Example - 1

Consider a web application that supports multiple languages. It selects messages for an appropriate language by using the lang parameter.

include("$dir/$lang");
echo "You shall not pass!\n";$dir = "/home/cwe/languages";$lang = $_GET['lang'];if (preg_match("/[A-Za-z0-9]+/", $lang)) {}else {}

The previous code attempts to match only alphanumeric values so that language values such as "english" and "french" are valid while also protecting against path traversal, CWE-22. However, the regular expression anchors are omitted, so any text containing at least one alphanumeric character will now pass the validation step. For example, the attack string below will match the regular expression.

../../etc/passwd

If the attacker can inject code sequences into a file, such as the web server's HTTP request log, then the attacker may be able to redirect the lang parameter to the log file and execute arbitrary code.

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.