Incorrect Parsing of Numbers with Different Radices

Description

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The below demonstrative example uses an IP validator that splits up an IP address by octet, tests to ensure each octet can be casted into an integer, and then returns the original IP address if no exceptions are raised. This validated IP address is then tested using the "ping" command.

import subprocess

def validate_ip(ip: str):

  split_ip = ip.split('.')
  if len(split_ip) > 4 or len(split_ip) == 0:

    raise ValueError("Invalid IP length")


  for octet in split_ip:

    try:

      int(octet, 10)

    except ValueError as e:

      raise ValueError(f"Cannot convert IP octet to int - {e}")



  # Returns original IP after ensuring no exceptions are raised
  return ip


def run_ping(ip: str):

  validated = validate_ip(ip)
  # The ping command treats zero-prepended IP addresses as octal
  result = subprocess.call(["ping", validated])
  print(result)

If run_ping() were to be called with one or more zero-prepended octets, validate_ip() will succeed as zero-prepended numerical strings can be interpreted as decimal by a cast ("012" would cast to 12). However, as the original IP with the prepended zeroes is returned rather than the casted IP, it will be used in the call to the ping command. Ping DOES check and support octal-based IP octets, so the IP reached via ping may be different than the IP assumed by the validator. For example, ping would considered "0127.0.0.1" the same as "87.0.0.1".

Example Two

This code uses a regular expression to validate an IP string prior to using it in a call to the "ping" command.

import subprocess
import re

def validate_ip_regex(ip: str):

  ip_validator = re.compile(r"((25[0-5]|(2[0-4]|1\d|[1-9]|)\d)\.?\b){4}")
  if ip_validator.match(ip):

    return ip

  else:

    raise ValueError("IP address does not match valid pattern.")



def run_ping_regex(ip: str):

  validated = validate_ip_regex(ip)
  # The ping command treats zero-prepended IP addresses as octal
  result = subprocess.call(["ping", validated])
  print(result)

Since the regular expression does not have anchors (CWE-777), i.e. is unbounded without ^ or $ characters, then prepending a 0 or 0x to the beginning of the IP address will still result in a matched regex pattern. Since the ping command supports octal and hex prepended IP addresses, it will use the unexpectedly valid IP address (CWE-1389). For example, "0x63.63.63.63" would be considered equivalent to "99.63.63.63". As a result, the attacker could potentially ping systems that the attacker cannot reach directly.

Example Three

taki.example.com 10.1.0.7
night.example.com 010.1.0.8

The entry for night.example.com has a typo "010" in the first octet. When using ping to ensure the servers are up, the leading 0 causes the IP address to be converted using octal. So when Kelly's script attempts to access night.example.com, it inadvertently scans 8.1.0.8 instead of 10.1.0.8 (since "010" in octal is 8 in decimal), and Night is free to send new Tweets without being immediately detected.

See Also