Example - 1

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.

raise ValueError("Invalid IP length")


int(octet, 10)

raise ValueError(f"Cannot convert IP octet to int - {e}")try:except ValueError as e:
// # Returns original IP after ensuring no exceptions are raised// 
split_ip = ip.split('.')if len(split_ip) > 4 or len(split_ip) == 0:for octet in split_ip:return ip

// # The ping command treats zero-prepended IP addresses as octal// 
validated = validate_ip(ip)result = subprocess.call(["ping", validated])print(result)import subprocessdef validate_ip(ip: str):def run_ping(ip: str):

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 - 2

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

return ip

raise ValueError("IP address does not match valid pattern.")ip_validator = re.compile(r"((25[0-5]|(2[0-4]|1\d|[1-9]|)\d)\.?\b){4}")if ip_validator.match(ip):else:

// # The ping command treats zero-prepended IP addresses as octal// 
validated = validate_ip_regex(ip)result = subprocess.call(["ping", validated])print(result)import subprocessimport redef validate_ip_regex(ip: str):def run_ping_regex(ip: str):

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 - 3

Consider the following scenario, inspired by CWE team member Kelly Todd.Kelly wants to set up monitoring systems for his two cats, who pose very different threats. One cat, Night, tweets embarrassing or critical comments about his owner in ways that could cause reputational damage, so Night's blog needs to be monitored regularly. The other cat, Taki, likes to distract Kelly and his coworkers during business meetings with cute meows, so Kelly monitors Taki's location using a different web site.Suppose /etc/hosts provides the site info as follows:

taki.example.com 10.1.0.7night.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.