Net::CIDR::Set versions through 0.20 for Perl did not validate IP addresses. The add method called the _encode method to parse addresses. If the addresses did not look like netmasks or network ranges, then they were assumed to single IP addresses and passed back to itself as a 32-bit or 128-bit netmask. If the argument was not a well-formed IP address, then this would lead to indefinite recursion. An attacker could use this to cause a denial of service.