Filtering false positives based on server-side behaviors

Reducing the rate of false positives is of vital importance in enhancing the usefulness of signature-based network intrusion detection systems (NIDSs). To reduce the number of false positives, a network administrator must thoroughly investigate a lengthy list of signatures and carefully disable the ones that detect attacks that are not harmful to the administrator's environment. This is a daunting task; if some signatures are disabled by mistake, the NIDS fails to detect critical remote attacks. We designed a NIDS, TrueAlarm, to reduce the rate of false positives. Conventional NIDSs alert administrators that a malicious message has been detected, regardless of whether the message actually attempts to compromise the protected server. In contrast, TrueAlarm delays the alert until it has confirmed that an attempt has been made. The TrueAlarm NIDS cooperates with a server-side monitor that observes the protected server's behavior. TrueAlarm only alerts administrators when a server-side monitor has detected deviant server behavior that must have been caused by a message detected by a NIDS. Our experimental results revealed that TrueAlarm reduces the rate of false positives. Using actual network traffic collected over 14 days, TrueAlarm produced 46 false positives, while Snort, a conventional NIDS, produced 818.