of the resource.
400 Bad Request indicates that the client submitted an invalid HTTP request. You will probably encounter this when you have modified a request in certain invalid ways, such as by placing a space character into the URL.
401 Unauthorized indicates that the server requires HTTP authentication before the request will be granted. The WWW-Authenticate header contains details on the type(s) of authentication supported.
404 Not Found indicates that the requested resource does not exist.
405 Method Not Allowed indicates that the method used in the request is not supported for the specified URL. For example, you may receive this status code if you attempt to use the PUT method where it is not supported.
413 Request Entity Too Large — If you are probing for buffer overflow vulnerabilities in native code, and therefore are submitting long strings of data, this indicates that the body of your request is too large for the server to handle.
414 Request URI Too Long is similar to the 413 response. It indicates that the URL used in the request is too large for the server to handle.
500 Internal Server Error indicates that the server encountered an error fulfilling the request. This normally occurs when you have submit- ted unexpected input that caused an unhandled error somewhere within the application’s processing. You should closely review the full contents of the server’s response for any details indicating the nature of the error.
503 Service Unavailable normally indicates that, although the web server itself is functioning and can respond to requests, the application accessed via the server is not responding. You should verify whether this is the result of any action you have performed.