CWE-405: Asymmetric Resource Consumption (Amplification)

This code listens on a port for DNS requests and sends the result to the requesting address.

This code sends a DNS record to a requesting IP address. UDP allows the source IP address to be easily changed ('spoofed'), thus allowing an attacker to redirect responses to a target, which may be then be overwhelmed by the network traffic.

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)sock.bind( (UDP_IP,UDP_PORT) )while true:                         data = sock.recvfrom(1024)if not data:break                            (requestIP, nameToResolve) = parseUDPpacket(data)record = resolveName(nameToResolve)sendResponse(requestIP,record)

This function prints the contents of a specified file requested by a user.

This code first reads a specified file into memory, then prints the file if the user is authorized to see its contents. The read of the file into memory may be resource intensive and is unnecessary if the user is not allowed to see the file anyway.

function printFile($username,$filename){                                                                                 //read file into string                            $file = file_get_contents($filename);if ($file && isOwnerOf($username,$filename)){echo $file;return true;}else{echo 'You are not authorized to view this file';}return false;                      }

The DTD and the very brief XML below illustrate what is meant by an XML bomb. The ZERO entity contains one character, the letter A. The choice of entity name ZERO is being used to indicate length equivalent to that exponent on two, that is, the length of ZERO is 2^0. Similarly, ONE refers to ZERO twice, therefore the XML parser will expand ONE to a length of 2, or 2^1. Ultimately, we reach entity THIRTYTWO, which will expand to 2^32 characters in length, or 4 GB, probably consuming far more data than expected.

<?xml version="1.0"?><!DOCTYPE MaliciousDTD [<!ENTITY ZERO "A"><!ENTITY ONE "&ZERO;&ZERO;"><!ENTITY TWO "&ONE;&ONE;">...<!ENTITY THIRTYTWO "&THIRTYONE;&THIRTYONE;">]><data>&THIRTYTWO;</data>

This example attempts to check if an input string is a "sentence" [REF-1164].

The regular expression has a vulnerable backtracking clause inside (\w+\s?)*$ which can be triggered to cause a Denial of Service by processing particular phrases. To fix the backtracking problem, backtracking is removed with the ?= portion of the expression which changes it to a lookahead and the \2 which prevents the backtracking. The modified example is:

var test_string = "Bad characters: $@#";                var bad_pattern  = /^(\w+\s?)*$/i;                var result = test_string.search(bad_pattern);

This example attempts to check if an input string is a "sentence" [REF-1164].

The regular expression has a vulnerable backtracking clause inside (\w+\s?)*$ which can be triggered to cause a Denial of Service by processing particular phrases. To fix the backtracking problem, backtracking is removed with the ?= portion of the expression which changes it to a lookahead and the \2 which prevents the backtracking. The modified example is:

var test_string = "Bad characters: $@#";                var good_pattern  = /^((?=(\w+))\2\s?)*$/i;                var result = test_string.search(good_pattern);